The days are getting shorter in the northern hemisphere, and with the ongoing pandemic, most of us expect to be spending more time in our homes than normal. Creating a healthy home environment is more important than usual, and the light inside your home is an often underappreciated part of this.

There has already been some explicit discussion[1] about the importance of lighting for health and productivity, as well as many mentions of it in other places. Nonetheless, based on discussions I've had recently within the community, I get the impression that it is helpful for me to write up the results and tinkering that have done over the past few years.

First, I will cover some of the research on how our bodies respond to light, and which particular characteristics of natural light we want to mimic. Then I will explain solving this problem is hard and my overall strategy for solving it. Finally, I will give some specific advice on what to buy and how to arrange things.

I give quite a lot of background before offering any specific advice. Although I think the background information might help you make good decisions, you should feel free to skip the next section if you're in a hurry or if it seems uninteresting.


Note: My background is in optics, not physiology or psychology and I began researching and writing this document almost four years ago. My original draft, as well as many of my sources, have been lost in the intervening years, so what you're seeing here is based on a combination of my notes that survived, my recollection of the research, and a partial duplication of the research. To make matters worse, it does seem that new research has come along since I began this project, so this is likely out of date. My guess is that most or all of the practical conclusions still stand, but I am only moderately confident of this. As much as I would like to take the time to update the research, past experience suggests that I will never actually publish it if I try to put too much more work into it. I welcome corrections, and if there is sufficient enthusiasm around this topic, I may try to write an updated version.

Your body uses light to synchronize its internal clock and to modulate your mood and alertness[2]. While the particulars of the lighting in your environment are important, your only perceptual access to information about lighting in the moment comes from your visual system, which is poorly adapted to solving the problem of determining the intensity and spectrum of a light source. This is mainly because our vision is optimized more for accurately identifying materials, textures, colors, and other properties of our surroundings than it is for knowing details about sources of light. This has the consequence that some of our default intuitions about the nature of ambient light are wrong, so when we're building our lighting environment, it can be difficult to make accurate judgments just by looking at things. We can do better if we use quantitative measures and our scientific understanding of how things work to solve the problem.


In addition to visual photoreceptors that are used for seeing things, your eyes contain non-visual photoreceptors which serve non-visual functions. Melanopsin is a photopigment that is found in cells in the retina[3]. It is sensitive to blue light, and when activated, these cells send signals that help with things such as regulating our internal clocks[4]. Unlike the our visual photoreceptors, which are more densely packed in the center of our retina[5] than in the periphery, these photosensitive cells are distributed relatively evenly throughout the retina[6], so that light coming from both the periphery and the center of our vision is important. Although I have found contradictory accounts of how visual photoreceptors and melanopsin-sensitive cells interact, there does seem to be some evidence that activation of our visual photoreceptors can influence the functioning of our melanopsin-containing cells. This seems consistent with many people's experience that blue LED lighting is more relaxing than white light. On the other hand, it may just be that it takes a lot of light to activate these cells[7].

I find the literature on how our mood and circadian rhythm respond to light exposure throughout the day to be somewhat confusing, and I have not tried to unravel it in detail. As far as I can tell, it depends on the full history of exposure over a course of days, not something simple like "total light exposure over the past N hours" or "Exponentially-weighted average of light exposure". For this reason, I am not particularly optimistic about figuring out the details of the algorithm used by our bodies to decide when to be tired or when to be happy, or any other aspect of our lives that we might want to improve.

Given this, as well as my priors in favor of just letting our bodies have the regulatory mechanisms they're "designed" for, it seems that we should try to create lighting that is similar to natural light and well-synchronized to our preferred schedules for sleeping, working, and relaxing. This means that, in order to signal to our bodies that it is daytime, and therefore time to be wakeful, we should use light that is bright, white, and distributed over our full field of view.

When we want to signal to our bodies that it is time to start winding down for the day, we want to avoid bright, white light. I have not done as much research on the best timing for switching over or the optimum spectrum for this, but based on the fact that it is dark at night, sunsets are yellow, and on my personal experience and that of people I know, there should be several hours between bright daytime lighting and bedtime, and the difference between the best lighting for the late evening and the daytime is quite large, both in terms of intensity and color.

Natural vs Artificial Light

Creating light that effectively mimics natural light is difficult. The first problem is that natural light is almost always much brighter than artificial light. Even on a very cloudy day, you might notice that the insides of buildings look dark from the outside, the brightest parts of most indoor spaces are next to windows, and if you use a camera or a light meter to measure the amount of light, you'll find that things are more brightly illuminated outdoors than indoors.

The second problem is that natural light has a broad, continuous spectrum, which contains only a few gaps within the visual range of wavelengths, while artificial light almost always has large portions missing, especially at shorter wavelengths. This has to do with how artificial light is generated. Incandescent bulbs are blackbody sources, but they're not hot enough to contain very much light at shorter wavelengths. Incandescent bulbs are usually around 3000K while the sun is 5800K. Fluorescent lamps and LEDs generate UV and blue light, which is then absorbed and re-emitted at longer wavelengths by phosphors. This technique produces light that can have a correlated color temperature (CCT) anywhere from 2000K or less to over 6000K[8]. Unfortunately, this results in a very spiky spectrum in the case of fluorescent bulbs and a large dip between blue-ish and yellow-ish wavelenths for most LEDs. Here are some spectra from Wikipedia[9]:

Spectra of different light sources

All this missing light can be easy to overlook. Our visual system projects all the spectral content onto three dimensions (or two, depending on how you look at it), and then does a remarkable job of giving us accurate information about the color of objects in a variety of lighting conditions (the blue-black/white-gold dress phenomenon is remarkable because of our failure to reliably solve this problem), which is great for many tasks, but terrible for evaluating the quality of a source of light.

