ROCHESTER INSTITUTE OF TECHNOLOGY
Center for Imaging Science
SIMG 215 – Imaging Science Fundamentals
Laboratory: Color Television
Background:
In order to understand how a color television works, the principles of additive
mixing must first be understood. To create an additive mixture, two lights of
different color are shone at the same spot on a diffuse white screen. The
resulting light that reaches your eyes is an additive mixture of the these two
sources. In other words, at each wavelength the intensities of light add, and a
“new” color is generated (or perceived) from this combined wavelength
distribution. As a result, mixing green and red will create yellow, mixing blue and
green will create cyan, and mixing blue and red will create magenta. Mixing all
three primary colors will create white.
G
C
Y
W
B
R
M
Partitive mixing is a form of additive mixing that is accomplished by placing small
light sources close enough to each other so that your eyes cannot see them
separately. The concept of partitive mixing can be found in impressionist
paintings, and is easy to explain. To create a painting, artists such as Claude
Monet dabbed small bits of paint onto the canvas. When a person stands very
close to the painting, it appears to be a seemingly random pattern of dots. By
increasing one’s distance from the painting, the colors fuse together and the
picture becomes apparent.
Color television is also accomplished through partitive mixing. Three
electron guns are aimed at a screen coated with three different dots (or small
rectangles) of phosphors: red, green and blue. A color television is designed
such that each of these dots can only be hit by one of the electron guns. By
varying the intensity of the electron guns, the three phosphors can blend together
to form a large range of color – when viewed at a sufficiently large distance from
the screen.
1
Materials:
Convex lens with focal length of 2 inches or less (eyepiece from telescope kit)
Color television with reasonably large screen
Color monitor (desktop computer CRT)
Procedure:
1. With the screen turned off, use the convex lens to magnify & examine the
television screen. Make sure the magnified image is in good focus. It helps
to leave the eyepiece in its styrofoam holder, as this way you can put the
entire eyepiece assembly (lens in styrofoam holder) right against the
screen. You may have to push on the styrofoam a bit to get the lens close
enough to the screen to achieve best focus. Record your observations.
2. Turn the television on to a color broadcast and again examine the
television screen with the convex lens. Make sure the magnified image is
in good focus. Again record your observations, taking note of the relative
intensities of colors as the broadcast proceeds.
3. Now examine black and white televised images. There are two ways to
accomplish this: (a) find a channel that broadcasts in black and white (“I
Love Lucy” on Nick at Night?); or (b) if you can’t find such a show, turn
down the color control until the screen is in black and white. Again use the
convex lens to examine the television screen, and record your
observations, taking note of the relative intensities of colors as the
broadcast proceeds.
4. Repeat steps 1 and 2 for the color monitor (CRT) of a desktop computer.
For step 2, note the magnified screen’s appearance for different colored
regions of the screen (for ex., compare the background color with a
different color in a foreground window). Also note the magnified screen’s
appearance within a bright white region, and with what appears to be a
black region. (A Word document like this one makes a good subject for the
“black and white” comparison.)
Questions to be addressed in “Results” section of report:
1. Based on your observations, how do color CRT screens accomplish the task
of generating any color within the visible light spectrum? Give at least two
specific examples.
2. Use your observations to contrast the magnified appearance of the computer
screen for regions of different color, for white, and for black.
3. Based on your observations, can you explain the fundamental difference in
the way the TV is using the partitive color concept for B&W vs. color?
4. How does the color television set’s screen differ from the color computer
monitor? Can you explain these differences in terms of differences in the way
TVs and computer monitors are typically used?
2