How pinholes function. Pinhole glasses (also known as stenopeic glasses from the Greek words for "little opening") are not made of glass at all but of an opaque substance such as metal or plastic. The user looks through any of the many small holes in the material. These holes have the effect of reducing the width of the bundle of diverging rays (called a "pencil of light") coming from each point on the viewed object. Normally, the full opening of the pupil admits light. When using pinholes, the pupil may be wide open, but only the central portion is receiving light.

Look at this drawing of an eye with a pinhole lens in front of it. You will see that the eye does not need to deal with the rays that would need the most bending since they are blocked by the lens. It is said that Scheiner first described this effect in 1573. Those who are familiar with cameras will recognize that this is the same principle used to increase the depth of focus by decreasing the aperture. Pinhole cameras also operate by this principle. Since the glasses are so close to the eye, the material between the holes is greatly out of focus and is not as disturbing as one might think. After a period of getting used to the glasses, the brain tends to ignore the presence of the material.

Pinhole lens

Also of interest is that the farther away the viewed object is, the less the pinholes are noticed. The honeycomb effect of the holes is more noticeable when viewing a book held close to the eyes, because the eyes are focused just a short distance in front of the glasses. When looking at a distant TV, however, the holes are hardly visible at all since the eyes are focused much farther away. Also, because of the distance, you can view the entire TV screen through one hole, an obvious benefit.

Looking through the teeth of a comb held in front of the eyes is another way to simulate the pinhole effect. The native people of Alaska have long used this principle by wearing glasses with narrow slits to look through, thus blocking out much of the glare from the sunshine reflecting off the snow and ice. Anyone who remains in this environment for long periods during the summer without protection can suffer from snow blindness. This painful condition forces the person to discontinue the use of the eyes until healing can take place. This is nature's way of protecting the eyes from permanent damage.

Limitations. One limitation of pinhole glasses is that blocking some of the light makes it more important than usual to have good lighting on the viewed object. When looking at television this is not a problem since the set makes its own light. Another limitation is that peripheral vision is diminished, so they should not be used for driving or similar activities involving motion. As with any glasses, even ordinary sunglasses, they should not be used to stare at the sun, thinking that no harm can be done. For more detailed and somewhat technical information on the characteristics of pinholes, read Pinhole Details.

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