Nonetheless, this missing light is important. In addition to failing to provide the appropriate signals for time of day, poorly filled out spectra can make things look unappealing, give us inaccurate information about the colors of objects, and make an environment feel generally less pleasant.

What I am not trying to replicate (for now)

There are some properties of natural light that are less important or more difficult to replicate. Although these might be desirable in certain contexts, we'll ignore them for now.

  1. Non-visible light: We're not trying to add in ultraviolet or infrared light. Although getting some UV might be good for people with a vitamin D deficiency, doing this safely and without other consequences (like degrading plastics or bleaching colors in your office or bedroom) seems hard, and I don't recommend trying to do this.
  2. Spectrum and intensity that changes continuously throughout the day. There are lamps that do this, and they seem especially pleasant to me for waking up in the morning, but I think this is particularly hard to get right and less important than other things
  3. All the little details. Natural light has lots of subtleties that might contribute to making it seem pleasant. For example, the sky is blue and polarized and direct sunlight is a bit yellow. The anisotropy of the color of the light makes for pleasant photographs. Things like clouds and leaves can make soft, moving shadows. Again, it would be neat to replicate this, but it seems hard to get right and I think it's a much lower priority than other things.

Why this is a hard problem to solve

To illustrate how much your visual system can correct for variations in lighting, I took photos and illuminance measurements of a photographic color calibration card in a variety of environments, shown here:

Lighting in various environments

The camera and software are calibrated to the photo in the top-left with the green border around it, which I took during a typical cloudy day in San Francisco, so the lighting is reasonably close to a 6500 kelvin blackbody spectrum. I tried to get the brightness of each image the same, so the apparent brightness of the display on the lux meter should give you a sense of how much brighter some of the environments are than others.

The yellowest lighting in these photos has a red-to-blue ratio 15 times higher than the bluest lighting, and the intensity varies by five orders of magnitude. Nonetheless, the card mostly looked the same to me in nearly every context. I was aware that the incandescent bulb was dimmer and yellower than the light from the cloudy sky, but the effect is nowhere near as extreme as the photos suggest. The two main exceptions to this were the LCD photo, for which the light was very dim (though I did not try allowing my eyes to adjust), and the low-quality fluorescent lamp, in which many of the colors just didn't look quite right.

I hope this will give you a sense for why we should not rely on our visual systems to tell us how bright or how blue a light source is. Fortunately, we can measure these things in other ways.

General approach to solving the problem

Given everything that we have so far, we want light that is:

  1. As bright as is practical during the day, preferably at least 1000 lux over the full room, with a CCT close to that of the sun (5500K)
  2. Much dimmer in the evening, with a much lower CCT (2700K)
  3. Covers all or most of our visual field
  4. Has a full spectrum, with few holes at visible wavelengths
  5. Does not have other annoying characteristics like sharp shadows, flickering, or the generation of unnecessary heat or noise

Natural light meets all of these criteria, except in some circumstances being annoying (direct sunlight in your eyes) or being too bright in the evening (if you live very far from the equator). I always try to work near a window if possible, and get as much natural light as I can. That said, at the moment I'm writing this, it has been dark outside for over an hour and it's only 6:25pm. The whole reason this is a problem to be solved is because we can't just use natural light all the time.

Getting enough light

As I have mentioned already, getting your indoor daytime light to be anywhere near as bright as outdoor lighting takes effort. If you install some lighting, and you still experience seasonal depression, the first thing I would try is adding more light. Most people will be limited by how many bulbs they can reasonably install in a room, either due to space, power, or heat, before they'll have too much light. Or they'll have a bunch of lights and think "Surely this is enough! Look at how many bulbs I have!", but until it is sufficiently bright to make your computer screen look dim, it is probably not as bright as natural light. In my small living room, I find that my setup which provides 20,000 lumens, and gives me a relatively uniform 1000 lux[10] is sufficient. I wouldn't mind more light (and I intend to add more once I'm in a more permanent living situation), but it is enough for me to feel alert during the day and avoid or at least reduce the effects of seasonal depression, so long as I'm able to spend time in there later in the day. For a larger room, you'll need more light, in a way that should scale roughly with the inside area of the walls, ceiling, and floor.

Getting the right spectrum

The spectrum of your daytime light is the trickiest part, but I have found two strategies that seem to work:

  1. Combining lots of different bulbs so they can fill in each other's gaps.
  2. Finding bulbs with a very high color rendering index, especially those marketed for photography.

For a while during grad school, my office had, in addition to the fluorescent tubes installed in the ceiling fixtures, three different kinds of LED bulbs and two different kinds of very high lumen compact fluorescent (CFL) bulbs. A disadvantage of variety is that you can't just buy a huge pack of all one kind of bulb, which is usually the most convenient and inexpensive option. An advantage of ordering many different bulbs is that if you get a particular one that you don't like, you only bought a few of them (for example, I had some which made an annoying buzzing sound and others that had an annoying green tinge). For high CRI bulbs marketed to photographers look for CRI of at least 85-90. A safer bet is to go with 95+. Once you know what you like, when you need to replace bulbs or build a lighting system in a new room, you be able to get it right more quickly.

