the Prescription/RX

A lens is made up of two prisms connected either base to base (plus lenses) or apex to apex (minus lenses). A minus lens will diverge light, and a plus lens will converge light onto the retina.

The Rx is measured in dioptic power, usually in quarter diopters starting at 0.00 and going in a positive or minus direction. The minus or plus power indicates if light needs to be either converged or diverged so that it lands directly on the retina. Additional refractive errors including astigmatism or presbyopia will mean the RX will need additional powers to be added to the lens to refract all light onto the retina.

An RX for Glasses contains several parts:

Sphere (SPH): Spherical power of a lens.

Cylinder (CYL): This is an additional power curve added to the lens to correct for astigmatism.

Axis: The degree of axis tells us where the CYL curve must be placed to correct refract light onto the retina.

ADD: Additional plus power added to the lenses to correct for presbyopia.

PRISM: This is added to the lenses to reduce diplopia.

PRISM Direction: This tells us where the BASE of the Prism lens is in relation to the pupil.

An RX for Contact Lenses may have the above corrections but sizing information is important because these lenses fit directly onto the eye.

Base Curve (BC): How steep the curve of a lens is.

Diameter: The size of the lenses.

Brand/Type: There are several manufacturers of contact lenses that all offer different materials for contact lenses, wear time, and care instructions.

Transpose an RX

Transposing an RX is done in 3 steps:

1. Add the Cylinder power to the Spherical power which gives you your new Spherical power.

2. Change the sign of the cylinder (do not drop the power), this gives you your new cylinder power.

3. Add or subtract 90 degrees from the axis. (remember the rx must be between 0 & 180 degrees). This gives you your new axis.

Example

-1.00 -1.00 090

Step one says to add the Sphere power and the CYL power. -1.00 + -1.00 = -2.00 (this is your new sphere power)

Step two says to change the sign of the CYL power. -1.00 is now +1.00

Step three says to add or subtract 90 degrees to the axis. 090 + 090 = 180

So your new RX is written as -2.00 +1.00 180.

You can check your work by transposing the new RX again. The answer should be the originally transposed RX. Let’s work through it.

Step one: -2.00 + +1.00 = -1.00

Step two: +1.00 is now -1.00

Step three: 180 - 090 = 090

So your new rx is -1.00 -1.00 090.

The Optical Cross

The optical cross shows the power of a lens in different meridians, mainly 090 degrees apart from each other. This is power, not necessarily prescription. You can put a prescription into an optical cross or you can understand the prescription of a lens from an optical cross. For example a spherical lens with a power of -5.00 will have the same power in all meridians and will look like this on an optical cross. Because there is no change in power at any point in this lens it is safe to say that there is no CYL power on this lens.

When you compare a spherical lens to a lens that has Cylinder power (to correct for astigmatism) you will see that a cylinder lens will have two different powers and thicknesses 090 degrees apart from each other. The lenses thinnest point will be 090 degrees away from it’s thickest point and will look like this on an optical cross. Let’s use this power, -4.00 -1.00 045 as an example. The difference in power 090 away from each other gives the CYL power of the lens. In the example here you see a difference of -1.00 when you go from -4.00@045 to -5.00 @135 and you see a +1.00 CYL power when you go from -5.00 @135 to -4.00 @045.

A Quote from John Seegers that may help you understand the optical cross and something great to memorize.

“The Shorter the Radius the Steeper the Curve, The Steeper the curve the higher the power, The Higher the power the thicker the lens.”

Example 1

A lens with a power of -2.50 @ 060 and a power of -1.50 090 degrees away (150) can be written onto an optical cross like this.
The power of the lens has a difference of +1.00 090 away when moving from -2.50 @060 to -1.50 @150

-2.50 to -1.50 is +1.00 (this difference is the CYL power) and can be written out as;

-2.50 +1.00 060

The power of lens has a difference of -1.00 090 away when moving from -1.50 @150 to -2.50 @060

-1.50 to -2.50 is -1.00 (this difference is the CYL power) and can be written out as;

-1.50 -1.00 150

Example 2

Let’s say you have these two powers -12.00 @045 and -6.00 @135. How would you put this into an optical cross and how would you write this out in RX form? The difference in power in either direction 6.00 diopters. It is plus 6.00 diopters when moving from -12.00 to a -6.00 and minus power when moving from -6.00 to -12.00. It can be written as

-6.00 -6.00 045 or -12.00 +6.00 135 and on an optical cross it would look like this.

Example 3

Now let’s work backwards from an optical cross into written form. Let’s say you have the following optical cross and you want to know how to write out the power.

Start with either meridian and write the power in that meridian as the sphere power and your axis at that point. For this example you see that there is -3.75 @045 so we now have two parts, the sphere power and the axis. We just need the CYL power and we are done! So far we have -3.75 _______ 045.

Next, calculate the power difference moving 090 degrees away and write that out as your CYL power. At 090 away the power is -4.25, because this power is MORE minus the CYL power is minus. There is an additional -0.50 power going from -3.75 to -4.25. You can write the -0.50 into your power so you now have -3.75 -0.50 045.

Let’s also try starting from the power at 135 degrees. We have -4.25 @135 and this can be written out as -4.25 ________ 135. When moving from -4.25 to -3.75 you are becoming LESS minus (or adding plus power) so your CYL is now in PLUS form. The power can be written out as -4.25 +0.50 135

Checking your work!

Transpose your written power to see if it matches up to the optical cross. This is a great way to double check your work. Another tip is to make sure you work from both meridians just to double check your math and power signs.

Concept of Axis from Laramy-K

https://www.youtube.com/watch?v=z6cHAQbych4

Concept of Axis from Laramy-K

https://www.youtube.com/watch?v=z6cHAQbych4