4 ms·

The Color of White Light

98 points by xk3 2mo ago

miladyincontrol 2mo ago
CRI of lights is definitely a large factor in my purchasing these days. The muting, compressive response of some lights almost pains me at this point. As this post's author noted white LEDs can be pretty solid. Bulbs that have a mix of white LEDs at various temperatures to be "bi-color" tend to be pretty solid as well. Its once you go towards RGB light bulbs that you tend to get a fairly iffy CRI. I've referenced https://optimizeyourbiology.com/light-bulb-database https://optimizeyourbiology.com/light-bulb-database and https://optimizeyourbiology.com/smart-light-database https://optimizeyourbiology.com/smart-light-database before when looking at bulbs but if anyone has a better resource, absolutely please drop a reply.
- StilesCrisis 2mo ago
  I'd be curious to understand exactly how CRI equates to the charts in this post!
  - esafak 2mo ago
    You basically measure the light source with a spectrophotometer, then perform the procedure in https://en.wikipedia.org/wiki/Color_rendering_index https://en.wikipedia.org/wiki/Color_rendering_index Alternatively you buy a ColorChecker chart and measure the reflected light with a colorimeter. Basically, the finer the color separation, wider the gamut, and whiter the light, the higher the CRI.
- roblh 2mo ago
  Consider looking at TM-30 as well as CRI, it's a more useful metric that better takes R9 into account. I've gotten a little bit obsessed with colour in the last few months, and have been buying different bulbs for the purpose of making DCP profiles and doing film scanning. What you want, specifically, is violet pumped LEDs. They're expensive and it seems like there are only a handful of them on the market, but they're a lot more balanced spectrally and tend to have low flicker (probably just because they're a premium product).
  - brookst 2mo ago
    Any preferred sources you get these from? Prepping for winter in a dark place and it sounds like CRI should no longer be my guide.
somat 2mo ago
I sort of wonder is there a "true" way to represent color? Would a intensity over frequency curve do it? I probably lack the terminology to express myself properly, but color ends up being a very human thing, with a huge focus on primary colors(the colors that match the frequency response of the human eye) This is fine, we are human after all, but then I always feel color theory dives off the deep end, with color wheels and complimentary colors and triplets, etc. why? there is nothing physically cyclic going on. It never makes any sense to me that patterns based on it being cyclic would be any better than just picking colors. Any way, silly rant aside, if we wanted to define an objective physical color format, how would it be done?
- dmd 2mo ago
  “I’ve thought about this for five minutes, and it seems so simple. It must be the tens of thousands of vision scientists and psychophysicists who have worked on this for the last century who have all missed how simple it is.”
- phito 2mo ago
  Colors only exist in the human brain, they're qualia, interpreted from the mixture of wavelengths our eyes detect.
  - deleted 2mo ago
    [deleted]
  - analog31 2mo ago
    I'd extend this to saying they exist in our nervous and sensory system, which includes the physical structure of our eyes and retinal cells, because that structure is definitely related to features of the world outside of our brains.
- RedNifre 2mo ago
  You could go a long way by specifying the reflectiveness over the visible light spectrum and then do the same for light sources. That way, you could do GDR style truly monochrome sodium vapor lamps, which makes telling colors apart impossible. To do neon colors, you would need some sort of frequency redirect in your texture data as in "this pixel absorbs 80% infrared, but reemits it as visible red". Then you could have black light light sources and cool poster textures as well. I think doing textures in this format would take too much space, but doing a 256 color palette that way might be fine.
deadlast2 2mo ago
Sun is white light. Wonder what that means. Brightest object in out awareness is modeled as clean white. Or is this the actual basic color.
- andsoitis 2mo ago
  [dead]
- Sharlin 2mo ago
  There are no objective "basic colors". What "white" means is "excites our different types of photoreceptors roughly equally". And the photoreceptors have the sensitivity curves that they do because of our sun’s surface temperature and a convenient "window" in the opacity of our atmosphere. Organisms evolved under a different star, within a different atmosphere would have a different idea of what a "neutral" color is. (Because of additional processing done by our visual system, what looks "white" to us is very dependent on the color of the ambient light. But the ~5800 K blackbody spectrum could be called the "most neutral" white to our eyes because that’s what evolution has optimized them for.)
- PeterWhittaker 2mo ago
  Sure, the sun is white, more or less (cf comments re human perception and what "white" really means). But when did humans first perceive the sun as white? We can only reliably say "sometime on or after 12 April 1961", the day Gagarin went into space. Prior to that, as far as we know, all humans saw the sun as some other colour, most often yellow or red, depending on the time of day. Since Newton's prism, we had pretty good reason for believing it was likely white, reasons that got stronger in the 19th C with improved theories of optics, the discovery of atomic spectra, etc. I'm curious to know when we confirmed for sure its whiteness, and whether this was before October 1957 (Sputnik) or after Gagarin or somewhere in between.
  - adrian_b 2mo ago
    The direct light from the Sun is yellowish, because the blue light of the sky is subtracted from it. If the sky is covered with clouds, then the light is white, because the blue sky light is mixed again with the direct Sun light. If the sky is clear and there is an unobstructed view of it, the light reflected by a white object will be the sum of the light coming from the sky to the object and directly from the Sun to the object, so it will be white. For example this is true for white clouds and white snow. This, i.e. that the Sun outside the atmosphere is perfectly white, has been understood for a long time, since the 19th century.
flimflamm 2mo ago
Thanks for this comparison. Very interesting and surprising!
tbirdny 2mo ago