For daytime color temperature, I recommend 5300-6000K. Many people dislike the light from high color temperature light bulbs, but I think this is because there is a lot of terrible indoor lighting out there, including light from bulbs that have a high color temperature but poor color rendering. A CCT of 6000K is very close to the color temperature of natural daylight through a window. If you get it right, people might even mistake the light coming out of your room for daylight! Still, some people seem to do alright with significantly lower color temperature. I've heard of some people doing well with as low as 4000K.

Evening light should be less than 3000K. Incandescent and LED bulbs both work well without much need to combine bulbs to fill in gaps at these color temperatures. If you do find low CCT bulbs to be unpleasant somehow, you might try either incandescent bulbs or high CRI bulbs.

My preferred light for the last hour before bedtime is an orange LED bulb with so little blue light that I can't tell blue objects apart from black.

Covering your full visual field

It might be tempting to get a therapy lamp or to get fewer bulbs and just put them in the corner of the room where you're sitting all the time. But this can be hard to get right. For starters, remember that the photoreceptors you're trying to activate cover your entire retina, so you want to illuminate your full visual field if possible. This covers a very wide angle of 210 degrees, which extends slightly behind you. Once you account for turning your head some of the time, this can easily extend to well over half the room. Another problem, at least for me personally, is feeling confined to one part of the room. This is more of a problem at home than it is at my office. When possible, I recommend just illuminating the whole room.

Still, this may not be an option for everyone. If I'm working in a shared space, I don't want to impose my preferences on everyone else, nor do I want to spend hundreds of dollars on lights that I'll only use for a couple months. Sometimes I have lights arranged just to illuminate my own part of the shared space. Similarly, some people use therapy lamps in these settings.

Evening and nighttime lighting

Evening lighting is easier. The main difficulty is in having multiple sets of lights for different parts of the day. Some people will only need daytime lights for their workplace. I use inexpensive low CCT LED and fluorescent bulbs, usually 2700K, and I switch from my bright daytime lights to those at 7pm. In my bedroom, I also have an orange LED bulb that is very dim, but sufficient for reading, and I switch over to this for the last 30-60 min before going to sleep.

The little things

I said I'm not trying to solve all of the problems with reproducing natural light indoors, but there are a few small things that can get it a little closer, and which can be nice.

Lightbulbs can cast annoying, sharp shadows, and if they are in your visual field, they can be annoyingly bright. Diffusers help with this. Most light fixtures have a lampshade or some other device to increase the effective size of the source, softening shadows and reducing the intensity of the light fixture itself. If you want diffusers that are spectrally neutral (that is, they do not absorb some colors more than others), you can find those in photography stores.

Having a bit of variety in the spectrum can be nice. Outdoors on a sunny day, shadows are blueish, and reflected sunlight is yellowish, for example. Because of this, even if you had bulbs with a perfect 6000K blackbody spectrum, it may still seem unnatural. One thing that can help with this is to add in some lower color temperature lights. Personally, I find that one ~3000K bulb per three ~5500K bulbs is very pleasant and less artificial-feeling than just 5500K[11].

Specific recommendations on what to buy

Since links for buying things online are only useful geographically and for a limited time, I will recommend brands that I have been happy with and explain my overall strategy, along with links to specific products.

Lux meter

Being able to measure how much light is in your house is useful and inexpensive. I use a $30 Uceri meter that I ordered from Amazon.

Light bulbs

I have found that fluorescent bulbs that are designed for photography are usually a good bet for color rendering. In particular, I have been using these very large bulbs from Alzo (note that these bulbs are huge and fragile, will not fit in most fixtures, and probably should not be used with strings of light sockets). For LED bulbs, I have been reasonably happy with bulbs from Alzo and Cree. Phillips and GE make bulbs that I have been happy with, but they also make bulbs that I have been quite disappointed with, so be careful.

For LEDs, I know one person[12] who has had success with Yuji corncob bulbs, for both high and low color temperature. He was kind enough to give me one of the 3200K bulbs, which I find to create very pleasant light, and which I now mix in with my 5500K bulbs during the daytime.

As I mentioned earlier, evening bulbs are generally easier, since high CRI, low CCT bulbs are simpler to make. Any incandescent should have a nice spectrum, and most LEDs with high CRI should be okay.

Bedtime bulbs

I've been satisfied with my orange LED bulb from Sunlite. One person I know prefers red bulbs over orange. I do not recommend incandescent bulbs with red/orange filters, as they are less efficient, get hot, and have a filament that still looks white-ish through the filter, which find I to be annoying.


Installing these bulbs can be a bit tough, especially the gigantic CFLs. Usually you'll want to have them above eye level, or at least have a diffuser. They need to be somewhere that they don't get too hot, and where they cast light in a way that illuminates the room evenly.

If you want to build your own fixtures with neutral diffusers, I purchased a couple of generic diffuser "socks" from Amazon several years ago, and I've been happy with them. They are no longer available, but if you want to see what I'm talking about they're here.

If you have many fixtures or fixtures in inconvenient locations, you may want to get a remote like this one from IKEA. I would imagine that a smart lighting setup could help, but I have not tried one myself.

