Anatomy & Refractive States of the Eye
Identifying ametropia with the optical cross

Identifying Ametropia with The Optical Cross
A skilled optician is capable of reading a prescription and identifying which ametropia it is designed to correct. The optical cross is graphical device useful in illustrating the powers of the front and back surfaces of a lens in order to arrive at its total power and can be useful in analyzing a prescription. Let’s look at the example of a set of optical crosses illustrated in Figure 8 below. The first cross represents the power of the front surface, the second cross the power of the back surface, while the third represents the total power of the lens. Here, we have the same lens previously mentioned lens with a +6.00 D spherical curve on the front and a -5.00 D spherical curve on the back. By utilizing the optical cross, we see the front surface power is a +6.00 sphere, or +6.00 in all directions or meridians. This is indicated by seeing the +6.00 written along both the horizontal and vertical lines. In a like manner the back surface contains a spherical power of -5.00 diopters in all directions. By combining +6.00 and -5.00 diopters algebraically, the resulting net power of the lens is equal to + 1.00.

As we have illustrated, by combining the powers of the front and back surface of a lens algebraically, it is possible to determine its overall power. This relationship between the power of the two surfaces and the net power of the lens may be expressed mathematically as:

Fl + F2 = FT

where F1 stands for the power of the front surface of the lens,

F2 the power of the back surface and

Ft the total lens power.

The Optical Cross