> It's probably a good idea to buy a few samples of several different types and brands, test them all, and then decide which to buy in quantity for the home. I'm looking forward to that. I own a bunch of Philips WhiteDial bulbs that are rated CRI90. It would be nice to test that and know how they really compare. No doubt there's a lot of dishonest CRI ratings out there.
Zak 2mo ago
Some additional notes on the topic: The author describes color rendering index as a percentage; it is not. It's 100 - [a calculation of average color error on 8 color samples]. Reporting the color error directly would probably be better; 90% and 95% would both be pretty good grades on most tests, but a rendering error of 10 sounds a lot worse than a rendering error of 5. Being based on only 8 samples, CRI Ra doesn't tell the whole story. There are other standards like TLCI that use more samples, and six supplemental samples some sources report. R9 (strong red) and R12 (strong blue) are particularly relevant to modern lighting, as LED sources often struggle with them despite high CRI (Ra) ratings. CRI is a comparison to blackbody radiation (like an incandescent bulb) or simulated daylight at the same correlated color temperature as the sample light source. An incandescent bulb and midday sunlight at the equator both have CRI of 100, but a CCT of 2700K and a CCT of 5700K will render colors very differently. White balance in a camera's settings or raw development software can make one look like the other. CCT is roughly blue-yellow balance, but light sources can deviate from blackbody radiation on a red-green axis. This is most often expressed as "delta u v" or "Duv", as it refers to distance in the CIELUV color space[0]. Duv numbers are small, with differences of less than 0.001 being noticeable in a side by side comparison and more than +/- 0.006 falling outside the ANSI standard for white light. Positive numbers are green, and negative numbers are red/pink. [0] https://en.wikipedia.org/wiki/CIELUV https://en.wikipedia.org/wiki/CIELUV
- cure_42 2mo ago
  Fascinating, thank you!
- wlesieutre 2mo ago
  Continuing on CRI’s limitations, since 2015 the new and improved metric is TM-30’s “fidelity index” (Rf) which is calculated based on 99 samples, among many other improvements. The larger number of samples makes it harder for an LED manufacturer to “cheat” by optimizing for the specific reflectance spectrums used to score CRI, getting a higher score while not actually rendering colors of most real world materials that well. https://www.energystar.gov/sites/default/files/asset/document/TM-30%20ES%20%28Final%29_0.pdf https://www.energystar.gov/sites/default/files/asset/documen... Unfortnately it’s still much less popular than CRI.
moultano 2mo ago
This is awesome. Are ordinary objects bright enough to register their spectra in it? You might also enjoy this, a project to find real world things with all the cyans you can't display on a computer screen. https://moultano.wordpress.com/2026/06/19/where-to-find-the-colors-your-screen-cant-show-you/ https://moultano.wordpress.com/2026/06/19/where-to-find-the-... After doing this I was looking for some way to do spectrophotometry with a small enough amount of equipment that my wife would approve of, so that I can add more things to the list and confirm their color.
- phyzome 2mo ago
  Certainly. Just illuminate them with sunlight and you should be good to go.
monster_truck 2mo ago
This rules. Much like designing fireworks, I wish it were somehow more accessible to mess around with mixing our own phosphors. I'd like to see this done for kit like ADJ / Eliminator / Chauvet, they do some wild near-UV shit to cause really cool/blown out color definitions.
zamadatix 2mo ago
White light discussion tends to focus around spectrums of perceptual/blackbody/the sun importance because... well, it's just the natural choice. Is there a name for a light source which has a constant emission intensity across a given range of wavelengths? Does such a thing have any practical purpose (beyond being interesting to think about) and do we actually make anything capable of producing such light output? I suppose one way might be to take some incandescent source and then stick a really well tuned filter in front to "level out" the normal curve, but that punts the problem to constructing such a well tuned filter.
- bradrn 2mo ago
  > Is there a name for a light source which has a constant emission intensity across a given range of wavelengths? The issue with this is that you can’t necessarily define ‘constant’ in a meaningful way. Being a density function rather than a point function, the energy irradiance from a source changes changes depending on whether you measure in terms of wavelength or frequency. I recommend [0] as an introduction to the problem. [0] https://oceanopticsbook.info/view/light-and-radiometry/level-2/common-misconception https://oceanopticsbook.info/view/light-and-radiometry/level...
  - zamadatix 2mo ago
    I'm not sure I follow why the density function can't be constant over the interval any more than it can be the usual slanted bell shape it is? I'm more than fine that the actual light is sampled out of this in quantized bins and amounts, that shouldn't be any different than any other light.
zenNeko 2mo ago
It's fascinating that js13k is out with rainbow in the theme, leading me to study light and rainbows and hn gets this posts.
ajot 2mo ago
Unrelated to TFA, this site is spectacular. I recommend any other HNer here to give a look to the posts in the "andinensis" section, where the author describes homesteading in a semi-remote area in Chile, and generating his own electrical power with a water stream that runs through his property. It has been shared here before as well.
keane 2mo ago
Full frame Foveon when?
hobzcalvin 2mo ago
I wanted to like this, but the “housewife” comments about sewing buttons and makeup are pretty awful. If we want an inclusive community, this sexist bullshit is not acceptable. It’s a bummer—I’m fascinated by color theory like this.
- II2II 2mo ago
  The sad thing removing the reference to housewives with respect to makeup would actually strengthen their point, since makeup is used in professional photography to restore more natural tones for both women and men.
peter_d_sherman 2mo ago
A copy of this page should be in every future Physics textbook... It shows to us just how skewed our (human) perception of light really is. It also shows to us just how much is going on, in terms of combinations of wavelengths present, with various light sources, and just how different, again, in terms of permutations of constituent waves, any two of them are... Anyway, a great page that should be included in any future Physics textbook!

