OPTICAL CROSS

Question 1

OPTICAL CROSS

you can…

Answer 1

Write a prescription from the optical cross

You can derive a prescription directly from an optical cross. You can also create an optical cross based on a prescription. We will learn and practice doing both. But what is it?

Question 2

The optical cross is a form of ....
its a simple...
what does it show?
its useful and understand... 
can also show...

Answer 2

The optical cross is a form of graphical notation for refractive error. It is a simple diagrammatic representation of the 2 principal meridians of curvature. It shows power and axis. It is very useful when trying to understand and visualize astigmatism and cylinder lenses. But it can also be used to show a spherical lens.

Question 3

In a spherical lens
all meridians have what?
what about power?

Answer 3

In a spherical lens, all of the meridians have the same curvature and thus the same power. We can plot this on an optical cross and it would show that 2 meridians have the same exact power (as do all the meridians in between).

Question 4

The prescription for this lens would be…



Answer 4



+6.00 sph

Question 5

With astigmatism
one meridian is … and the other is..
these are two pricipals..
the steeper curve is what?

Answer 5

With astigmatism, one meridian on the cornea has a flatter curve and one has a steeper curve. These are the 2 principle meridians. The steeper curve is the astigmatism. A cylinder lens is oriented at such an axis so that it increases the curvature of the flatter meridian. This creates an equal curvature all around.

Question 6

Here is a cylinder lens

Power is what?

Answer 6

Here is a cylinder lens oriented at 20 degrees. But remember, the POWER is 90 degrees away from the axis at 110 degrees. So this cylinder increases the power at 110 degrees so that it matches the amount of power found at 20 degrees.

Question 7

An optical cross is basically showing

Answer 7

The power of the sphere and the axis of the cylinder.

An optical cross is basically showing us the power of the sphere and the power and axis of the cylinder. We can figure out the prescription (in plus OR minus form) from the optical cross.

Question 8

This picture and optical cross

Answer 8

This picture and optical cross of a lens shows the 2 principle meridians being 90 and 180 degrees. We already know that the meridians in regular astigmatism HAVE to be 90 degrees away from each other. It also shows us that the POWER of the meridian at 90 degrees is -4.25 and the POWER at 180 is 
-5.75.

Question 9

In order to determine the prescription

Answer 9

In order to determine the prescription, we must pick one of the powers as our sphere. If you want your answer in plus cylinder, you should pick the number that would be the left-most number on the number line (most minus or least plus). This means the other number would be “higher” on the number line, or more to the right. You would be traveling in the plus direction. This indicates plus cylinder form.

Question 10

If you want your answer in minus cylinder

Answer 10

choose the number that has the highest negative number

If you want your answer in minus cylinder, you should pick the number that would be the right-most number on the number line (least minus or most plus). This means the other number would be “lower” on the number line, or more to the left. You would be traveling in the minus direction. This indicates minus cylinder form.

Let’s start with plus cylinder first for this example.

Question 11

Between -5.75 and -4.25, -5.75 is

Answer 11

Between -5.75 and -4.25, -5.75 is more minus. We will use this as our sphere
-5.75 ____ x _____

Question 12

We must remember that -4.25 and -5.75

Answer 12

We must remember that -4.25 and -5.75 are referring to the power in those meridians. It is not the same as the prescription. The cylinder would be the difference between 
-5.75 and -4.25 (or how far did we travel from one number to the next on our number line).

 5.75
- 4.25
 1.50

Our cylinder is +1.50 because we are working in plus cylinder and we went in the plus direction.

-5.75 +1.50 x______

Question 13

The axis that goes in the prescription is the axis

Answer 13

The axis that goes in the prescription is the axis that belongs to the sphere power that you used. In this case, we have -5.75 as our sphere. The optical cross showed the axis with a power of -5.75 is 180. So our axis is 180.

Question 14

So our prescription in plus cylinder is…

Answer 14


-5.75 +1.50 x 180

Question 15

For minus cylinder

Answer 15

For minus cylinder, we start with the least minus number as our sphere, -4.25. To get from -4.25 to -5.75, we have to travel -1.50. The axis that belongs to -4.25 is 090. 

-4.25 -1.50 x 090

Question 16

The prescription for this lens would be either…

Answer 16


-5.75 +1.50 x 180
or
-4.25 -1.50 x 090

Question 17

To plot a prescription

Answer 17

To plot a prescription on an optical cross, we are doing the same thing but in reverse. It is very simple.

Question 18

Put the sphere and the axis of the sphere on the cross.

Answer 18

• 4.50 +1.50 x 090
• 4.50 +1.50 x 090

To figure out the power that goes in the other meridian, just add 1.50 to -4.50.



-4.50

+1.50

-3.00


The other power is -3.00

Question 19

To determine the axis in the other meridian,

Answer 19

To determine the axis in the other meridian, just add or subtract 90 (whichever is appropriate) because the 2 principle meridians are always 90 degrees away from each other.

90 + 90 = 180

Question 20

The optical cross looks like this.

Answer 20

POST PIC

Question 21


+0.50 is our sphere with an axis of 175.

Answer 21

+0.50 -4.75 x 175

Question 22

To determine the power in the other meridian…

Answer 22

-4.75
+0.50
 -4.25

Question 23

The axis in the other meridian…

Answer 23


175

-090
TOTAL
 085

Question 24

The optical cross looks like this

Answer 24

POST PIC

Question 25

Let’s Practice

Answer 25

PRACTICE