Hey list members, is anything perceived by the standard observer as technically, absolutely white? If white is all colors, and assuming that means equally, it should be illuminated by the entire visible spectrum equally and reflect the entire visible spectrum equally (or be illuminated unequally and be unequally reflective in inverse proportion). Is there a combination of light source and material like that? Snow seems like a good candidate for a material, but illuminated by what? Our sun? Stars have different spectra. Is the spectral output of our sun at midday (on the equator during an equinox, or somewhere in the tropics at the right time of year) considered the reference "white" illuminant?
On Jun 9, 2014, at 1:45 PM, John Gnaegy <gnaegy@apple.com> wrote:
Hey list members, is anything perceived by the standard observer as technically, absolutely white?
What you are referring to is the "perfect reflecting diffuser" which is a theoretical material that reflects all visible light frequencies in all directions equally. It does not exist in a physical form. A real world material which comes close is Fluorilon (Avian Technologies), or Spectralon (Labsphere), both of which are made from sintered PTFE and reflects about 99% throughout the visible spectrum. Both of these would appear white to the standard observer. The most common "white" colorant uses titanium dioxide, which is actually a very pale yellow but is perceived as white. When placed next to a Fluorilon reflectance standard its true yellow color is easily perceived.
If white is all colors, and assuming that means equally, it should be illuminated by the entire visible spectrum equally and reflect the entire visible spectrum equally (or be illuminated unequally and be unequally reflective in inverse proportion). Is there a combination of light source and material like that?
A light source that emits all visible wavelengths equally is known as the equal-energy illuminant and is standardized as Illuminant E. It also does not exist as a physical light source.
Snow seems like a good candidate for a material, but illuminated by what? Our sun? Stars have different spectra. Is the spectral output of our sun at midday (on the equator during an equinox, or somewhere in the tropics at the right time of year) considered the reference "white" illuminant?
As for using snow as a reflectance standard, water has a slight red absorption which gives it a cyan-blue appearance. The absorption is so small that the water must be very thick for the blue color to be noticeable. Think of icebergs or diving underwater for examples. Robin Myers
My take is that some good reference material come very close to what Robin Myers described, as reflecting equal amounts of light across the visible spectrum. A few years ago, I purchased such a white material for dirt cheap from BabelColor. Been using it successfully in all my reproduction work ever since. But you raise a very interesting and difficult question, John, what is "white"? Is there such thing as a true, absolute "white"? Sun at midday would intuitively seem like a "natural" candidate of white, if only it could be observed directly, but at billions of candelas/m2 (I have no idea what the true value is) it's not a really good useful "candidate of white". Yet, white is so important in all color work. We speak of "white point" all the time. Photoshop has a "white point" tool. Our monitors are pegged at a certain white point and so on. White is truly everywhere, as a color. I spent many agonizing hours, days, weeks, months, years of my own time trying to naively measure "sun at midday" or what I call "direct sunlight", because I was convinced that this exact type of light could only be the "true" measure of white, since humanity has thousands of years of evolution under its belly, judging the world around us, people, animals, oceans, nature using that exact light. To me, this light is the most important "reference" for the visual system. It's abundant, it's ubiquitous and it's stable. All qualities I thought were rock-solid and immovable until I started to get some good spectra, measured by my hand, which, at first, didn't make sense. And as a true skeptic, I still have doubts about the limits of my instrumentation. Yet, many useful conclusions and interpretations could be drawn from my data and, boy!, did it make me "humble". I had no idea what absorption bands were before, yet they were there, under my eyes, oxygen, nitrogen, all kinds of atmospheric particles and whatnot. Still, despite all the technical and conceptual difficulties, I could only conclude that "direct sunlight" is indeed, for me, the "best" white out there, for reproduction purpose. If we're going to be talking about some "reference white", why not go with what nature has provided in abundance, I am convinced of? I know the idea of white is just that, an abstraction. To physically "realize" that idea, to me, only standard reference material such as Robin suggested could ever be used for that purpose. That's one part of the "recipe" of white. The other part, is the light under which this perfectly reflecting material is observed, there is no getting around that. I tend to be biased and still think of sun at midday as the best we have, regardless of all its pitfalls, that it varies during the day, how it changes relative to the position around our hemisphere, time of the year and so forth... To me, the best place to observe a fresh ColorChecker chart is still under direct sunlight, on June 21st, at noon -- see you there! / Roger Breton -----Original Message----- From: colorsync-users-bounces+graxx=videotron.ca@lists.apple.com [mailto:colorsync-users-bounces+graxx=videotron.ca@lists.apple.com] On Behalf Of John Gnaegy Sent: 9 juin 2014 16:46 To: 'colorsync-users?lists.apple.com' List Subject: is anything white? Hey list members, is anything perceived by the standard observer as technically, absolutely white? If white is all colors, and assuming that means equally, it should be illuminated by the entire visible spectrum equally and reflect the entire visible spectrum equally (or be illuminated unequally and be unequally reflective in inverse proportion). Is there a combination of light source and material like that? Snow seems like a good candidate for a material, but illuminated by what? Our sun? Stars have different spectra. Is the spectral output of our sun at midday (on the equator during an equinox, or somewhere in the tropics at the right time of year) considered the reference "white" illuminant? _______________________________________________ Do not post admin requests to the list. They will be ignored. Colorsync-users mailing list (Colorsync-users@lists.apple.com) Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/colorsync-users/graxx%40videotron.ca This email sent to graxx@videotron.ca
On Jun 10, 2014, at 8:04 AM, Roger Breton <graxx@videotron.ca> wrote:
A few years ago, I purchased such a white material for dirt cheap from BabelColor. Been using it successfully in all my reproduction work ever since.
