Re: is anything white?
Re: is anything white?
- Subject: Re: is anything white?
- From: Robin Myers <email@hidden>
- Date: Mon, 09 Jun 2014 14:29:25 -0700
On Jun 9, 2014, at 1:45 PM, John Gnaegy <email@hidden> 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
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