98 points by xk3 2mo ago

CRI of lights is definitely a large factor in my purchasing these days. The muting, compressive response of some lights almost pains me at this point. As this post's author noted white LEDs can be pretty solid. Bulbs that have a mix of white LEDs at various temperatures to be "bi-color" tend to be pretty solid as well. Its once you go towards RGB light bulbs that you tend to get a fairly iffy CRI. I've referenced https://optimizeyourbiology.com/light-bulb-database https://optimizeyourbiology.com/light-bulb-database and https://optimizeyourbiology.com/smart-light-database https://optimizeyourbiology.com/smart-light-database before when looking at bulbs but if anyone has a better resource, absolutely please drop a reply.

StilesCrisis 2mo ago
I'd be curious to understand exactly how CRI equates to the charts in this post!
- esafak 2mo ago
  You basically measure the light source with a spectrophotometer, then perform the procedure in https://en.wikipedia.org/wiki/Color_rendering_index https://en.wikipedia.org/wiki/Color_rendering_index Alternatively you buy a ColorChecker chart and measure the reflected light with a colorimeter. Basically, the finer the color separation, wider the gamut, and whiter the light, the higher the CRI.
roblh 2mo ago
Consider looking at TM-30 as well as CRI, it's a more useful metric that better takes R9 into account. I've gotten a little bit obsessed with colour in the last few months, and have been buying different bulbs for the purpose of making DCP profiles and doing film scanning. What you want, specifically, is violet pumped LEDs. They're expensive and it seems like there are only a handful of them on the market, but they're a lot more balanced spectrally and tend to have low flicker (probably just because they're a premium product).
- brookst 2mo ago
  Any preferred sources you get these from? Prepping for winter in a dark place and it sounds like CRI should no longer be my guide.

somat 2mo ago

I sort of wonder is there a "true" way to represent color? Would a intensity over frequency curve do it? I probably lack the terminology to express myself properly, but color ends up being a very human thing, with a huge focus on primary colors(the colors that match the frequency response of the human eye) This is fine, we are human after all, but then I always feel color theory dives off the deep end, with color wheels and complimentary colors and triplets, etc. why? there is nothing physically cyclic going on. It never makes any sense to me that patterns based on it being cyclic would be any better than just picking colors. Any way, silly rant aside, if we wanted to define an objective physical color format, how would it be done?