Photography stands

I like using photography lighting stands because they're inexpensive and tall enough to get the bulbs well-above eye level. If you're worried about them being ugly, they can probably be decorated (I intend to add some fake ivy to mine, for example). I use these from Amazon

DIY fixtures

It is also easy to build something that can sit on top of a shelf. One way to do this is to use cable ties to attach a power strip to a wooden board, and stick socket adapters in the power strip. Unforunately, I cannot share a photo of the last one that I built, because the pandemic ate it.

Wrapping up

I hope this has been helpful. The most important things to remember are:

  1. More light is good, and it is difficult to have too much light.
  2. Try to cover your full visual field
  3. The spectrum of your light matters a lot, and during the day, you probably want >5000K
  4. During the evening, you probably want <3000K
  5. If you normally find high color temperature (bluish) light to be unpleasant, try using higher quality lighting. Mixing bulbs or using stuff made for photographers helps with color rendering and overall pleasantness

I welcome any comments on your experiences! I know that others have spent time researching and experimenting, and many have probably done a better job of tracking down or constructing good lighting equipment than I have. I am also happy to add links to other people's write-ups on this.

Acknowledgements: Many people helped me along the way while I was researching and writing this. In particular, thanks to my former lab mates at UT Austin for putting up with my lighting experiments, Meredith Johnson for her encouragement to start the project in 2016, Katja Grace for reminding me that I should just post stuff instead of worrying so much about whether it is bad, and everyone that I've shared notes with over the past few years, especially those at the FHI office.

  1. See: ↩︎

  2. ↩︎

  3. The retina is the photosensitive part of the eye, analogous to a CCD or CMOS sensor in a camera ↩︎

  4. ↩︎

  5. ↩︎

  6. See fig 1E: This diagram seems to suggest there are actually more melanopsin-sensitive cells in some parts of the periphery of our visual field. It is plausible to me that you could look at this map, and determine where to put lights in your house, for example, ensuring that there is more light in the upper portion of your visual field, but I have not taken the time to decipher that diagram into such a map of where the illumination should be in our visual field. ↩︎

  7. A potentially stupid and probably not very informative experiment I did once: I used to work in a lab where I had to wear laser safety glasses all the time. These glasses effectively cut out all of the blue light, in addition to severely reducing overall light transmission. Wearing these all day is really bad for people who suffer from seasonal depression, so I tried attaching some blue LEDs to the temples of the glasses so that the light would reflect off the inner surface of the lenses and into my eyes. When I tried them in the lab, the first thing I noticed was that the lab seemed way less depressing than usual. After maybe 30 minutes, I noticed that I was feeling somewhat less drowsy and less compulsion to go outside than I often did in the lab. Unfortunately, the lights gave me a terrible headache, so I never used them enough to see if this was a real effect or not.

    Note that although melanopsin-sensitive cells are referred to as "non-visual" there does seem to be more recent research suggesting that melanopsin may play a role in visual perception as well. ↩︎

  8. Remember that this is the temperature of a blackbody source that is most similar to the the light source in question. Be careful, because the spectrum is often very different from a blackbody! ↩︎

  9. has a nice diagram showing spectra for various light sources ↩︎

  10. Lumens are a unit for luminous intensity (the amount of light per time, weighted by how bright it appears to humans)

    Lux is lumens per area, or how brightly-lit a surface is, according to human vision. ↩︎

  11. H/T to Ben Weinstein-Raun for bringing this to my attention and giving me a bulb to try it out ↩︎

  12. These are the bulbs that Ben recommended to me which I found useful for adding some more yellow light to my daytime lighting. ↩︎

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First, some meta-level things I've learned since writing this:

  1. What people crave most is very practical advice on what to buy. In retrospect this should have been more obvious to me. When I look for help from others on how to solve a problem I do not know much about, the main thing I want is very actionable advice, like "buy this thing", "use this app", or "follow this Twitter account".

  2. Failing that, what people want is legible, easy-to-use criteria for making decisions on their own. Advice like "Find something with CRI>90, and more CRI is better" is better than "Here's a big, complex description of the criteria you should try to optimize for that will inevitably tradeoff against each other".

  3. The technical background is important, but in a somewhat different way than I'd thought when I wrote it. When I was writing it, I was hoping to help transmit my model of how things work so that people could use it to make their own decisions. I still think it's good to try to do this, however imperfectly it might happen in practice. But I think the main reason it is important is because people want to know where I'm coming from, what kinds of things I considered, and how deeply I have investigated the matter.

On the object level:

  1. As far as I know, the post does not contain any major errors in the technical background.

  2. Some of the practical advice is probably skewed too far toward my personal preferences. A lot of people seem to prefer lower color temperature light than I do, for example.

  3. I now think that getting LEDs with a high-quality spectrum is fairly easy with the right budget, and the harder part is figuring out where to put them to illuminate your visual field without doing something annoying like having a bright cornbulb at eye-level. The Lightcone team seems like they're making good progress on this and doing good experiments.

  4. My impression from talking to people over the last 14 months is that it would be a pretty huge public service for someone to keep an up-to-date list of what the best lights are on the market(s) for various budgets and circumstances, as well as a bunch of guides/photographs for helping replicate specific solutions people have found that work for them.

  5. By far my favorite new-to-me lighting product is the Yuji adjustable-color-temperature LED strips/panels. I'm excited to experiment with them and hopefully publish some useful results.

Quick question: 
When you say, "Yuji adjustable-color-temperature LED strips/panels"

Do you mean these guys?

