Monday, May 22, 2006

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Types of Visual Acuity Testing

a. Minimum Visible Acuity : measures brightness discrimination; the person's ability to detect small differences in the brightness of two light sources. Minimum visible acuity is determined by the brightness of the object relative to its background illumination as opposed to the visual angle subtend by the object.

b. Minimum Perceptible Acuity : measures detection discrimination. Minimum perceptible acuity is concerned with simple detection of objects, not their identification or naming. An example of this type of acuity testing is determining if a child can see and grasp a small candy bead held in the examiner's hand.

c. Minimum Separable Acuity : measures the resolution threshold, or smallest visual angle at which two separate objects can be discriminated. Landolt C, and grating acuity are examples of minimum separable tasks.

d. Vernier Acuity (hyper acuity ): a precise form of visual discrimination still under study. Hyper acuity has been coined to classify the high precision (within a few seconds of arc) with which vernier alignment task can be performed. This level of precision is well above resolution or recognition acuity thresholds.

e. Minimum Legible Acuity: measures the individual's ability to recognize progressively smaller objects (letters, numbers or objects) called optotypes. The angle that the smallest recognized letter or symbol subtends on the retina is a measure of visual acuity. This type of acuity testing is used most often clinically.

f. Snellen Acuity uses a notation in which the numerator is the testing distance (in feet or meters) and the denominator is the distance at which a letter subtends the standard visual angle of 5 minutes. A 20/20 letter (6/6 in meters) subtends an angle of 5 minutes when viewed at 20 feet (6 meters).

Question: What are the dimensions of a 20/20 size “E” from an eye chart that is meant to be viewed at 20 feet? What is the visual angle?

Answer: A 20/20 “E” from a chart meant to be viewed at 20 feet is about 9 mm tall. Each leg and space between the legs is about 1.7 mm tall. The 20/20 “E” subtends 5 minutes of visual angle; each leg and space is 1 minute (1/60 degrees) = 0.017 degrees of visual angle.

For the Landolt “C”, the opening in the “C” is about 1.75 mm (1 minute of arc).

Question: How many minutes does the “E” on the 20/20 line of the Snellen eye chart subtend?

Answer: 5 minutes at 20 feet. Snellen eye chart measures the minimum legible acuity.

Question: What is the optimum size of pinhole used to measure “pinhole acuity”?

Answer: The optimum size is 1.2 mm. Larger pinholes do not effectively neutralize refractive error and smaller pinholes markedly increase diffraction and decrease the amount of light entering the eye.



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