Start with the high contrast grating. Show the gratings at a distance of 2.5m starting with the 0.5cpcm grating. Turn the gratings in different orientations before exposing them from behind the grey cover. Do not move the grating when presenting it. Ask the person to respond by showing the orientation of the lines with his/her hand or with the ruler that is included in the test. Threshold value is reached when three out of five presentations lead to correct response.

If the broadest lines could not be seen at 2.5m distance, move closer until they are seen. Step back a little, turn the grating behind the cover and present it again. If a person has uncorrected astigmatism, grating will be seen at different distances when presented in different orientations. Thus you learn about the person's refractive error while measuring grating acuity.

If the broadest lines were seen at a distance of 2.3m, grating acuity is 2 cpd (0.5cpcm equals 0.5cpd at 57cm, 1cpd at 114cm and 2cpd at 230cm). This is a low value.

When testing normally sighted persons one starts by showing the finest grating at about 1.5m distance, moves backward until the lines cannot be discerned and then moves toward the person until they become visible again. At this distance make the presentations while slightly changing the distance in order to find the threshold value. Test first with the large stimulus and then with the two smaller ones.

Nomograms: Grating acuity (cpd) as a function of testing distance, A. 0.5cpcm grating, B. 2.0cpcm grating and C. 8.0cpcm grating. D. Diameter of the stimulus as a function of testing distance.

The distance needs to be measured accurately. It is easy to measure if the person sits with his/her head supported on the head rest of a corneal microscope and a tape measure is fixed on the head rest. When the distance is measured the result can be read on the corresponding nomogram (Diagram A). For example, if a person saw the 8cpcm grating at 1.15m distance, grating acuity is 16cpd with a 10 degree stimulus (the grating is 20cm in diameter, thus 20 degrees at 57.2cm and 10 degrees at 115cm distance, Diagram B).

Grating acuity at the lower contrast levels is measured similar to the measurement at high contrast level.

It takes some time to get accustomed to locating the correct value on the nomogram. It is best to mark down the result on a diagram immediately. If an error is made in one of the measurements the resulting curve looks strange. Then the strange value should be measured again. If the approximate multiple values of 57.2 cm are used (115, 170, 230, 285 and 340) grating acuity is easier to calculate. It is 2,3,4,5,or 6 times the cpcm value of the grating or between two of these values. This helps in locating the exact value on the nomogram. The result is marked on the answer sheet the same way as the values measured with optotype tests. Grating frequencies are on the bottom line.

Result of a measurement at the three contrast levels. The person saw the 8cpcm grating at 270cm at high contrast, which corresponds 38cpd (F). The 10% contrast 8cpcm grating was seen at 110cm distance, which corresponds to 15cpd (G). The 2.5% contrast 2cpcm grating was seen at 174cm distance, which corresponds to 6cpd (H). The line H-G-F depicts the slope of the contrast sensitivity curve. It is Type III curve.


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