Aim
Stenopaic slit is used to determine the refractive error in each of the meridians individually. It is a great help when the patient needs high astigmatic correction.
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Fig:- Stenopaic Slit |
Test
The stenopaic slit consists of rectangular aperture ranging from 0.50–1.00mm in width and upto 15mm in length. It is assumed to limit the admission of light to one meridian. A target with a large letter may be used if the standard chart seems insufficient, or the patient may be asked to move closer to the test chart.
Illumination
Room illumination has to be sufficient.
Test Distance
Once the primary meridians are located, the test is done at a standard distance of 20 feet.
Patient Position
The patient may assume an unusual posture to look through the slit aperture.
Step by Step Procedure
1. To begin the patient is fogged and unfogged monocularly until the best possible initial acuity is recorded.
2. Now place the slit before the eye being tested and slowly rotate while the patient’s attention is towards the chart.
3. The patient is instructed to point out if at any position the visual acuity improves or better than other position.
4. If such a position is found, it is refined and bracketed by slight rotation of the slit on either side.
5. To be sure rotate the slit by 90° to opposite meridian to compare the acuity. If astigmatism is significant, the second position should reveal poor acuity than the first one. The disparity of acuity relates to the magnitude of the cylinder correction.
6. Correct the better acuity meridian first with spherical lens and then correct the opposite meridian again with spherical lens.
7. Achieve the best possible acuity in both the meridians separately and then remove the slit from the trial frame.
8. Transform the meridian values so derived into spherocylinder form and refine the same.
9. Record the results.
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