Pinhole glasses, also known as stenopeic glasses, are eyeglasses

Glasses

Glasses, also known as eyeglasses , spectacles or simply specs , are frames bearing lenses worn in front of the eyes. They are normally used for vision correction or eye protection. Safety glasses are a kind of eye protection against flying debris or against visible and near visible light or...

 with a series of pinhole-sized perforations filling an opaque sheet of plastic in place of each lens. Similar to the workings of a pinhole camera

Pinhole camera

A pinhole camera is a simple camera without a lens and with a single small aperture – effectively a light-proof box with a small hole in one side. Light from a scene passes through this single point and projects an inverted image on the opposite side of the box...

, each perforation allows only a very narrow beam of light to enter the eye which reduces the size of the circle of confusion

Circle of confusion

In optics, a circle of confusion is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source...

 on the retina and increases depth of field

Depth of field

In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...

. In eyes with refractive error, the result is claimed to be a clearer image. However, a second effect may appear at the common bridge between each two adjacent holes, whereby two different rays of light coming from the same object (but each passing through a different hole) are diffracted

Diffraction

Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

 back toward the eye and onto different places on the retina. This leads to double vision (objects having doubled edges) around the rim of each hole the eye is not focussing on, which can make the overall image disturbing and tiring to look at for prolonged periods of time.

Unlike conventional prescription glasses, pinhole glasses produce an image without the pincushion effect around the edges (which makes straight lines appear curved). While pinhole glasses are claimed to be useful for people who are both near- and far-sighted, they are not recommended for people with over 6 diopters of myopia

Myopia

Myopia , "shortsightedness" ) is a refractive defect of the eye in which collimated light produces image focus in front of the retina under conditions of accommodation. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in...

. Additionally, pinhole glasses reduce brightness and peripheral vision

Peripheral vision

Peripheral vision is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the field of view that is included in the notion of peripheral vision...

, and thus should not be used for driving or when operating machinery.

Merchants state that after prolonged use, the plastic grating should become easy to ignore. However, each time the user blinks, the horizontal lines of the grating will briefly appear to be thicker. This is because the eyelid moving over the pupil will reduce the amount of light falling onto the retina and thus will briefly remove the lateral inhibition effect which normally makes all the holes appear bigger (and the grating appear thinner). So, as long as the user keeps blinking, they will be constantly reminded of the dark grating covering their eyes.

Pinhole glasses have been marketed by various companies on the claim that—combined with certain eye exercises

Eye exercises

Orthoptics is a discipline dealing with the diagnosis and treatment of defective eye movement and coordination , binocular vision, and amblyopia by eye care professionals...

—they could permanently improve eyesight. Skeptics argue that no scientific evidence has been found to support them. Due to a lack of formal clinical studies to substantiate this type of claim by companies selling pinhole glasses, this type of claim is no longer allowed to be made in the United States under the terms of a legal settlement with the Federal Trade Commission.

The pinhole occluder

Pinhole occluder

A pinhole occluder is an opaque disk with one or more small holes through it, used by ophthalmologists and optometrists to test visual acuity. The occluder is a simple way to focus light, as in a pinhole camera, temporarily removing the effects of refractive errors such as myopia...

, a device used by ophthalmologists and optometrists for diagnosis of refractive errors, works on the same principles, but is not intended for use outside of diagnosis.

As viewing through a pinhole can much improve clarity of vision of people with refractive error, in an emergency a clear view can sometimes be obtained by looking through a single improvised pinhole.