Primary Colours and Black

[daamon]

We all know that the three primary colours are red, blue and yellow.

So why is it that TV screens and (I believe) PC monitors have red, blue and green dots?

Also, the combination of red, blue and yellow creates white (that still amazes me, even though I've seen it done). So, how is black produced - not just on screens but in general? What combination of colours makes black? None that I can see...

[BJ_M]

right - black is the absence of color




Black can also be made by mixing equal amounts of red, yellow and blue.

[BJ_M]

That RGB is used, is because the human eye is not equally sensitive to all three colors. It's necessary to include more information from the green pixels in order to create an image that the eye will perceive as a "true color."

In a digital camera for example - there is in the Bayer filter pattern an alternate row of red and green filters with a row of blue and green filters. You may be surprised to find that the pixels are not evenly divided. In fact, there are as many green pixels as there are blue and red combined.

[BJ_M]

red yellow blue

Those are `subtractive' primaries, good when colours are produced by subtracting light, as in printing, or when mixing colours from a paintbox.

A better set of subtractive primaries is
magenta yellow cyan



There are `additive' primaries, good when colours are produced by adding light, as in TVs and computer monitors,
red green blue



The magenta-yellow-cyan and red-green-blue primaries are good in the sense that their combinations yield a wide range of colours from only three primaries. However, as seen on the chromaticity diagram on the spectrum Where's purple? (see below) , three primaries can produce only colours within their gamut, leaving many colours, such as disco purple, beyond the pale.








Suppose that you could, by some magic, tickle each of the three types of cone in your eye individually, or in any proportion, at will. Clearly you could produce any possible colours - along with some impossible colours - by tickling your cones in the appropriate combination, the colours that correspond to tickling each cone separately are the cone primaries, or fundamental primaries,
extreme red - psychedelic aquamarine - extreme purple



It should be emphasized that the colours you actually perceive are coloured by processing in the brain, and do not depend simply on how much each cone type is tickled. The complicated wiring that converts what you see on your retina to what you get in your brain is traced for example in the lucid text by D. Falk, D. Brill & D. Stork `Seeing the Light' (Wiley, 1986).



btw - a computer CRT screen can not show Disco purple, at the shortest monochromatic wavelength. It comes out wrong. This is due to the phosphors used in most pc screens (not all)..

[daamon]

right - black is the absence of color

So, when black is "displayed" on the screen, it's actually a lack of electrons being fired at the screen where black's required and relies on the blackness of the screen to show black?

[BJ_M]

yes -- the absence of the 3 `additive' primaries

[offline]

Think of it in terms of the technology used and the
base colour simulated. Most printers do not print
white, for example. Degrees of whiteness are simulated through the contrast between it and
black. Brightness levels extend the range in CRT's just like
glossy-matt media with printers.

[daamon]

Black can also be made by mixing equal amounts of red, yellow and blue.

OK... Based on the additional info you gave above (very interesting and comprehensive, by the way) - this now makes sense.

What I saw was an experiment in school where split white light (into red, yellow and blue) was recombined to form white again. But what you provided above expands on that when it talks about additive and subtractive primaries.

Cheers BJ_M... That's cleared up something that's been something I've wondered about (not all the time) for years.

[Shadowmistress]

Disco purple? That's a little before my time.
Since it can't be displayed on a screen is it found anywhere in nature?

[daamon]

Disco purple? That's a little before my time.
Since it can't be displayed on a screen is it found anywhere in nature?

Yeah - It's the colour your contact lenses go when you're in a disco and under a UV light...

[yoda313]

Hi,

So if black is the absence of electrons does that mean a picture taken in extremely low light would take up less disk space than a daytime picture??? (since it would have less photographic material recorded???).

Kevin

[waheed]

Hi,

So if black is the absence of electrons does that mean a picture taken in extremely low light would take up less disk space than a daytime picture??? (since it would have less photographic material recorded???).

Kevin

Depends on the resolution

[BJ_M]

we were talking about a crt tube in fact, electrons themselves do not produce color, only when they hit something - but ....

yes it does take less room - depending on the type of compression used (no compression takes the same amount of space regardless) , but then - so does a pure white picture ...

[yoda313]

Hi,

Thanks waheed and Bj_M

Kevin