The Retina

The Retina, or light sensitive part of the eye, covers the back of the eyeball and is the final destination of the light. The lens and cornea actually invert or turn the image displayed on the retina upside down in the process of providing a clear image that is in focus.

How Do We See Upside Down?

Since we have been seeing things upside down since birth, this really isn't any problem at all. In fact, the American psychologist G. M. Stratton experimented with a pair of glasses that inverted the image to make it right side up and found that he had to "relearn" how to see, a process that took days. In a later unrelated experiment, the participant actually reached the point where they could ride a bicycle while wearing the glasses. The challenge, however, is that the participant's world is again turned upside down when they cease wearing the glasses. Fortunately it takes less time to return to to the normal upside down world.

Myopia and Hyperopia

Myopia, or nearsightedness, and hyperopia, or farsightedness, result from a failure of the eye to focus the image correctly on the retina at the back of the eyeball. In myopia, the focal point of the eye is too short, making it difficult to clearly see distance objects, but objects can be seen clearly up close. This condition can be aided with a diverging lens which lengthens the focal distance.

In hyperopia, or hypermetropia as it is sometimes called, the focal length is abnormally long, making it difficult or impossible to focus on objects up close. A converging lens is needed to help bring the image into focus.

Rods and Cones

On a microscopic level, the retina is made up of two types of light sensitive cells, rods and cones. Rods are best at scotopic or low-light-level night vision, while the cones are best at photopic or high light level, high resolution color vision. Each retina has about 120 million rods, and 6 to 7 million cones, each is about 1 to 3 micrometer in diameter.

The human eye has three types of cones which receive short (S), medium (M) or long (L) wavelengths. They are also known as the blue, green and red receptors. We see color because these cones are stimulated.

Adaptation is the process that takes place when converting from photopic (high-light levels) to scotopic (low light levels) vision. This process, which takes approximately 20 to 30 minutes results in a 10,000 times increase in light sensitivity. You can see a single match at 50 miles on a clear night. The strange thing is that adaptation back to high light levels is accomplished in less than a second.

This change back and forth is due to a bleaching of the visual pigment. The visual pigment is easily bleached by light, but takes time to regenerate into an unbleached state to respond at full light sensitivity. Red low frequency light does not cause bleaching of the rods and, therefore, does not reset the visual pigment.

Mesopic Vision

Mesopic vision occurs at light levels where photopic and scotopic vision overlap. This level occurs between about a quarter moon and twilight. Mesopic vision seems to create a somewhat eerie heightened awareness sensation that you may feel when out at night.

Fovea

This is the only area of eye where we can see clearly, but because the eye moves around freely, it seems as though you can see everything with equal clarity. The fovea is surprisingly small. The fovea contains only cones, which are best at high light levels. That partially explains why peripheral vision is better at night. This also explains why it is impossible to see with the same clarity at night no matter how close you bring the object.

Blind Spot

The optic nerve enters the eye about 5 mm from the center of the fovea. This produces a blind spot at this position that our brain automatically fills in. Rarely is this a problem, since the eyes work as a team, each covering for the missing information in the other eye's blind spot. It is especially interesting to notice how the brain automatically fills in the blind spot with what you expect to see in that area.

Look at the optical illusion below. Simply focus your right eye at the "X" with your left eye closed and move in and out. You will notice that at a certain distance, the RED "O" disappears. You can do the same test with your left eye by looking at the "O".

Spectral Response

The photopic vision is more sensitive to lower frequency (reds and yellows) colors than scotopic vision which is more sensitive to the higher frequency colors like blue and green. This is known as the Purkinje effect, as it was discovered by the Czech scientist Johannes Purkinje (1787 - 1869.) Specifically, the scotopic vision is most sensitive at 555 nm and the photopic at 507 nm.

This explains why red light does not cause bleaching of the rods and, therefore, can be used without resetting the visual pigment. That is why instrument panels are often reddish and why pilots and astronomers use a red filter over their flashlight to read at night.

Temporal Response

The rods and cones in the eye integrate, or add together, all of the light they detect for a period known as the integration time, action time or storage time. This time is estimated at about 1/5 to 1/10 of a second. This summing process allows the eyes to be more sensitive to light.

The critical flicker frequency, or CFF, is the minimum frequency that a light must be flashed in order for us to not detect any flicker. The CFF ranges from 5 to 55 Hz depending on the size and brightness of the source. Movies, which show only 24 frames per second are interrupted several times per frame, or held in place, and quickly switched to the next frame to avoid flicker. Television, on the other hand, which has 25 to 30 frames per second, displays every other line 50 to 60 times per second to avoid creating a detectable flicker.

Red Eye

Have you ever noticed the reflection of a cats eyes in the headlights or seen the effects of Red Eye on a photograph? This is because the rods and cones of the retina are actually pointed away from the pupil and toward the reflective surface at the back of the eye. Scientists don't seem to have any explanation for this. Normally the eye appears dark, because you block the light when you look into someone's eye. That's why ophthalmologist used a ophthalmoscope to look through a partially silvered mirror that simultaneously projects light directly into the eye.

Color Optical Illusions

The structure and characteristics of our eyes cause some interesting optical illusions. Click to see examples.