In article ,
Szczepan Białek wrote:
Sometimes the screen on TV or cinema is perfectly white. This in cinema
reflect. This reflected light splitted with the prism has only three
frequences?
They're likely to be three bands of frequencies rather than three
narrow single-frequency lines, because the technologies used to create
the frequencies aren't narrow-band. But, yes, what you are seeing as
"perfectly white" under these circumstances is often *not* a smooth,
continuous spectrum.
In the case of a TV screen, you're seeing either:
- The mixed emissions of a set of red, green, and blue phosphors,
individually excited by electron beams [for CRT displays], or
- The emission from the phosphors of a cold-cathode fluorescent
backlighting lamp (a complex spectrum with multiple peaks) filtered
through red, green, and blue pixel-sized filters (for most LCD
tubes).
In traditional film cinema, you're seeing the emissions of an
incandescent or halogen bulb (fairly continuous spectrum) filtered
through three colors of dye in the film print.
The fact that these complex mixtures of overlapping color spectra can
look "pure white" to our eyes, is due in large part to our complex
nervous systems. Our eye/brain systems adapt to the mix of colors
present under differnet lighting conditions, and interpret different
combinations as "pure white" depending on what's available at the time.
This is why, for example, indoor fluorescent lighting can actually
look half-decent to our eyes once we get used to it (we "see" a fairly
complete range of colors there) but what looks "white" to use under
fluorescents will actually have a distinctly greenish cast to a film
or digital camera.
It's also why a rather curious phenomenon can be demonstrated. The
*exact* same mix of color emissions may look very different to us,
under different ambient lighting conditions... what might look
greenish outdoors will look pure white or even slightly pinkish under
indoor fluorescent lighting, because our brains *interpret* that input
differently due to the different surroundings.
--
Dave Platt AE6EO
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