Basically, the human visual system assumes the brightest thing it sees is white; thus you can project color slides, that under daylight have a color cast, in a dark room and they will appear OK. Viewing slides/transparencies in a room with other lights present will negate this.
Thank you benjiboy for your answer, that color temperature would make sense since it is approximately neutral in color (not too cool, not too warm, so it should render a neutral color for grey areas in the film slide when viewed.
And what Prof-Pixel is reffering to is called chromatic adaptation.
There is no one fixed answer. It's more important to have a full spectrum mid-range light source with a very high CRI. But the industry standard
when comparing on sample to another is 5000K. But a 5000K bulb does not produce a 5000K viewing light because the diffuser inevitably has
a slightly warm cast to it. Color pros will have different light sources available, since they want to match the display or reproduction conditions
of a specific client's needs. I use a variety of light sources in the darkroom so I can evaluate prints analogously. But my personal standard is
4200K because it is a nice compromise between daylight and tungsten illumination. Might lightbox for critical colorwork is very close to true
5000K, with are CRI around 98. Macbeath markets color booths, lightboxes, and targets for industry standard work, which might be informative to study. But casual viewing does not need this kind of accuracy. Critical viewing for precise color reproduction might.
Hello again Mr Pixel ... back before I could afford my own color darkroom, or even large format film, one trick I'd use when projecting slides was
was to use a slightly gray neutral gray background rather than an ordinary white projector screen. By tricking the eye in this manner, the
saturation of hues seemed to pop a lot more. One still thought white was white, but it no longer glared and competed. ... same reason many
museums choose a light neutral gray wall paint, though in this case they have to absorb the cold tones of ambient surrounding light, so generally add something to created an subtle warm neutral, which cumulatively comes out looking neutral gray, even though it isn't. During my
ole color consultant days I worked up formulas for a few museums. In one instance they failed to rotate their inventory and had an ancient
5-gal bucket which they wanted to use up before ordering more. So they decided to use it in the back room just in case there was some kind
of mismatch. Now it turns out the paint had gone completely rancid and stunk like sour milk, and that the room they used it in held the
central air conditioner for the entire museum, where the air intake was! So they had to shut the whole thing down for two weeks, paint over
the stinky paint with shellac, and air everything out. I was pretty humorous (not for them, and I'm sure the painter was looking for a new