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OPTOMETRY
ACCURATE MEASUREMENT OF THE ASTIGMATIC AXIS AND ASTIGMATIC
POWER (WHEN USING THE CROSS CYLINDER)
1 Set the dot chart as the target. The cross cylinder status "1" is set for the right eye (dial
mode A) and the accurate measurement mode of the astigmatic axis is accessed.
2 Press "1" or "2" to reverse the cross cylinder. (As Lens "1" is bordered in green, and Lens
2 in red, the patient can answer with the color.) As the cross cylinder is reversed each time
the switch is pressed, let the patient compare how the chart looks. Stop reversing when
the patient tells the chart is seen better. The screen displays the "+" or "-" status. If it is
"+", add 5° to the astigmatic axis to make it 95° with the [+] switch or dial.
3 Reverse the lens again and let the patient compare how the chart looks. He says the "+"
status is better.
Increase the astigmatic axis by 5° to make it 100°.
4 Reverse the lens again. Mr. A is hesitating to tell which chart surface is seen better. If
judgment is difficult or if each surface of chart looks almost the same, a correct astigmatic
axis is obtained. (The axis is set as 100°.)
5 Next, measure the accurate astigmatic power. Press [C]. The dial operation display is "C"
and the accurate measurement mode of the astigmatic power is accessed. In the cross
cylinder measurement, when changing the astigmatic power by -0.50D, the spherical
power is automatically changed by +0.25D to keep the equivalent spherical power con-
stant.
6 Use the same dot chart. Reverse the cross cylinder as Step 2 and let the patient compare
how the chart looks. Mr. A answers the "-" status is seen better. Increase the astigmatic
power by -0.25D with the [-] switch or dial. (The astigmatic power of Mr. A is -0.75D.)
7 Reverse the lens again and let the patient compare how the chart looks. Mr. A answers
the "+" status is seen better. Decrease the astigmatic power. (The astigmatic power of Mr.
A is -0.50D.)
If his answer is "-", -0.25D should be added.
8 Reverse the lens again. Mr. A answers the "-" status is seen better. The astigmatic power
is to be increased by -0.25D but it makes the astigmatic power 0.75D again.
By the above results, it is judged that -0.50D is not enough and -0.75D is excessive. So,
the accurate astigmatic power is -0.62D. However, such a lens needs to be specially
ordered. In general, the weaker -0.50D is applied.
9 The accurate measurement of the astigmatic axis and astigmatic power is finished.
Remove the cross cylinder.
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