Exploring Color Vision with LED’s Mort Sternheim, Rob Snyder, Chris Emery March, 2014

1. Exploring Color Vision with LED’s Mort Sternheim, Rob Snyder, Chris Emery March, 2014

2. 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 . (Infrared extends from 700 nm to 1 mm = 1,000,000 nm) Most of the UV is absorbed in the atmosphere Source: Adapted from http://www.ucar.edu/learn/imgcat.htm

3. 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

4. Energy emitted vs wavelength at various Kelvin (absolute) temperatures. * Total grows rapidly with T. * Wavelength at peak increases as T decreases. • Total energy emitted • ~ area under curve • ~T4 • Wavelength • at peak ~ 1/T

5. Blackbody radiation • A “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/T • Surface temperatures: • Sun: ~ 6000 K • Human: ~ 27 ◦C = (273 + 27) K = 300 K

6. Blackbody radiation, cont. • Temperature Ratio, human/sun • 300/6000 = 1/20 • Stefan – Boltzmann: E ~ T4 : • Human radiation rate smaller than the Sun by (1/20)4 = 1/16,0000 • Wien: Peak wavelength ~ 1/T • Sun peaks at ~ 500 nm • Human radiation peak wavelength • (500 nm) x 20 = 10000 nm = 10 µm = 0.01 mm

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

8. Infrared thermometer • No contact • Measures infrared emission • Laser indicates spot of measurement • ±2◦ accuracy

9. 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.

10. 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” conesdetect 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?

11. Primary colors • 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.

12. 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.) Magenta  Yellow   Cyan

13. Complementary colors • Cyan is complementary to red because • cyan + red = white • Also: • magenta + green = white • yellow + blue = white Magenta  Yellow   Cyan

14. Color Addition Spotlights from Arbor Scientific ~ $250

15. 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.

16. Paints and 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. • Combining lights is an additiveprocess, but combining pigments is a subtractiveprocess.

17. Lights Paints

18. 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 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.

19. Solderless breadboard connections • Yellow lines show how sockets are connected • Vertical columns with + are connected, as are columns with a – • Each horizontal row of five sockets is connected

21. White LED activities • 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?

22. Features of white LED breadboards • Advantages: • Everyone can explore with “hands-on” setup • Extensions to circuits, etc. • Low cost - about $30 per setup • Disadvantages: • Fragile • Takes time to assemble • Needs soldering for switches

23. Summary of color basics • Eyes 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.