As far as my testing of such products, it was the best of that type available. In terms of neutrality and Lstar value, it was (is) amazingly good! Too bad it was so expensive to produce and thus is no longer available. Andrew Rodney http://www.digitaldog.net/
On Jun 10, 2014, at 7:14 AM, Andrew Rodney <andrew@digitaldog.net> wrote:
On Jun 10, 2014, at 8:04 AM, Roger Breton <graxx@videotron.ca> wrote:
A few years ago, I purchased such a white material for dirt cheap from BabelColor. Been using it successfully in all my reproduction work ever since.
As far as my testing of such products, it was the best of that type available. In terms of neutrality and Lstar value, it was (is) amazingly good! Too bad it was so expensive to produce and thus is no longer available.
I have a BabelColor WatchYourWhite target, and it really is quite good. Anybody who wants something comparable...for a mere $500, you can buy a 2" square of Spectralon. And for less than $5,000, you can buy a 5" x 5" multi-step Spectralon target with 12%, 25%, 50%, and 99% swatches. (Prices have increased radically. I seem to remember just a few years ago that the multi-step target was closer to $1,000.) However...both the WatchYourWhite target and Spectralon are overkill for almost all photographic purposes. A styrofoam coffee cup is similarly spectrally flat, but about 80% reflective. Better, because of its near-conical shape, you can use it to sample light from all angles with a single exposure. And you can fit it over the lens to integrate the light from the entire scene. If you're making your own profiling target, PTFE thread seal tape, layered sufficiently that it's opaque, makes a great patch to include in the target. It's not Spectralon and it's quite delicate...but your camera won't know the difference. But, lastly...well, the most effective objective way to normalize white balance and exposure both is by shooting a chart (the ColorChecker Passport is perhaps ideal), profiling a UNIWB gamma 1.0 development of that chart, and querying the resulting profile to determine where the white point lies. You can then re-develop the image with the figures the profile tells you and have a perfectly white balanced, perfectly exposed photograph. ...but, alas, not with Adobe's RAW development engine.... Cheers, b&
Ben,
However...both the WatchYourWhite target and Spectralon are overkill for almost all photographic purposes.
At the unbelievable low price WatchYourWhite was selling for, it was a no brainer. Besides, it is a useful tool against overexposure since, as you know, if the target is blasted in any of the three RGB channels, all bets are off. And I agree, a piece of Styrofoam has incredible value :-) Best / Roger
On Jun 10, 2014, at 2:22 PM, Roger Breton <graxx@videotron.ca> wrote:
However...both the WatchYourWhite target and Spectralon are overkill for almost all photographic purposes.
At the unbelievable low price WatchYourWhite was selling for, it was a no brainer.
Agreed. It may have been the most expensive such tool on the market...but it was also the only one that actually outperforms a styrofoam cup....
Besides, it is a useful tool against overexposure since, as you know, if the target is blasted in any of the three RGB channels, all bets are off.
You have no idea how much I long for a true RAW histogram + blinkies on a DSLR. (I imagine MagicLantern provides one, but it scares me.) Armed with such a tool, a *real* white reference like the WatchYourWhite is fantastic for quick-and-dirty exposure control in the field. Take a picture, make sure it's not quite clipping, and you're set for anything that's not specular or its own light source. Or, decide how much headroom you want for specular reflections and light sources and expose so the WatchYourWhite target is that much below clipping. Want two stops for highlights? Expose the WatchYourWhite two stops below clipping. Alas, the camera makers have not seen fit to give us such capabilities.... b&
On Jun 10, 2014, at 7:04 AM, Roger Breton <graxx@videotron.ca> wrote:
I tend to be biased and still think of sun at midday as the best we have, regardless of all its pitfalls, that it varies during the day, how it changes relative to the position around our hemisphere, time of the year and so forth...