It looks kind of intimidating to setup, and is pricey, but maybe is worth it.

Yeah, those or these:

Or the long 2700K/6500K ribbons.

They're not as bad to setup as I'd feared, though they are a bit of a hassle. I'm experimenting with them now, and I will write about it if I come up with a good way to build a light fixture with them.

Just checking; I think you might have sent the wrong link though?

Yes, sorry, I somehow missed your reply until now. I probably meant this:

(That link I did share is pretty interesting, BTW. It describes some stuff during the Wild West days of medical research, including the use of a frighteningly primitive laser to kill a tumor in a kid's eye)

Interesting. Thanks!

Do you have any updates on this? I'm interested in this.

Yeah! I made some lamps using sheet aluminum. I used hot glue to attach magnets, which hold it onto the hardware hanging from the ceiling in my office. You can use dimmers to control the brightness of each color temperature strip separately, but I don't have that set up right now.

See also Ben Kuhn: and me: (both of these are from about a year ago)

My update since my blog post is that I have a Yuji "high bay" luminaire, which I had linked to from my blog post but not tried last year. Now I can confirm that it is pretty bright, good color spectrum and easy to use -- I place it on top of a bookshelf facing towards the corner.

Thanks, I've added those links.

I actually did consider trying to start a bright light company or trying to make an indoor lighting consulting company in late 2016 or early 2017, but decided against it.

This is really cool!

I thought about how to do this a while back, but I couldn't think of a good way to do it without it being huge, expensive, or a fire hazard. At the time, I do not think there were bright enough LEDs to really make this work, and I simply hadn't thought of a good way to solve the Rayleigh scattering problem.

I also hadn't thought of the trick with a bunch of boxes with separate LEDs. It seems like you could make a bunch of smaller versions of his larger one (that is, using smaller LEDs with reflective collimators and a layer of colloid for scattering), and get a similar effect without needing such a large volume. I may try this out.

I really appreciate you writing this!

Just wanted to add that my informal impression from a few experiments is that the difference between 90 CRI and 95+ CRI is actually large. 


Sounds about right for CRI. I think there are a couple things going on with it:

  1. CRI is a mediocre measure to begin with, as far as the subjective quality of the light is concerned
  2. As far as I know, there's no oversight, third party measurement, etc

I'm not sure how much of it is bad measurement and how much of it is CRI being a poor metric, but the best 85 CRI bulbs I've seen are substantially better than the worst 90 CRI bulbs, which is why I'm hesitant to tell people to rule out 85 CRI bulbs entirely. I've not encountered any 95 CRI bulbs that are bad, so maybe the better advice is just to go for 95+ CRI whenever possible.

Yeah, makes sense. Fwiw, I have encountered one purportedly 97+ CRI lamp that looked awful to me. 

You mentioning this years ago at a meetup triggered me to write a blog post about my approach to this. Mostly it addresses automation and CCT correlation to time of day.

If somebody wants to throw money at this problem look at Hue bulbs and augment the lumens with some corncob LED bulbs. Automate the bulb color temperature based on time of day at the switch (time-slot based scene rules) or with a timer rule (or both). If you get a warm and a cool temp corncob bulb, you can power them each with a smart plug (e.g., the Zooz ZEN25 double switch, which has two switchable plugs in one unit). Set your home automation rules to toggle the smart plug with the Hue switch and power the cool bulb, the warm bulb, both, or neither depending on the time of day.

One thing I'd emphasize is that, if you're optimizing for as much light as possible, adding diffusers will make your end lumen count take quite a hit. I've never measured it, but I would guess that the loss is on the order of tens of percents. This is pretty unfortunate since naked bulbs are both pretty unpleasant to have in your periphery, and also very unaesthetic. But if you're going as far as erecting structures that power bulbs that are circumnavigating your living room, I'd advocate putting a lot of thought into diffusing the light while minimizing lumen losses.

Forgive me if I missed it in the post, but might it be cheaper to use some kind of visor if you're only illuminating for personal benefit? Much easier and cheaper to fill your FOV with light that way.

Visors I've seen do not illuminate all that much of your field of view. Plus I'd prefer not to have to wear a thing all the time. But maybe there are better products like this now? Or were you thinking of building something?

Thank you, great post! I especially liked your insight on the color rendering index.

There are some reports that LED lighting can damage your eyes. From a quick glance at Wikipedia, the evidence does not seem very convincing, but I'm not an expert. What do you think about those claims?

And if there is no particular danger from LEDs: Is there an inherent danger in looking at high-lumen lights (regardless what type of light it is)? At which point do we have so much brightness in one small spot that it becomes dangerous?

Two relevant links:

The first Reddit link bottoms out at this study. The key detail not mentioned in the abstract is that this is in rats whose pupils have been dilated with atropine, and the rats are in conditions where a human with pharmaceutically-dilated eyes would not be able to function without sunglasses. This makes the paper's comparison between different types of light sources and wavelengths pretty uninformative.

The rest are all about what level of brightness is acceptable. But we have a pretty good point of comparison: we know sunlight is safe (as long as you don't look at the sun directly); and the indoor lighting solutions under consideration are all significantly dimmer than sunlight.

Curated. I think having better indoor lighting is a surprisingly tractable and important problem. I think it's great to not only get a timely reminder to prioritize it, but to bring in a lot of domain-specific knowledge of how to go about it.

