All right, I’ve always understood that when it comes to pigment, the color black is all the various colors mixed together, but when it comes to light, the color black is the <i>absence</i> of any colors.
Assuming that is actually true, how do projectors (movie, slide, computer, etc.) display the color black onto a white screen? I would assume that anything black would just display as the background color of the screen (in other words, it would be transparent). Which would make sense if movie screens were black. But movie screens tend to be white.
My only theory is that what I said above is correct, but the white screen only appears black in comparison because the rest of the screen is being lit up to a great degree. I have heard that sunspots, for example, are actually incredibly bright but only appear dark in comparison with the surrounding sun. Of course, I suppose it helps that movies are generally shown in darkened rooms, but still, it’s an awfully powerful optical illusion if true.
Go into a room with no windows and turn out the lights. What color do the walls appear to you? Even if the wall (or screen) is white, if there is no light falling on it, it will appear black.
The color of an object is determined by the light it reflects back to our eyes. A red object reflects only red light to us and absorbs all other colors. A white object (like a screen) will reflect all light back, so that if you project a red image onto the white screen, all of the red will be reflected and we will see red. For a black image, no light is projected onto that portion of the screen, so nothing is reflected back to us.
Everything in the theater is black if there’s no light shining on it. In order for that part of the screen to remain black, they don’t shine any light on it, and they do that by blacking out (go figure) the film so that light can’t shine through it and illuminate the screen.
I should also mention that the brain will compensate and make correct interpretations. For example, most theaters are not completely dark, so some ambient light falls on the screen.
In the case of a projector at home being shined on a beige colored wall, our brains can make the adjustment to realize that the “bright beige” shirt the businessman is wearing with his suit is supposed to be white.
The brain also figures out that the man’s suit, which is really the same color as the rest of the wall outside of the picture area is supposed to be black.
How about for an example of the OPs description a normally lit room, a white wall, and an overhead projector.
Now lay a transparent sheet with black text and a big black dot on the projector and project onto the white wall.
Does the picture on the wall now show a bright white square with black text and a big black dot?
Or does it show a bright white square with text and big dot the color of unilluminate white wall?
It shows the second, of course. A more brightly lit white box, with the ambient light colored wall that your brain interprets as black forming the text and the “black” dot.
Don’t believe your brain can fool you like that? Remember this illusion?
Neither. There is no bright white square. There is a brigthly illuminated wall-colored square, and the text and dot are also wall-colored, but illuminated the same as the surrounding wall. Our brain interprets this as white and black.
That is the freakiest thing I have ever seen! I had to copy the image and check it in Photoshop just to make sure it wasn’t a trick. I think my brain just asploded…
The parts of the screen or wall in the brightly lighted room have less light on them than the other areas and so they look darker. How dark is a matter of the ambient light, but the contrast still makes them look quite dark. A case in point might be sun spots. Against the general brackground of the sun they look black. Actually they are quite bright being at a temperature of some 4000 K. However the rest of the surface is at something like 7000 K.