Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum

# Science in Focus: Shedding Light: Lights, Camera, Action

## 3D Photography and Anaglyphs

The Science

The images in the activity are called anaglyphs, two images created by photographing the same object through two different color filters, each taken from a slightly different angle and then superimposed on each other. Anaglyphs take advantage of two key concepts: 1) color filters and 2) human depth perception.

The filters provide a specific set of information by blocking certain colors from passing through them. The left eye sees one image through a red filter. The right eye sees another image through a green filter. The images are seen from slightly different angles. The brain combines the two images and a sense of depth is perceived.

### How Filters Work

Filters are transparent materials that selectively absorb photons of certain colors. See the diagram below. Visible white light consists of the full "rainbow" spectrum of colors. A blue filter only passes photons associated with the color blue; it absorbs all other photons. A white light viewed through a blue filter will thus appear blue. And a blue light viewed through a blue filter will also appear blue. The filter allows only blue photons from the white light to reach your eye, and does the same screening process for the blue light, which is only emitting blue photons anyway. The distinction between the two lights as seen through the blue filter is undetectable by your eye. If you were to look at a red object through the blue filter, the object would look black because there are no blue photons available to pass through the filter: The absence of any visible photons is perceived by the brain as black.

Here you can see the same image separated in to two layers. On the left is the image that contains no red light. On the right is the image that contains no blue or green light.

In the anaglyphs, the red filter blocks all parts of the image that do not reflect red photons: if it's not red, you don't see it. Similarly, the green filter blocks all non-green portions of the image; if it's not green, you see black. If you look closely at the image, you will notice these effects. This filtering process allows your left eye, which is looking at the image through the red filter, to see only red; your right eye receives only green photons from the green filter. Your brain receives these two differently colored images and merges them into a single scene that contains a sense of depth, a "3-D" effect, even though the images are flat. How does the 3-D effect occur?

### Human Depth Peception

Your eyes are about 6.5 cm apart; therefore each eye has a slightly different view of the world. You can demonstrate this effect to yourself: Hold up one hand at about arm's length and extend one finger. Close one eye and look at your finger; note what appears behind your finger. Now perform the same observation with your other eye. Did you notice how the position of your finger appears to move compared to background objects? Next try looking at objects other than your finger using the same method. Do you observe how the farther away the object is from you, the less it seems to move compared to its background? Try looking at very distant objects. Note that very distant objects don't seem to move compared to the background. This phenomenon, which results from having two eyes spaced slightly apart, provides your brain with two somewhat different views of every scene. With this information, your brain can assemble "3-D" views and the world literally takes shape!

The images for the anaglyphs were taken in a way that mimics the spacing of our eyes. The images were taken either by a camera that was moved sideways 6.5 cm between photos. The addition of the colored filters, one for each eye, insures that only one image goes to each eye. If you simply viewed the images side by side, you'd just see two images.

### Extending the Content Learning

You have just looked at stereoscopic images intended to provide a three-dimensional effect. These images can be produced in different ways, all based on the same science.

The first technique for producing 3-D views was the stereoscope. First proposed by Rollman in 1853, it was developed into a successful application by Hauron in Paris in 1891 and quickly became all the rage. By the early 1900s, stereoscopes were about as common as television sets are today. The stereoscope requires two photographs taken from slightly different angles to be positioned on a stiff mounting. This "stereo card" was then placed in a viewer that uses prisms and lenses so that each eye sees only one of the photos; the brain does the rest. A modern Viewmaster® uses a similar technique, with the pair of photos mounted opposite each other on a disk that inserts into the viewer.

Another method for producing three-dimensional images is to photograph an image twice with a polarizing — rather than a color — filter over the lens. One picture is taken, then the filter is turned 90 degrees and the camera moved 6.5 cm for the second photo. These two images can be projected onto a screen so that they are superimposed. You view the images through glasses that have a polarized filter over each lens, one filter turned 90 degrees to the other. This type of stereo-photography can produce excellent color "3-D" images since color filtering is not involved.

Try making your own anaglyphs and sharing them with your Light Buddy. Read on in III. Do It Yourself.