Lux meter

Being able to measure how much light is in your house is useful and inexpensive. I use a $30 Uceri meter that I ordered from Amazon.

Is there much benefit in getting the Uceri meter rather than using a free phone app like Light Meter which uses my phone's camera to measure the lux?

I just compared my phone (Pixel 3a) to my Uceri meter, using the app that you link to here.

It seems okayish. The main issue with it is that the camera has a limited field of view, so it does not capture light coming from every direction. With the lux meter and the phone both pointed directly at the lamp near my desk, the phone reads about 35% higher than the lux meter, and when they're both pointed at an angle that barely excludes the lamp from the field of view, the phone reads 45% lower than the lux meter. The reading on the lux meter only changed by 4% between these two angles.

I'd say if you're okay with getting within a factor of two, the phone app is fine. The phone and the lux meter seem to agree the best when the lamp is just barely inside the field of view of the camera.

I have compared a professional luxmeter with the free app "GPS Status" which also displays a Lux value. The readings were within 30% tolerance for normal lighting situations. A smartphones light sensor will clip at some (high) value though. This clipping value can easilly be tested by pointing the sensor directly towards the sun.

It might well depend on the smartphone model. My tests were done with an older Motorola and a Oneplus 6. I'd guess a free app will be sufficient for most purposes.

Another thing that might be worth considering is that if you want to estimate your body's response to light, a camera-based illuminance meter with a very wide field of view and some amount of clipping is probably better than a standard lux meter with a diffuser, since it will account for whether the light is spread over your full field of view or not.

An interesting article and timely given the lengthening nights in the northern hemisphere. I tackle the indoor lighting challenge in several ways. First, I have a large number of multi-bulb chandelier light fixtures. I put a different bulb in each fitting, and let the large number of crystals blend them together for me. (This also tends to create faint shadows and color variations throughout the room which is helpful for hiding evidence of children touching walls with dirty fingers....) I don't worry too much about what color any particular light fixture has at any given time as long as there is a good variety. As you pointed out, filling in the gaps in the spectrum seems to be helpful. In terms of brightness, I place high output bulbs in parts of the house I frequent during the course of the day (kitchen, living room), and low output bulbs in places I tend to only go in the evening (dining room, bedrooms, bathrooms). Actively managing beyond that seems like more work than I'm interested in. 

I also installed much larger windows on the north and east sides of my house and chopped down trees blocking my view of the sky on the north side of the house. I think that is more helpful than all the lighting combined.

Spectrum and intensity that changes continuously throughout the day. There are lamps that do this, and they seem especially pleasant to me for waking up in the morning, but I think this is particularly hard to get right and less important than other things

My current setup uses a single 800 lumen lifx bulb for continuously changing light in the morning and evening, and 24 normal bulbs to get high amounts of light during the day (that automatically turn on/off with a socket timer). I think that captures the main benefits of continuous lights, without needing more than one special bulb.

One thing to consider is not just the effect that lighting has on you, but what it has on others. For example, when I think of the quality of different friends and family members' houses, one of the defining components is how and how well they are lit. My aunt with giant windows and balanced, bright lights easily beats out my friend who has two reading lamps and a kitchen light that get turned on during game nights. 

One thing I would ask, Richard, is how do you manage your lights? My current set up has me turn on four different pairs of lights across my room (plus a giant window which I leave always open). Turning the lights off at night is easy- I just go down in temperature because otherwise it makes the TV/monitors annoying to look at- but actually turning on four different lights when I wake up is hard to do. I'm tired.

Buying is easy! Managing is hard!

I agree that it can be hard to manage a large number of fixtures. I know that other people have come up with more clever solutions than I have, but one thing that I have found helps is to plug as many lights as is practical into the same outlet, and use a remote like this one from IKEA: (I'll add this to the article)

I've been using this multi-plug remote controller set for several years, and it's great.

Thanks! I’ve been noticing the world getting darker and this post is quite timely.

Has anyone here looked into metal halide lamps? They seem to be cheap, similarly efficient as LEDs, and since they are available in basically arbitrary brightness they solve the problem that it's really hard to find space for all these fixtures for LEDs. I'm moving in a few months and I'm currently planning to outfit my new place with at least 1–2 of them in addition to LED, so any advice is welcome.

Although getting some UV might be good for people with a vitamin D deficiency, doing this safely and without other consequences (like degrading plastics or bleaching colors in your office or bedroom) seems hard, and I don't recommend trying to do this.

I have been thinking about this recently (prompted again by vitamin D being a significant factor in protecting against COVID, but also it gradually making its way to the top of the "well-supported health things I ought to try" list), and am curious for details here. That is, suppose I get a bunch of RayVio 293 nm LEDs, and I want to figure out how to use them in a way that's safe and minimally does other bad things. What tools should I be buying to measure things, what sorts of damage should I be looking for, what sorts of things might make sense?

The biggest reasons why I do not try to do this myself are:

  1. I haven't done enough research to know what a good dose is in terms of wavelength, intensity, and time
  2. It seems hard to build a source that gets the dose right, without basically building a tanning bed
  3. I think it's hard to know what dose you're actually getting

It's also possible that I am unreasonably worried about shining artificial UV light on myself.

