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A Little Background (skip to demonstration)
The retina is the part of the eye that controls color vision. The retina contains two kinds of light-sensitive receptors: rods and cones. The rods are sensitive to detail and therefore are active mostly in the dark. The cones, however, are sensitive to color and are most effective in bright light.
The light that we see is called the visible spectrum; it contains light with wavelengths from around 400 to 800nm. Each of these wavelengths corresponds to a different color and different energy level. There are three types on cones in the eye, one sensitive to each red (r), green (g), and blue (b) light. The red cones, detect light with wavelengths between 475nm and 700nm, and peak at about 580nm. The green cones, detect light with a wavelength between 435nm and 635nm and peak at about 540nm. Blue cones are sensitive to light with wavelengths between 400nm and 550nm and peak at about 450nm.
Figure 1: The color spectrum for cones.
http://www.photo.net/photo/edscott/vis00010.htm , 8/3/01
Our demonstration allows an observer to look at a spectrum of light and see how sensitive their eyes are to its different colors. The demonstration consists of a box, 3ft long by 1ft wide. There is a light source at one end of the box that sends light through a slit system and a diffraction grating. When an observer looks into the front of the box and through the spectrum, he can see a visible spectrum projected on the side of the box.
The slit system is the key to this demonstration. The slit system consists of two slits that an observer can switch between. The first slit is a rectangular slit and when observer looks at light coming through this slit, he sees a nice rectangular spectrum with constant intensity from the bottom to the top. The second slit is triangular so the intensity of the light coming through it decreases linearly, in the vertical direction. Attached to the triangular slit is a linearly decaying filter.
When an observer looks through the slit/filter combination, she sees the spectrum vertically decaying in intensity. As a result, the observer can actually see how sensitive his eyes are to different colors of light.
Figure 2: The Spectrum Through the Rectangular Slit. Figure 3: The Spectrum Through the Triangular Slit System.
It was extremely important that we had all the proper measurements before building the box, because if any angle was off the spectrum would be difficult to see. The box we constructed was approximately 12"x12'"x48" in size and made out of 1/2" oak veneer. Wood runners were placed along the top and bottom pieces to allow for easy attachment, due to the thickness of the paneling. The inside was colored in black velvet to detour any excess light from hitting the diffraction grating. The plans for the inside of the box were designed on AutoCAD LT and are shown below. To see the individual parts, click here.
Our light source consisted of a 500 watt halogen flood lamp that was placed in the very back of the box and was screwed into place. The glass on the front of the lamp was ground to help scatter light evenly to the slit. Two holes were cut in the back for two 3" cooling fans that are capable of moving 32 cubic feet per minute. This would help the air flow inside and to reduce the risk of sudden combustion.
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The vision box before final setup. The bell shaped end was discarded after we realized it would not be needed. |
Approximately 6 inches from the lamp we placed a 12"x12" board with a 3"x.5" whole directly in the center. This would only allow a direct amount of light through. Next we constructed a 5"x22" board that would slide back and forth through the box. This would have two holes about the same size as the other board and would also have a "slit" placed in front of them. One slit is linear and made from black cardboard and brass siding. On top of it was placed a 3"x1" linear filter that would help reduce the spectrum so the viewer can see their own sensitivity. The other slit is a rectangular slit used to see the normal spectrum. |
At the very end of the box we placed another 12"x12" board with a 6"x6" hole cut out in the center for the diffraction grating. To mount the diffraction grating we cut 2 pieces of plexi glass 7"X7" to "sandwich" the grating. Next we painted two picture mattes and and drilled 4 holes through both on all four corners. We then sandwiched the plexi glass in between the two mattes and screwed that fixture to the back of the wood.
The wiring of the box was fairly simple. Using a 25 foot extension chord, we split the the wire and created a push button that would be enable the light to be turned on. On an and off switch was wired to the fans and the push button. When turned on the fans would run and allow for the push button to operate. The circuitry is as follows.
A diagram of the circuitry of the vision box.
The final product was given a handle on the top for carrying along with two handles on the sides near the rear of the box for easy transportation. The box was given a cherry stain, and a coat of finish was applies. To the front of the box where the the grating is put a cover was made. A piece of wood was cut 13"x13" and then one rubber stopper was screwed into each corner. The end was then buckled in place on two sides.
The vision box viewed at an angle. The hand is holding up the frame
containing the diffraction grating. |
In this picture you can see the light source located at the back of the vision box. |
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This pictures looks straight at the front of the vision box.
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This is a picture of the spectrum taken through the rectangular slit with a digital camera. |
This is a picture of the spectrum taken through the linear slit and linear filter with a digital camera. |
The vision demonstration has many interesting applications. Apart from showing how the eye is more sensitive to certain colors than to others, people can look at the box through different colored filters and see how that affects what they see. People who are colorblind also see vastly different spectra than those who are not. This box is a very useful tool for investigating color vision, how it works, and how people with different visual conditions see the world.
Click here to read more about how this works