Small slit lenses refer to a special lens in the lens box. It includes a set of 15mm long slits. The width of the slits is 1mm, 3mm and 5mm. It is used to subjectively judge the degree of correction added to the two main meridians of the corrective lens. Small slit optometry is actually to obtain the correction degree by separating each meridian. It is also a technique for detecting irregular astigmatism in the absence of necessary facilities in clinical practice. Irregular astigmatism may be secondary to keratoconus, pterygium, corneal trauma or suture, etc.
The basic principle of small slit refraction is similar to that of pinhole lenses, but the small slit allows us to test each meridian separately. Small slit refraction is performed using trial frames and trial lenses or lens strips. Cover one eye, add +1.00D to +1.50D spherical fog mirrors to the original optometry, and place a 1mm wide slit in front of the fog mirror. When the subject looks at the distance vision chart, the examiner rotates the slit until the best vision is found. In this position, the slit is parallel to the axis of the corrected negative cylinder.
Place the slit in this position, place a lens in front of the slit, and defog the vision to obtain the MPMVA. The sum of all lenses in the trial frame will be the final refractive error degree in this axis. Remove all additional lenses from the trial frame and rotate the slit to the position with the worst vision. If the position is not 90° to the first meridian at this time, it is irregular astigmatism. Apply additional lenses again and defog the vision on this meridian until the best vision is obtained.
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Post time: Oct-11-2024