Re: [Phys-l] About black zones in screens

["John Clement" <clement@hal-pc.org>]

Actually this is a question involving perception and optics.  The LCD
projectos work by having a light source which is filtered through an LCD
screen.  Essentially each pixel has a variable filter which controls the
light intensity.  There are only 3 types of pixels with red, blue, green
filters.  The LED projector otoh has individual elements that produce light
at specific wavelengths of the 3 colors and when black they are turned off.
As a result the LED projectors can produce deeper blacks because the LCD
projectors do not completely block the light source.  Of course the ambient
light in the room is also reflected off the screen so you never get a "true"
black.  So I doubt that the black has a different wavelength by design, but
the ambient light would have a different spectral composition from the white
areas on the screen, so if you measure the black and white areas you may
find different spectral composition of the light.  If you measure the black
and white in a totally darkened room with an LCD projector you may get
different spectral compositions because the RGB pixels probably do not go to
the same level.  With an LED black should measure as almost no light.  To
make an extremely accurate measure of the black you probably need a black
room, and be dressed in black to avoid any stray inscattering with the
measuring device behind the screen looking through a hole.  But this might
not be necessary for practical purposes.

But the eye doesn't perceive absolute values, rather it compares across
boundaries and reports only a "cartoon" to the brain.  A uniformly
illuminated area sends essentially no signal to the brain.  So if the
difference between the black and white areas is large enough you will see
them as true black and true white.  But even then it is often possible to
generate effects such as whiter than white perception.

But my question now is why is this of concern?  Optics has two components
the physical optics and the perception.  The usual textbook treatments
assume that what you perceive is what is measured, but that is never true.
The variation between people's eyes and the neuron suppression which
produces edge perception make individual perception extremely variable.  In
addition to different relative densities of the 3 color cones and the B&W
rods, some people have genetically different color filters in their cones.
It is even possible to fool the eye into "seeing" full color with just a 2
color projections in a totally darkened room.

John M. Clement
Houston, TX

> I would like to ask a question to the experts in optics. We are
> considering the screen image  generated by a classroom projector in the
> case of a picture with black and white boxes, like the chess table. Are
> the black zones of the screen generated by absence of light (relying in an
> eye illusion), or they have some particular color ?. I have heard that
> some new LCD or LEDs TVs are using a particular light length.