exam Flashcards
convert cross spectacle to power cross diagram
+1.50/-2.50 by 130 degrees
To convert the prescription +1.50 / -2.50 × 130° to a power cross, we need to calculate the power at the two principal meridians that are 90° apart.
Step 1: Understanding the Prescription
The prescription consists of:
Spherical power (S): +1.50 D
Cylindrical power (C): -2.50 D
Axis: 130°
Step 2: Identify the Principal Meridians
Since the cylindrical axis is at 130°, the two principal meridians are:
130° (Axis meridian): This meridian has the spherical power only.
130° + 90° = 40° (Meridian perpendicular to the axis): This meridian has the spherical power + cylindrical power.
Step 3: Calculate the Power at Each Principal Meridian
Power at 130° (Axis meridian):
This meridian has the spherical power, i.e., +1.50 D.
Power at 40° (Perpendicular meridian):
This meridian has the spherical power plus the cylindrical power.
Powerat40°
=
+
1.50
+
(
−
2.50
)
Powerat40°=+1.50+(−2.50)
Powerat40°
=
+
1.50
−
2.50
=
−
1.00
D
Powerat40°=+1.50−2.50=−1.00D
Step 4: Represent as a Power Cross
The power cross will show the powers at these two meridians:
mathematica
Copy code
-1.00 D @ 40°
—————–
+1.50 D @ 130°
This power cross indicates that:
The power at 40° is -1.00 D.
The power at 130° is +1.50 D.
This representation helps visualize the distribution of refractive power across the two main meridians.
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