dmd 2mo ago
“I’ve thought about this for five minutes, and it seems so simple. It must be the tens of thousands of vision scientists and psychophysicists who have worked on this for the last century who have all missed how simple it is.”
phito 2mo ago
Colors only exist in the human brain, they're qualia, interpreted from the mixture of wavelengths our eyes detect.
- deleted 2mo ago
  [deleted]
- analog31 2mo ago
  I'd extend this to saying they exist in our nervous and sensory system, which includes the physical structure of our eyes and retinal cells, because that structure is definitely related to features of the world outside of our brains.
RedNifre 2mo ago
You could go a long way by specifying the reflectiveness over the visible light spectrum and then do the same for light sources. That way, you could do GDR style truly monochrome sodium vapor lamps, which makes telling colors apart impossible. To do neon colors, you would need some sort of frequency redirect in your texture data as in "this pixel absorbs 80% infrared, but reemits it as visible red". Then you could have black light light sources and cool poster textures as well. I think doing textures in this format would take too much space, but doing a 256 color palette that way might be fine.

deadlast2 2mo ago

Sun is white light. Wonder what that means. Brightest object in out awareness is modeled as clean white. Or is this the actual basic color.

andsoitis 2mo ago
[dead]
Sharlin 2mo ago
There are no objective "basic colors". What "white" means is "excites our different types of photoreceptors roughly equally". And the photoreceptors have the sensitivity curves that they do because of our sun’s surface temperature and a convenient "window" in the opacity of our atmosphere. Organisms evolved under a different star, within a different atmosphere would have a different idea of what a "neutral" color is. (Because of additional processing done by our visual system, what looks "white" to us is very dependent on the color of the ambient light. But the ~5800 K blackbody spectrum could be called the "most neutral" white to our eyes because that’s what evolution has optimized them for.)
PeterWhittaker 2mo ago
Sure, the sun is white, more or less (cf comments re human perception and what "white" really means). But when did humans first perceive the sun as white? We can only reliably say "sometime on or after 12 April 1961", the day Gagarin went into space. Prior to that, as far as we know, all humans saw the sun as some other colour, most often yellow or red, depending on the time of day. Since Newton's prism, we had pretty good reason for believing it was likely white, reasons that got stronger in the 19th C with improved theories of optics, the discovery of atomic spectra, etc. I'm curious to know when we confirmed for sure its whiteness, and whether this was before October 1957 (Sputnik) or after Gagarin or somewhere in between.
- adrian_b 2mo ago
  The direct light from the Sun is yellowish, because the blue light of the sky is subtracted from it. If the sky is covered with clouds, then the light is white, because the blue sky light is mixed again with the direct Sun light. If the sky is clear and there is an unobstructed view of it, the light reflected by a white object will be the sum of the light coming from the sky to the object and directly from the Sun to the object, so it will be white. For example this is true for white clouds and white snow. This, i.e. that the Sun outside the atmosphere is perfectly white, has been understood for a long time, since the 19th century.

flimflamm 2mo ago

Thanks for this comparison. Very interesting and surprising!

tbirdny 2mo ago

> It's probably a good idea to buy a few samples of several different types and brands, test them all, and then decide which to buy in quantity for the home. I'm looking forward to that. I own a bunch of Philips WhiteDial bulbs that are rated CRI90. It would be nice to test that and know how they really compare. No doubt there's a lot of dishonest CRI ratings out there.

Zak 2mo ago

Some additional notes on the topic: The author describes color rendering index as a percentage; it is not. It's 100 - [a calculation of average color error on 8 color samples]. Reporting the color error directly would probably be better; 90% and 95% would both be pretty good grades on most tests, but a rendering error of 10 sounds a lot worse than a rendering error of 5. Being based on only 8 samples, CRI Ra doesn't tell the whole story. There are other standards like TLCI that use more samples, and six supplemental samples some sources report. R9 (strong red) and R12 (strong blue) are particularly relevant to modern lighting, as LED sources often struggle with them despite high CRI (Ra) ratings. CRI is a comparison to blackbody radiation (like an incandescent bulb) or simulated daylight at the same correlated color temperature as the sample light source. An incandescent bulb and midday sunlight at the equator both have CRI of 100, but a CCT of 2700K and a CCT of 5700K will render colors very differently. White balance in a camera's settings or raw development software can make one look like the other. CCT is roughly blue-yellow balance, but light sources can deviate from blackbody radiation on a red-green axis. This is most often expressed as "delta u v" or "Duv", as it refers to distance in the CIELUV color space[0]. Duv numbers are small, with differences of less than 0.001 being noticeable in a side by side comparison and more than +/- 0.006 falling outside the ANSI standard for white light. Positive numbers are green, and negative numbers are red/pink. [0] https://en.wikipedia.org/wiki/CIELUV https://en.wikipedia.org/wiki/CIELUV

