RET
Optics Research Workshop
”Demos”
Dr. Mike Nofziger
Professor
College of Optical Sciences
University of Arizona
Dr. Mike Nofziger 2013
Demo #1 -- “Color Mixing”
Purpose: Demonstrate additive color mixing using inexpensive materials:
Red, Green, and Blue LED flashlights and a styrofoam cup.
Materials Needed:
- Red LED flashlight
- Green LED flashlight
- Blue LED flashlight
- Styrofoam cup
(or Red keychain LED light)
(or Green keychain LED light)
(or Blue keychain LED light)
(larger size cup, 16-20 oz., easier to see)
• The use of LED’s is a much cheaper alternative to using glass colored filters.
Dr. Mike Nofziger 2013
Optical Demonstrations
Demo #1 -- “Additive Color Mixing”
Theory:
As humans, we see color because we have 3 different types of light-sensing
cells (cones) in our retinas. Our cones inherently respond to light in either the
blue, green, or red part of the visible spectrum. This is the basis for our
trichromatic color vision.
- Additive color mixing is how we see colors from any device that emits light:
(LCD screens—TV’s, iPads, cell phones, computer screens, etc., computer
projectors, laser light shows…..)
● Additive color mixing works by physically overlapping (“adding”) 2
or more different light sources in the object space of our eyes. The
image of this region falls on our retina, where the various lights
overlap onto the cones. Our brain interprets the varying signals from
the “red”, “green”, and “blue” cones as various colors.
Dr. Mike Nofziger 2013
Optical Demonstrations
Demo #1 -- “Additive Color Mixing”
Theory:
- Subtractive color mixing is how we see colors using paints, inks, pigments,
dyes, etc.)
● Subtractive color mixing works by using inks (etc.) printed on
white paper that physically absorb (“subtract”) red, green, and/or
blue light. If white light shines on the paper, the light that is
reflected is now missing varying amounts of red, green, or blue,
creating the sensation of other colors.
- In second grade we all learned that the “primary colors” were red, blue, and
yellow. What we actually learned about were the subtractive primary colors,
and technically these are:
Cyan
(a ‘turquoise’ form of ‘blue’)
Magenta (a ‘pinkish’ form of ‘red’)
Yellow (‘yellow’ is yellow!)
NOTE: C, M, and Y are the ink colors used in ink jet printers!
Dr. Mike Nofziger 2013
Optical Demonstrations
Demo #1 -- “Additive Color Mixing”
Theory:
“Color Math” (a.la. Dr. Mike):
(an easy way to remember how
additive and subtractive color mixing works)
R + G + B = White
|
|
|
C + M + Y = Black
► Cyan ink absorbs Red light.
Print Cyan dots on white paper. Shine white light onto the paper.
The reflected light is missing Red. It contains Green and Blue,
which additively looks like the color Cyan 
► Shine Green light onto a white wall. Overlap the area with Blue light.
The additive combination of Green and Blue light will look like the
color Cyan. (White minus Red)
► Cyan and Red are complimentary colors.
Red + Cyan
= White
Red + {Green+Blue} = White
Dr. Mike Nofziger 2013
Optical Demonstrations
Demo #1 -- “Additive Color Mixing”
Theory:
“Color Math” (a.la. Dr. Mike):
(an easy way to remember how
additive and subtractive color mixing works)
R + G + B = White
|
|
|
C + M + Y = Black
► Magenta ink absorbs Green light.
Print Magenta dots on white paper. Shine white light onto the paper.
The reflected light contains Red and Blue light, which additively
looks like the color Magenta 
► Shine Red light onto a white wall. Overlap the area with Blue light.
The additive combination of Red and Blue light will look like the
color Magenta.
► Magenta and Green are complimentary colors.
Green + Magenta
= White
Green + {Red+Blue} = White
Dr. Mike Nofziger 2013
Optical Demonstrations
Demo #1 -- “Additive Color Mixing”
Theory:
“Color Math” (a.la. Dr. Mike):
(an easy way to remember how
additive and subtractive color mixing works)
R + G + B = White
|
|
|
C + M + Y = Black
► Yellow ink absorbs blue light.
Print Yellow dots on white paper. Shine white light onto the paper.
The reflected light is missing Blue. It contains Red and Green light,
which additively looks like the color Yellow 
► Shine Green light onto a white wall. Overlap the area with Red light.
The additive combination of Red and Green light will look like the
color Yellow.
► Yellow and Blue are complimentary colors.
Blue + Yellow
= White
Blue + {Red+Green} = White
Dr. Mike Nofziger 2013
Optical Demonstrations
Demo #1 -- “Additive Color Mixing”
Comments on the Demo:
• Buy LED flashlights that have a constant on/off switch, not just a momentary push-button switch.
• The styrofoam cup acts like an optical integrating sphere: it “scrambles up” any spatial variations
in the light patterns from the LED flashlights, making the light appear very uniform in its spatial
appearance. The cup (or an integrating sphere) does NOT affect the colors in any way.
• The ability to achieve a “pure” white color depends on:
- the actual spectral distribution of each LED flashlight
- the relative amounts of the Red, Green, and Blue light, which with simple flashlights,
you have no control over.
The goal here is to produce what looks like a “convincing” version of white light. If you were trying
to make the “whitest” white light possible, you would have to spend a lot more money! Having said
that, you can control the relative amounts of the Red, Green, and Blue by moving the flashlights
closer to, or further away from, the bottom of the cup.
• When you buy an LED flashlight, you have no way of knowing what the actual spectral output is
(needed if you were trying to make a pure white). However, the central wavelengths of “Red”,
“Green”, and “Blue” LED flashlights are pretty consistent due to how they are manufactured.
• This demo works MUCH better if you use LED flashlights, vs. using colored cellophane as red,
green, and blue filters in front of white-light flashlights. Cellophane just doesn’t provide pure
enough colors to make a convincing white.
Dr. Mike Nofziger 2013
Optical Demonstrations