Astigmatic fan block technique is used in subjective refraction for determination of Astigmatism in a patient and to determine correct cylinder axis and power.
1. Astigmatic dial is a combination of fixed dial rotary dial.
2. The inner portion of the dial content a "V" and two sets of mutually perpendicular line. This dial is rotateable.
3. And the outer peripheral dial contains radial lines spaced at 10' interval and arranged after the manner of the rays of the rising sun.
4. "V" block can be rotated through go.
Procedure...
1. Perform retinoscopy and find the power of both eyes.
2. Starting with right eye ( left eye ocluded) remote the cylinder lens found on retinoscopy from the trial frame.
3. Determine the best vision sphere by fogging -unfogging technique or by duocrome test.
4.The expected BVS will be half of the cylinder power.
5. Direct the patient to fan and block chart and while looking at the fan add fogging lens of spherical power until patient report all the lines of the fan now blurred.
6. Now reduce the spherical lens power by 0.25DS.
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