cure_42 2mo ago
Fascinating, thank you!
wlesieutre 2mo ago
Continuing on CRI’s limitations, since 2015 the new and improved metric is TM-30’s “fidelity index” (Rf) which is calculated based on 99 samples, among many other improvements. The larger number of samples makes it harder for an LED manufacturer to “cheat” by optimizing for the specific reflectance spectrums used to score CRI, getting a higher score while not actually rendering colors of most real world materials that well. https://www.energystar.gov/sites/default/files/asset/document/TM-30%20ES%20%28Final%29_0.pdf https://www.energystar.gov/sites/default/files/asset/documen... Unfortnately it’s still much less popular than CRI.

moultano 2mo ago

This is awesome. Are ordinary objects bright enough to register their spectra in it? You might also enjoy this, a project to find real world things with all the cyans you can't display on a computer screen. https://moultano.wordpress.com/2026/06/19/where-to-find-the-colors-your-screen-cant-show-you/ https://moultano.wordpress.com/2026/06/19/where-to-find-the-... After doing this I was looking for some way to do spectrophotometry with a small enough amount of equipment that my wife would approve of, so that I can add more things to the list and confirm their color.

phyzome 2mo ago
Certainly. Just illuminate them with sunlight and you should be good to go.

monster_truck 2mo ago

This rules. Much like designing fireworks, I wish it were somehow more accessible to mess around with mixing our own phosphors. I'd like to see this done for kit like ADJ / Eliminator / Chauvet, they do some wild near-UV shit to cause really cool/blown out color definitions.

zamadatix 2mo ago

White light discussion tends to focus around spectrums of perceptual/blackbody/the sun importance because... well, it's just the natural choice. Is there a name for a light source which has a constant emission intensity across a given range of wavelengths? Does such a thing have any practical purpose (beyond being interesting to think about) and do we actually make anything capable of producing such light output? I suppose one way might be to take some incandescent source and then stick a really well tuned filter in front to "level out" the normal curve, but that punts the problem to constructing such a well tuned filter.

bradrn 2mo ago
> Is there a name for a light source which has a constant emission intensity across a given range of wavelengths? The issue with this is that you can’t necessarily define ‘constant’ in a meaningful way. Being a density function rather than a point function, the energy irradiance from a source changes changes depending on whether you measure in terms of wavelength or frequency. I recommend [0] as an introduction to the problem. [0] https://oceanopticsbook.info/view/light-and-radiometry/level-2/common-misconception https://oceanopticsbook.info/view/light-and-radiometry/level...
- zamadatix 2mo ago
  I'm not sure I follow why the density function can't be constant over the interval any more than it can be the usual slanted bell shape it is? I'm more than fine that the actual light is sampled out of this in quantized bins and amounts, that shouldn't be any different than any other light.

zenNeko 2mo ago

It's fascinating that js13k is out with rainbow in the theme, leading me to study light and rainbows and hn gets this posts.

ajot 2mo ago

Unrelated to TFA, this site is spectacular. I recommend any other HNer here to give a look to the posts in the "andinensis" section, where the author describes homesteading in a semi-remote area in Chile, and generating his own electrical power with a water stream that runs through his property. It has been shared here before as well.

keane 2mo ago

Full frame Foveon when?

hobzcalvin 2mo ago

I wanted to like this, but the “housewife” comments about sewing buttons and makeup are pretty awful. If we want an inclusive community, this sexist bullshit is not acceptable. It’s a bummer—I’m fascinated by color theory like this.

II2II 2mo ago
The sad thing removing the reference to housewives with respect to makeup would actually strengthen their point, since makeup is used in professional photography to restore more natural tones for both women and men.

peter_d_sherman 2mo ago

A copy of this page should be in every future Physics textbook... It shows to us just how skewed our (human) perception of light really is. It also shows to us just how much is going on, in terms of combinations of wavelengths present, with various light sources, and just how different, again, in terms of permutations of constituent waves, any two of them are... Anyway, a great page that should be included in any future Physics textbook!