Assuming you have an answer for 1 (and that paper looks promising for having a good answer to this), you need to build a source that illuminates your skin in some reasonably even way. This can be hard, since most light sources radiate over a wide angle, so that you get 1/r^2 drop off in intensity. For example, just now I used my lux meter to measure the intensity of the light near my desk on my forehead and my stomach, and they varied by almost a factor of two. One potentially pretty neat way to fix this would be to use some kind of large collimator like the one in that video that Robert Miles mentioned. The intensity will drop off much more slowly with distance from the source, which should make it easier to get a predictable dose, plus our intuitions about dose from the sun will will still sort of work (for example "The part of my skin that is farther from the sun but is normal to the sun's rays is getting more"). If I did build a big sunlight simulator like that, I would be tempted to add in a little UV.

I recently learned that a common way to measure UV dose is to paint something with a white pigment that absorbs UV and reflects visible/IR light, and see how much it heats up when illuminated. I'm not sure how well this would work when the dose rate is less than 10 mJ/cm^2/minute as in that paper. You probably would just want to use a UV index meter, like they did in the Nature paper you linked.

What would be good is to have some kind of inexpensive cumulative dosimeters that you could place on yourself and around in the area where the UV is so that you can check that you're not inadvertently getting way more or way less than you want. A quick Google search for "UVB dosimeter" looks like there are options, but I have not looked at any of them enough to know if they're any good.

As your background is in optics, can I get your opinion on this paper on the intersection of indoor lighting, optics and our responses to light wavelengths, and public health:

To me it's on the knife's edge separating woo and legitimate niche research, I would like an educated opinion.

Interesting, thanks for sharing.

I don't think I'd take the paper itself seriously. Melatonin Research doesn't seem to be a real journal and the paper looks very unprofessional. One of the paper's authors is a lighting engineer who I think is trying to get people to use indoor lighting with more NIR, while the other seems to be a late-career melatonin guy who probably did some good research at some point. This isn't to say that we should dismiss things unless they're written by Credentialed Scientists® in a Serious Journal® and formatted in The Right Way®. But peer review does provide some protection from nonsense and we should be cautious about stuff that's trying to look like it's part of the academic peer-review system when it's not.

As for the content, I'm not able to say much about the biology. I didn't notice anything while skimming it that I know to be wrong, but I know very little biology, so that's not surprising.

The optics is suspect. I'm pretty dubious about the thing with a "light guide" and the idea that the structure of a persons head/brain is supposed to distribute NIR in some way. I almost dismissed the stuff about NIR and the brain out of hand, but it turns out a human skull/scalp can diffusely transmit something like a few percent 830nm light, at least according to this paper with a simple but reasonably compelling experiment.

I'll admit that paper does leave me wondering if longer wavelengths do matter somehow. It looks like a person standing in sunlight or next to a campfire probably does get some non-negligible NIR illumination through a substantial portion of their body. But I wouldn't take that paper as strong evidence for or against much of anything.


Thanks for taking the time to read.

Here's a 4 time board certified MD's primer on light that mentions the paper (and related research): 

Other highlights:

  • Sun exposure decreases all cause mortality, including from melanoma
  • Sun exposure has beneficial effect on brain volume in multiple sclerosis, independent of consequent vit D

Have you thought about flood lights? They seem to be relatively cheap per lumen and can be pretty powerful, meaning they don't take up much space. Also, they can be usually attached to a wall or ceiling.

I have thought about it, and I am somewhat considering one for my office. Heat and power consumption used to be a concern, but now that there are good LED floodlights available, the main issue for me, personally, is that in the spaces where I work they will tend to cast sharper shadows that I want, since most floodlights behave like small sources and you only need one or maybe two to light an area.

I'd be interested to hear about any particular arrangements you've considered!

I spent 5 minutes searching for replacements to the various products recommended and my search came up empty.

Is there someone who has put together the needed list of bright lighting products on I tried doing it myself and ended up hopelessly confused. What I'm asking for is eg. two desk lamps and corresponding light bulbs that live up to the criteria.

I'll pay $50 to the charity of your choice, if I make a purchase based off your list.


If I insist on only using existing ceiling light fittings -- so I need bulbs with (as I'm in the UK) standard bayonet mounts and form factor similar to that of traditional incandescent bulbs -- are there any good options? I'm having trouble finding anything with both decent brightness and good CRI, but maybe I'm looking for the wrong thing (I've been assuming LED is best these days) or in the wrong places.

Just ran across an awesome YouTube video that shows how to build a light that mimics artificial sunlight, including parallel rays and the color effects from Rayleigh scattering.  Seems like a pretty cool project, and I would definitely make one if I was planning staying at my current location long term.

There's a (very reasonable at this time, I think) emphasis in this high-preformance day and evening lighting on trying to match natual brightness and spectrum.

Have you come accross any good data or models for late-day/early dusk?

I ask because while there's a lot of good data and models for mid-day solar illumination, I don't know enough to know how much to trust them towards the end of the day (which I suspect is not really the designed use case), nevermind stuff after sunset—and there's over an hour between sunset and astronomical dusk.

(But yes, for midday... lots of models to choose from. The SMARTS model seems like a good choice. Good-enough accuracy, covers spectrum from 280–4000nm, easy setup, and has relatively few inputs and outputs to worry about. Big comprimise is that the diffuse illumination is treated as uniform, and it doesn't do clouds.)

This is a good question. I have not looked, and the closest thing I have done is to measure the color temperature and intensity according to my camera for direct sunlight just before sunset (seems to be about 2700K, 1900lx).

