Exploring Color Vision with LED’sMort Sternheim, Rob Snyder, Chris Emery

March, 2014

Most of the sun’s electromagnetic radiation is Ultraviolet (UV), Visible & Infrared (IR) . It peaks at 502 nm (green), near 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

.

Source: Adapted from http://www.ucar.edu/learn/imgcat.htm

Most of the UV is absorbed in the atmosphere

(Infrared extends from 700 nm to 1 mm = 1,000,000

nm)

Visible light ranges is a small slice of the solar radiation spectrum. It extends from 400 nm (violet) to 700 nm (red).

1 nm = 10-9 m

Total energy emitted ~ area under curve ~T4

Wavelength at peak ~ 1/T

Energy emitted vs wavelengthat various Kelvin (absolute) temperatures.

* Total grows rapidly with T.

* Wavelength at peak increases as T decreases.

Blackbody radiationA “blackbody” is a perfect absorber and emitter of

thermal electromagnetic radiation. Stefan-Boltzmann law :

Electromagnetic energy radiated by blackbody E ~ T4

Wien Displacement Law: Peak wavelength ~ 1/TSurface temperatures:

Sun: ~ 6000 KHuman: ~ 27 ◦C = (273 + 27) K = 300 K

Blackbody radiation, cont.Temperature Ratio, human/sun

300/6000 = 1/20Stefan – Boltzmann: E ~ T4 :

Human radiation rate smaller than the Sun by (1/20)4 = 1/16,0000

Wien: Peak wavelength ~ 1/TSun peaks at ~ 500 nmHuman radiation peak wavelength

(500 nm) x 20 = 10000 nm = 10 µm = 0.01 mm

Infrared jargon*Designation Abbreviation Wavelength

Near-Infrared NIR

780 nm – 3000 nm0.78–3 µm

0.00078 mm – 0.003 mm

Mid-Infrared MIR3000 nm – 50,000 nm

3–50 µm0.003 mm - 0.050 mm

Far-Infrared FIR

50,000 nm – 1,000,000 nm50–1000 µm

0.05 mm – 1 mm

(human) 0.01 mm*Definitions vary with the source. This is “ISO 20473.” See http://en.wikipedia.org/wiki/Infrared for others, some with more subdivisions.

Infrared thermometerNo contactMeasures infrared

emissionLaser indicates spot of

measurement±2◦ accuracy

Color vision

Our eyes have two main types of photoreceptors, rods and cones, located at the back of the eye in the retina.

Cones allow us to see colors. They are not as sensitive as the rods and only work in bright light.

Rods are used to see in dim light and only show the world to us in shades of gray with poor resolution.

There are 3 types of cones, each with pigments sensitive to a specific range of wavelengths. “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

•Which cones respond when you look at a yellow wall?•Can you tell if the light is a spectral yellow or a green-red mixture?

Primary colorsRed, 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.

Inexpensive (Home Depot) red, green, and blue spotlights can produce white and complementary colors - Cyan, Magenta, Yellow. (The circles are never as sharp as in this figure.)

Cyan

Magenta

Yellow

Complementary colorsCyan is complementary to

red because cyan + red = white

Also:magenta + green = whiteyellow + blue = white

Cyan

Magenta

Yellow

Color Addition Spotlights from Arbor Scientific

~ $250

Digital cameras have three types of filters that cover an array of sensors. The filters select red, green, or blue light. The data is stored on a memory card.

Paints and pigmentsYou 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.

Combining lights is an additive process, but combining pigments is a subtractive process.

Lights Paints

Exploring colors with “white” LED’sLED’s (light emitting diodes) emit

light with a specific color or wavelength when a current passes through.

A “white” LED is 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.

Variable resisters and switches let you vary the intensities.

Solderless breadboard connectionsYellow lines show how

sockets are connectedVertical columns with +

are connected, as are columns with a –

Each horizontal row of five sockets is connected

White LED activitiesStart 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?

Features of white LED breadboardsAdvantages:

Everyone can explore with “hands-on” setupExtensions to circuits, etc.Low cost - about $30 per setup

Disadvantages: FragileTakes time to assembleNeeds soldering for switches

Summary of color basicsEyes and digital cameras detect light in the red, green

and blue ranges of the visible spectrum.Primary colors of light are different from primary

colors of paints or pigments.Mixing three primary colors of light is additive, producing a

brighter color that we see.Mixing three primary colors of paints is subtractive,

producing a darker color that we see.