During a routine eye exam, the eye doctor often needs to check one eye's acuity compared to the other. In order to do this, the examiner uses a set of special eye covers known as occluders. Some occluders cover the patient's non-tested eye area completely and allow absolutely no light to penetrate. Other occluders have pinholes which allow only a certain amount of light to enter the eye. There are also transparent occluders which create a blurry effect over the non-tested eye. The eye doctor may use different pinhole occluders with different strengths in order to gauge a patient's level of distorted vision, whether it be nearsightedness or farsightedness.
The concept of using a pinhole lens or pinhole glasses in order to strengthen a person's vision naturally has been around for several decades, but there is little scientific evidence to back up such claims. Pinhole occluders limit the amount of available light which reaches the viewer's eye, which can slightly sharpen his or her focus without corrective lenses. The act of squinting also has the same effect, creating a temporary sense of sharp focus until the eyelids relax again. Eye doctors use pinhole or opaque occluders in order to obtain more accurate measurements without the patient voluntarily or involuntarily squinting. The non-tested eye can remain open in its natural position, but the patient cannot use it to read the lines or describe the images.
A typical optical occluder is a handheld device with a small curved cup on the end of a pen-sized stick. Some examiners will use the same occluders for multiple patients, while others will use disposable versions for each patient. The curved cup is designed to fit over the patient's eye socket area but not compress the eyelashes or eyelids. Some examiners allow their patients to hold the occluder in a comfortable position over each eye during the exam. The same light or visual blocking effect can also be achieved while the patient looks through the binocular eye examination instrument. An eye doctor will commonly block each eye in order to determine differences in visual acuity or signs of astigmatism.
There are also medical devices called occluders which are used primarily by cardiovascular surgeons. These occluders are essentially plugs which are surgically introduced into a leaking blood vessel and directed to the source of the leak. Once in place, the occluder is carefully opened like an umbrella, plugging both sides of the tear securely. This cardiac form of occluder serves a vastly different function than optical occluders.