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SIDE TRIPS FROM THE ROUTINE DISTANCE SUBJECTIVE REFRACTION
Clock Chart (Sunburst Dial)
PURPOSE To determine the cylindrical component of the refractive error by a subjective technique．
INDICATIONS
The clock chart is performed if 20/20 VA cannot be achieved with spherical lenses and the starting points of the refraction do not indicate a need for cylinder．It is also indicated if there is reason to believe that the tentative cylindrical correction from the static retinoscopy is inaccurate．Clock chart is done one eye at a time．
STEP-BY-STEP PROCEDURE
1．Remove any cylinder that may have been in the phoropter and fog the eye to a VA of 20/40 with spherical lenses．
2．Show the patient the clock chart(see Figure 3-17)．
3．Ask the patient to identify the darkest，sharpest set of lines in the clock chart according to their position on the face of a clock (eg.2 and 8 o’clock，3 and 9 o’clock)．If all the lines appear equally blurry, the test is terminated．Return to step 12 of the routine distance subjective refraction．
4．If one set of lines appears clearer or darker than the others，set the AXIS of the minus cylinder in the phoropter to 30 times the smaller o’clock from the patient’s report(eg, if the patient reports the 2 and 8 o’clock lines are darkest, set the cylinder axis to 30×2 = 60° on the phoropter)．This is known as the “rule of thirty．”
5．If two sets of lines seem about equally dark or sharp, select an axis value midway between(eg, if 1 and 7 and 2 and 8 are equally sharp, set the cylinder axis to 30×1.5=45°on the phoropter)．
6．Add -0.25 D cylinder power．Again ask the patient if one set of lines is clearer or darker than the others．
7．If the patient still reports that one set of lines is clearer or darker than the others，repeat step 6．If the patient reports that all of the lines are about equally sharp and dark or if a different set of lines becomes sharper and/or darker，return to step 12 of the monocular distance subjective refraction．
Note：In the clock chart test it is not necessary to maintain the spherical equivalent of the starting point． |
