9.1: Making Color Photographs

Today, you will read about how cameras make pictures in color. What you learned in class about color mixing can help you think about movies and about photographs.

What Happens When I Mix Colors of Light?
In class, you learned how the eye detects colors. You also learned that two things happen when you mixed colors of light. First, when you mixed colors, you ended up with a new color. Using two sources, you mixed red light with green light and got yellow. When you added blue light from a third source, you got white.

Second, when you mixed colors from two sources, the new color was brighter. The color in the area lit by all three sources was the brightest of all the colors. You might also have used a computer simulation on the Internet to see that whenever two colors of light are mixed, the new color is brighter than the old ones.

This happens because as you increased the amount of light, more light was available to reach your eyes. On the Internet, if you see a simulation about “mixing light” or “mixing colors of light,” you will see that when you mix maximum amounts of red, green, and blue light, you see white. On the other hand, when you mix no red, green, or blue light, you see black. These activities help you learn that white light is the brightest of all colors, and it is a mixture of many colors. When you see black, it means that there is no light present.

How Do People See Different Colors?
In the previous activity, you learned how people see different colors. The retina, in the back of the eye, contains cells that are sensitive to light. Two kinds of cells are in the retina: cone-shaped cells and rod-shaped cells. The rod-shaped cells tell you how bright or how dark something is. The three different kinds of cone-shaped cells are each sensitive to a different color of light. Some are sensitive to red light, some to green light, and some to blue light. All these cells send signals to your brain. Your brain processes these signals as images. In Lesson 9, you learned about digital cameras and found out that they work much like your eyes—they detect the light coming into them. Some cameras use film instead. Cameras with film also detect the light coming into the camera and to the film to record it. You may have taken pictures on a camera that uses film. Once you have used up your film, you have to take it to a store to be developed. Only then can you see the pictures that you took.

How Does Film in a Camera Record Different Colors of Light?
Just like your eyes, a camera has a hole that lets light inside. In your eye, this hole is your pupil. In a camera, this hole is the shutter. When you press the button to take a picture, you open the shutter. In a split second, light enters the camera and travels through it. The light reaches the back of the camera where the CCD is located. In a camera that uses film, this is where the film is located. The film absorbs light, which causes it to change color. Red light causes the film to change in a different way than blue light. In black-and-white film, all colors of light will cause the film to change in the same way. Instead of producing areas with different colors, black-and-white film only produces areas that are lighter or darker.

Color film is made of three layers. These layers of film are like the cone cells in the retina of an eye. Some layers sense red light, others green light, and others blue light. The difference is that in the retina, the different cone cells are located side by side, and in film the different layers are one under the other. If color film is hit by red light, the layer that is sensitive to red light does not become red—it becomes light blue. Green light makes the layer sensitive to green light become light purple. Blue light makes the layer sensitive to blue light become yellow. So if you look at film after it has been exposed to light, what you will see will look very different from the object that you took a picture of. It will have the same shape, but all the colors will be wrong. This photo shows what film would look like if you took a photo of a castle. It is interesting to look at the negatives from film that has been developed to see the difference between the negatives and the photos!

After photos have been taken, film needs to be protected so it will not get exposed to any more light. Film can still absorb more light. If more light were to strike the film, it would record this light as well as the light that hit it while taking the pictures. This would damage the first picture it recorded.

The film is rolled and placed into a canister that does not let in any more light. The person who develops the film opens the canister in complete darkness and puts the film into a liquid that makes the film no longer sensitive to light. The film can then be taken out into the open without any worry of damaging it. The store then uses the film to print photos. The machine that prints pictures on film changes the colors on the film back to the original colors, so the photos look like the original objects.

You have learned how camera film combines only three colors of light to make all the colors that your eye can see. For example, you know that red and green light can mix to make yellow light. Think of it next time you see something yellow like a flower. Are you actually seeing yellow light, or is it a mixture of red and green light that looks yellow to your eyes? In the next lesson, you will learn how you can tell whether you are seeing a pure color or a mixture of different colors of light.