I'm quite partial to the Sun, as well...but your post is really getting at the heart of the way our visual systems work. Superficially, one might think that our visual systems would function like (imprecise) spectroradiometers -- that they tell us the absolute brightness and color of the light reaching our eyes. After all, that's pretty much how the mechanics of our eyes work: a photon hits one of the photoreceptors in the eye, and that creates an electrochemical impulse in the optic nerve associated with the location and color of the photoreceptor. The more photons in a shorter amount of time, the stronger the nerve signal. However, the "software" in our brains is much more sophisticated than that. It's able to integrate the entire visual field into a pretty good estimation of the color and intensity of the illuminant, and it then adjusts our perception of objects such that what we actually see is much closer to what a contact spectrophotometer reads. As such, so long as the illuminant falls in an huge range of spectral power distributions, the stuff we look at always looks pretty much the same. From a practical perspective as an human living in the real world, this is fantastic. That piece of fruit you picked off the tree in the morning still looks like the same piece of fruit in the evening by the campfire. However, it's also the source of so many of the practical problems people have to deal with in critical color applications. For, of course, the whole system is only roughly approximating constancy of perception; once you start to examine it critically, the deviations not only become apparent, but can even be a bit jarring. Just yesterday somebody sent me and at least an hundred other people a bunch of photos shot indoors with a generic digicam whose white balance algorithms suck royally. Everything was horridly yellow...and, yet, I'll bet that most people didn't even notice, and the one or two who might have likely weren't particularly bothered. ...and, yet...the light in the space itself didn't at all look weird or unusual, <i>and</i> those photos are likely displaying absolute XYZ values closer to those in the original scene than a properly white balanced photo would. So which rendering is correct? The one that (perhaps) faithfully reproduces the original colors in the scene, complete with non-Sun illuminant, or the one that faithfully reproduces the normalized appearance of what the scene would look like if viewed under the illuminant that's dominant for you right this moment -- your computer display? I think we may eventually see the day when cameras include a spectroscope designed to record the illuminant, and that information getting added to the file's metadata and used for white balancing. There's no better solution to the problem, and it's not *that* big a technical challenge, all things considered. Since I'm not keen on hauling my i1 Pro and a laptop around with me on photographic excursions, I'm personally toying with the idea of trying to use an iPhone for that sort of thing. Lots of people are using iPhones for spectral analysis for chemistry, but I don't know of anybody doing it for graphic arts.... Cheers, b&
Ben Goren wrote:
reaching our eyes. After all, that's pretty much how the mechanics of our eyes work: a photon hits one of the photoreceptors in the eye, and that creates an electrochemical impulse in the optic nerve associated with the location and color of the photoreceptor. The more photons in a shorter amount of time, the stronger the nerve signal.
Actually, the adaptation starts happening right in the photo-receptor. That's why if you stare at one image for a length of time, you get an after image when you eventually move your eyes. Our white point adaption can largely be explained as the consequence of this "automatic gain control" of each receptor cell type (short, medium, long). Graeme Gill.
On Jun 9, 2014, at 1:45 PM, John Gnaegy <gnaegy@apple.com> wrote:
Hey list members, is anything perceived by the standard observer as technically, absolutely white?
Superlative question! There are two parts to your question: surfaces and illuminants. There are some substances that, for all pragmatical porpoises, equally and proportionately reflect all visible light that reaches their surfaces. Spectralon is the expensive optical-grade stuff, but Teflon (PTFE) thread seal tape (with enough layers that it's opaque) is close enough that your eyes aren't going to be able to tell the difference. Barium sulfate, from which baryta papers get their name, is another good example. Polystyrene doesn't tend to be 99.9% reflective the way Teflon is; its reflectance is usually in the 80% range. However, its reflectance characteristics are almost perfectly flat as well, making a styrofoam coffee cup a technically better white balance target for a camera than almost any gadget sold as such that you can buy. The other half of the equation, that you allude to, is the illuminant. Our visual systems are incredibly effective at adjusting for different illuminants. As such, something that's equally reflective will appear white regardless of the actual spectral power distribution of the illuminant. That is, it'll appear white until you look critically at it...but, even then, it's not that foam cup doesn't look white, but that _everything_ looks off. Objectively, that foam cup is going to look much yellower in direct sunlight and much bluer in open shade. Subjectively, it's going to look equally white no matter what. An excellent exercise to demonstrate this is to photograph a styrofoam cup in the first light of dawn or last light before sunset, and to compare the sunlit half with the shadowed half. Play around with the white balance, both using the eyedropper to click on each half and to manually adjust to see the changes. Also pay attention to the appearance of everything else in the scene! Now, repeat the exercise, but the next day and without the camera and just your eyes. In the graphic arts industry, the CIE has defined a set of standard illuminants. The D series are designed to be a close approximation of daylight. Though D50 (approx. 5000K) is what's used as the profile connection space, D65 (approx. 6500K) is the closest there officially exists to "generic" light. Widipedia is a good starting place for further research: http://en.wikipedia.org/wiki/Standard_illuminant Cheers, b&
participants (6)
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Andrew Rodney
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Ben Goren
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Graeme Gill
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John Gnaegy
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Robin Myers
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Roger Breton