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.

“Red” cones have a peak detection of greenish-yellow.

“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.

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 # Desc. Quantity Price Ea. Total

20758 Solderless Breadboard 1 $15.95 $15.952118803 Potentiometer, 1kΩ 0.5W 3 $1.25 $3.75315521 Pushbutton Switch SPST 3 $1.09 $3.272128500 LED, RGB Water Clear 1 $1.75 $1.75216452 9V Battery Snap 1 $0.32 $0.32661386 330Ω 0.5W Resistor 3 $0.99/10 $0.992152884 #22 AWG Hook-up Wire 1/25’ $3.95 $3.95

9V BatteryTotal: $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.