Exploring Color Vision with LED’s
Mort Sternheim, Rob Snyder, and Chris Emery
March, 2014
Objective: To learn how red, green, and blue lights can produce any color that we can perceive using an
inexpensive setup with “white” LED’s.
1. Light from the Sun
Most of the sun’s radiation is Ultraviolet (UV), Visible & Infrared (IR). It peaks at 502 nm (green), near
the peak sensitivity of the human eye. (1 nm = 10-9 m).
~ 43% is in the visible range
~ 49% is in the near infrared range
~ 7% is in the ultraviolet range
< 1% is x-rays, gamma rays, radio waves
Figure 1. The sun’s electromagnetic
radiation spectrum. Most of the UV is
absorbed in the upper atmosphere and
does not reach the earth’s surface.
Figure 2. Wavelengths
of ultraviolet, visible,
and infrared light.
Visible light ranges
from 400 nm (violet) to
700 nm (red).
The frequency f and wavelength  of light waves are related by the speed of light, c = 3.00 x 108 m/s,
f = c
Question 1. What is the frequency of violet light with a wavelength of 400 nm?
2. How do we detect light?
Our eyes have two kinds of detectors, cones and rods.
Cones allow us to see colors, but they are not very sensitive
and only work in bright light. Rods are used to see in dim
light, but only show shades of gray and have poor spatial
resolution. There are three types of cones, each containing
a different light sensitive pigment.
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

“Red” cones have a peak detection of greenishyellow.
“Green” cones have a peak detection of green.
“Blue” cones detect principally blue and violet
colors
Figure 3. Response curves for the three kinds of cones.
Question 2: (a) Which cones will respond when you look at a yellow wall? (b) Can you tell whether the
light is a mixture of red and green wavelengths or a pure spectral yellow?
Red, green, and blue lights are generally designated as primary colors,
although any three colors that are well separated can be selected as
primaries. Any color that we can see can be reproduced by some
mixture of these primaries. You can use three spotlights with these
colors to form the image in Figure 4. (In practice, it is not possible to
get results as sharp as in the figure.) The spotlights are available at low
cost in Home Depot.
Figure 4. Three colored spotlights produce white. Note that R and B
form magenta, G and B form cyan, and G and R form yellow.
Complementary colors. Magenta, cyan and yellow are called
complementary colors. If, for example, we add red to cyan, we form
white, so cyan is referred to as complementary to red.
A set of Color Addition Spotlights from Arbor Scientific
(http://www.arborsci.com/color-addition-spotlights) is available for
approximately $250 if you include the optional control box that lets you
dim one or more of the lamps. (Figure 5). They are somewhat better in
color than the Home Depot spotlights but produce rather small circles.
Figure 5.
Paints or Pigments. You can also produce all colors with three
primary paints or pigments, but the process is very different. For
example, a red pigment absorbs most of the light that is incident on
it except for wavelengths near the red part of the spectrum.
Combining three primary pigments produces black, not white.
(Figure 6). Combining lights is an additive process, but combining
pigments is a subtractive process.
Figure 6. Combining paints is a subtractive process.
Question 3. The typical inexpensive ink jet printer has 3 color cartridges and a black cartridge.
(a) What are those colors? (b) Most ink jets also include a black cartridge. Why?
3. Exploring colors with “white” LED’s
LED’s (light emitting diodes) emit light with a specific color or wavelength when a current passes
through. A “white” LED is actually a package of three LED’s: red, green and blue. It has four wires
coming out, a common anode and three cathodes which connect to the three LED’s. In Figure 7, each
LED is connected to a battery via a switch
and a variable resistor, which allows you to
vary their brightness. Each LED circuit also
contains a fixed resistance which prevents
too large a current from damaging the LED.
Figure 7. A white LED is a package of 3 LED’s.
The blue square objects are the variable
resistors or potentiometers.
The advantage of this inexpensive setup is that it allows you to explore mixing colors in many ways.
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
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Start by turning on only one switch, red, and varying the resistance. What happens? Turn it off
and do this with green and then with blue.
What happens when two switches are on and you vary the resistance? What complementary
colors do you see?
Turn on all three and vary the resistances. What combination makes the best “white”? Is white a
precisely defined concept?
Question 4. Identify the series and parallel parts of the circuit. (See Figure 8).
Figure 8. Circuit diagram for the white LED setup.
Materials for assembling the white LED circuits.
Parts List: Jameco Electronics -- www.jameco.com
Part #
20758
2118803
315521
2128500
216452
661386
2152884
Desc.
Solderless Breadboard
Potentiometer, 1kΩ 0.5W
Pushbutton Switch SPST
LED, RGB Water Clear
9V Battery Snap
330Ω 0.5W Resistor
#22 AWG Hook-up Wire
9V Battery
Quantity
Price Ea.
Total
1
3
3
1
1
3
1/25’
$15.95
$1.25
$1.09
$1.75
$0.32
$0.99/10
$3.95
$15.95
$3.75
$3.27
$1.75
$0.32
$0.99
$3.95
Total:
$29.98
Answers:
1. f = c, f = c/ = (3.00 x 108 m/s)/(400 x 10-9 m) = 7.5 x 1014 /s
2. (a) When we look at a yellow wall, the red and green cones respond. (b) You cannot tell whether the
light coming from the wall is a mixture of red and green wavelengths or a pure spectral yellow; the
cones will respond in the same way.
3. (a) Basic ink jets use the three complementary colors in Figure 4, cyan, magenta, and yellow.
Expensive models may use additional colors to make it easier to make high quality prints with saturated
(intense) colors. (b) Black cartridges are also included to make more economical use of ink instead of
using the three color cartridges to make black. Early inexpensive ink jets did, in fact, just use three color
cartridges and did not include a black one.
(4) There are three series components. Each consists of a fixed resistor, a variable resistor, a switch, and
an LED. These are all connected in parallel to the cathode (negative terminal) of the battery via wires
from the switches and to the anode (positive terminal) of the battery via a wire to the common anode of
the LED’s.