How does a projector project black light?

Question

ie how does black appear on a white wall?

even in a brightly lit room, when a projector is shone onto a white wall (or any colour i guess), there is still black text (etc.).

ie with plenty of light around, a white wall is reflecting/displaying black. How is this possible?

Answer 1

Black is the absence of light -- reflected light in this case.

With no other source of illumination, there is no light to reflect off the "white" (reflective) wall, so it appears black. When white or colored light shines on the wall, it is reflected and you can perceive the color.

Additional Details response:

The reason you see "black" even in a lit room is that your eyes are very sensitive to contrast (differences in illumination levels) and your brain and optical nerves automatically adjust to the average level of light that you can see and then amplify any differences so that you can easily identify fine details in things. Like when you look at the skin of an orange in broad daylight -- the little indentations are not significantly less well-lit than the rest of the orange, but your eyes/brain SEES the shadows in the indentations really well.

So in the lit room, every bit of the white (reflective) wall has some basic level of illumination just from the lights in the room. Then the projector shines even more white or colored light on parts of the wall, but not on other parts (because the transparency or film has black there to block the light from going through). Your eyes see the difference in illumination, and your brain adjusts its perception to match the new "average" level of illumination -- some parts of the wall will now be brighter than that average and are perceived as "white" or bright colors, some parts are not as bright as the average and are perceived as "black" or "grey" or darker colors.

This is the same thing that you experience when you go outside into a dark night after being inside a well-lit house. At first you can't see anything, the whole world looks "black as the inside of a cow". But then your eyes and brain adjust themselves to the new level of illumination, and you can see more details. Not as well as you could see in full light of course, but at least you can see things now.

Answer 2

Black Light Projector

Answer 3

it appears black by contrast with brigtly lit white areas.

I.e. you turn off the lights - room is black. You shine a light from projector - it's white. whatever part of screen got no light falling on it remains black.

Answer 4

Like the other two guys said. It's an optical illusion. (And a pretty cool one.)