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Distance PD is the separation between the visual axes of the eyes in their primary position, as the subject fixates on an infinitely distant object. [2] Near PD is the separation between the visual axes of the eyes, at the plane of the spectacle lenses, as the subject fixates on a near object at the intended working distance. [3]
b) the greatest distance at which lights of 1,000 candelas can be seen and identified against an unlit background. Note.— The two distances have different values in air of a given extinction coefficient, and the latter b) varies with the background illumination. The former a) is represented by the meteorological optical range (MOR).
Some clinics do not have 6-metre eye lanes available, and either a half-size chart subtending the same angles at 3 metres (9.8 ft), or a reversed chart projected and viewed by a mirror is used to achieve the correct sized letters. In the most familiar acuity test, a Snellen chart is placed at a standard distance: 6 metres.
An example of the Landolt C eye chart (also known as the Japanese eye chart.). Numerous types of eye charts exist and are used in various situations. For example, the Snellen chart is designed for use at 6 meters or 20 feet, and is thus appropriate for testing distance vision, while the ETDRS chart is designed for use at 4 meters. [16]
Near visual acuity or near vision is a measure of how clearly a person can see nearby small objects or letters.Visual acuity in general usually refers clarity of distance vision, and is measured using eye charts like Snellen chart, LogMAR chart etc. Near vision is usually measured and recorded using a printed hand-held card containing different sized paragraphs, words, letters or symbols.
In this test, an examiner shines a pen light at the patient's pupil and asks the patient to either point to the light source, or describe the direction that the light is coming from (up, out, straight ahead, down and out, etc.). If the patient is able to perceive light, the letters LP are recorded to represent the patient's acuity. If the ...
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When testing at near, the patient is to fixate on light source at 33 cm, which is held at eye level. When testing at distance, the patient is to fixate on a light source at 6m. Patient is instructed to fixate on the light source with both eyes opened. The Maddox rod is then placed over the fixating eye.