I would think that you could get a decent estimate using SMARTS if you adapted the input file to a ray going through a lot more atmosphere, but I'm not sure. I haven't looked at the code and it might do something like make approximations that only work for short path lengths or low optical densities or something.

The most 'natural light' LEDs I know of seem to still be too new to be commercially available in ready-out-of-the-box modules, but with a little time to engineer the setup I think one could assemble these into a usable thing. Significantly more deep purple wavelengths and less 'cyan dip' than the alternatives. Here is a quick comparison of spectra at 5000K, vs one of the Yuji 95CRI ones:

Of course, these are not smart bulbs, so there's an obstacle to automation again. I think solutions from the indoor agricultural field probably exist, but I haven't found them in a quick look. Hard for me to say if the difference here is worth the requisite fiddling, but if there's interest I will take a harder look at what it would require.

Having not used them yet, I can't personally attest to the light being higher quality, but the custom flashlight community is pretty discerning and they seem to be quite taken with them.

These look very promising, ship to Europe too. Extra high-CRI, very powerful (up 2600 lumens/m) and even dimmable? Wow. A bit unfortunate they are 5x times as much expensive than other high-CRI high-power led strip.

Yes, how much difference does the 400-450nm segment make? I can see the absence of the huge blue spike potentially leading to less circadian disturbance. Maybe someone who already has one of the lower-CRI high-intensity setups could try this out, exchange which one is operating in which room, and see if they have a preference for one or the other. If the gradual color shift wasn't important, the setup would be very simple to trial.

A couple notes:

1. In regards to dimming, I have seen (fairly well-regarded) third parties saying that the CCT/tint stays pretty stable for the Optisolis under current-regulated dimming (as opposed to PWM), but I don't think Nichia themselves make any claims about it. I don't even have hearsay about the Sunlike.

2. Just as a followup to my first comment, I have noticed that Yuji has claimed 98CRI stuff also, with two CCTs on separate circuits on the same board, which is convenient. (I was just thinking of buddying the strips) But the price is not competitive, and I have a little more trust in Nichia and Seoul SC just because they are major global players in LED manufacture.

I am glad there are more posts on this. Are there any reasons why not considering LED strips at all? When installed properly with the "milk" diffuser and as indirect lightning, it's IMO quite nice and effective. They seem to be powerful enough (20W are about 2k lumens/m), can be also found in high-CRI variants, less expensive, various CCT, dimmable, etc. I am considering using them in a new house. Basically, multiple parallel led (with different CCT, like 3000, 4500, 6400) strips diffused against a wall/ceiling, controlled via smart relays and incorporated in home automation (so physical switches turn e.g. just the one with 3000K after ~9pm).

I need to validate that in some studio, but my hope is that e.g. 50 000 lumens from 6x4 meters of high-CRI led strips of 3 different CCT diffused over a wall would provide sufficient daylight feeling + not taking any space, generate many shadows, looks nice and are OK to look at. If I could make it dimmable, it would be great, but there seem to be tradeoffs (dimmable ones are not that good on CRI apparently).

Btw. there are apps for phones with light detectors which seem to follow basic physics (like showing 4x times smaller number when being 2x times further from the source)

I just recently made my new ugly but sufficient lumenator (a combination of 6400K and 4500K CRI95+ 1500 lumen bulbs, still adding additional ones) and also managed to integrate a WiFi relay with Smart Bulbs to my home automation via Home Assistant with a fallback to manual switch. I also tried Home Assistant's "flux" component which sets the colour and brightness based on the sun position but replaced that idea with being less-sun-dependent. I want to have a lot of light until e.g. 8 pm regardless of the light outside and only then start dimming to red. Simple variant is plain automation with templating, hard but dynamic.

Thanks! The last time I was shopping for LED strips, they just weren't bright enough to really light up a room, but it looks like that has changed. I'll add something about this in the post.

BTW, what I was using LED strips for was "UFO lighting" around the base of my bed. The relatively dim, low CCT strips were attached around the bottom edge of my bed frame, with the LEDs pointing toward the floor. The scattered light was just enough to read by, and having the light mainly on the floor was good for not tripping over things if I had to get up in the middle of the night.

By the way, I asked about this setup on reddit. They also recommend some custom COBs, which seems to be the most powerful solution, but isn't as practical as strips.

That's a pretty good thread, and also reveals that there is an existing single-component solution for the scheduled dimmer potentiometer, from the aquarium sphere, which I hadn't thought of at all. A hundred bucks is a bit more than I was hoping to see, since you still probably need one for each room, but at the very least it simplifies things for the early adopters.

I thought the strips seemed pretty attractive for people making their own setups, since they can passively cool and are already somewhat diffuse. I do think the intensity is plenty high enough (20,000lm in a 56x26cm board), but these Bridgelux COBs the fellow mentions are 10-15% as expensive per lumen as the Optisolis strips, and outperform them too, at least according to their own measurements. As he says, it's not much more complicated electrically to set them up, but you do have to fasten them to a heatsink with thermal paste, and possibly mount a reflector, which makes things somewhat more tedious. Overall though, even with the expensive controller, a 20000lm device is probably still coming in under 200 USD for materials (not including however you mount it in the space), which is pretty similar to what you get out of the Hue bulbs.

Yeah, that's useful. Agree on the assessment, I want to give it a shot with one of those Bridgelux Vesta Thrive thing, it sounds like a good hobby project I would like to try. If that happens, I would do a post about it here.