When you go to the ophthalmologist (a medical doctor who specializes in the treatment of eyes) or optometrist (eye doctor who is trained to treat eyes, but is not a medical doctor), you usually take an eye test to check your vision. On the cart is the most common eye tests, called the Snellen Test. There are directions in the Eye Folder on the cart. Basically, you stand 10 feet away from the chart, cover one eye, and then determine the lowest line you can read. The numbers next to the line represent the visual acuity in the eye you didn't uncover. Then you determine your visual acuity in the other eye. Then use both eyes uncovered.

Visual acuity is measured with numbers 20/200, 20/50, 20/20 and 20/15. The numbers means that you can see at 20 feet what a person with normal vision can see at 200. That means that your distance vision is not as good as it is for most people. Usually, it means that you can just get it corrected with glasses. 20/50 means that you can see at 20 feet what a person with normal vision can see at 50 feet - still not good. 20/20 means that you can see at 20 ft what a person with normal vision can see at 20 ft - in other words, normal vision. And 20/15 means you can see at 20 feet what a person with normal vision can see at 15 feet - in other words, better than normal. For example, the big horse in the picture is something you should be able to see at 200 ft. The cat, rabbit, horse and seahorse in the 50/20 line are pictures that kids should see at 50 feet. And the giraffe, horse, monkey, dog, rabbit, goat and rabbit are things that kids should be able to see 20 feet away.

Assignment:

In your notebook, make an entry . The name of the project is "Visual Acuity."

The put the following chart in your notebook:

Left Eye Acuity: ____________

Right Eye Acuity: ____________

Acuity with both eyes: ___________

Then answer these two questions in full sentences: 

1) Is your visual acuity better, worse or the same when using both eyes?

2) Why do you think this is so?

There are different charts on the cart. Many of the carts are to test the visual acuity of kids.

3) Why do you think they use pictures of animals?

4) What are the problems with using pictures of animals?

5) There's one chart with the letter "E" turned rotated to 90º, 180º and 270º and 0º. What do you think this chart is used for?

Did you know you have a blind spot? You have one in each eye! So you really have two blind spots. The blind spot is there because where your optic nerve is, the retina has no rods or cones. So there is nothing there to sense the light.  Fortunately, the brain is very good at filling in missing parts of visual fields. The visual fields of the eyes overlap, so the brain is able to use what it sees with one eye to fill in missing pieces. But even with one eye closed, it fills in the missing piece.

You should know by now that we have three types of cones - red, green and blue. The cones have a special chemical that absorbs different colors of light, call pigments. Pigment means something that absorbs some colors of light and reflects others. Pigments give paints, ink and crayons their color. In the eyes, the pigments are called cone pigments. Normal vision is three-color vision.  When one of the cone pigments doesn't work, one has two-color vision. Those people usually can't tell the difference between different shades of green, yellow, orange and red that people with normal vision can. Finally, there is something called anomalous three color vision, which I have. Anomalous means not quite normal. I can't tell the colors apart quite as well as people with normal vision, but, if I were not tested for this, I would never know I have it. It doesn't seem to affect me in my day to day life.