Determining the correct lens blank is the first step to surfacing a lens.  This is a two step process, the fist step is to find the correct base curve and the second step is to determine the needed blank size.

The base curve is the curve from which all other curves will be determined.  This its usually the front curve.  Today’s labs will generally cut all curves on the back surface so if this is the case then your base curve is on the front surface.  If you happen to be cutting the front surface of a lens then your base curve is the back surface.  For our purpose we will call the base curve the front curve of the lens.

Many surfacing laboratories use a computer or a base curve chart.  The base curve chart is different for each manufacturer.  You could have the same Rx on a 6.00 base for one manufacturer and another manufacturer will call for a 5.50 base curve.  The reason for this is manufacturer X says that Rx Y will work best on a Z base curve.  This is because they have tested there lens and found that if you put Rx Y on base curve Z it will reduce marginal astigmatism and curvature of field.  The bottom line is, try not to deviate from the manufactures recommendations.

It is OK to change the base curve in order to fit into a frame, but try to keep this to a minimum and do not change the curve by more than a diopter.

If the base curve chart indicates two different base curves and the difference between these base curves is less than 2.00 D then you may use the same base curve for each eye.  Use the rules below.

Base Curve Rules

•Both Lenses Plus- Use the steeper (highest numerical value) base curve of the two.
•Both Lenses Minus- Use the flatter (lowest numerical value) base curve of the two.
•One Lens Minus, One Lens Plus- Use the steeper base curve of the two.

Keep in mind that this does not apply to progressives or any type of ashperic lens.  When dealing with these types of lenses it is best to always use manufacturers recommendations.  Also note that changing the base curve to fit into the wrapped frames will result in the patient having trouble seeing clearly.  It is beyond the scope of the paper as to the reason.  If you do come across these frames and want to put lenses into them use an outside lab, because there are advanced calculations that go into the process of making these types of glasses.

What if we do not have a base curve chart or a computer?  We will use Vogel’s Rule to determine the base curve.  Before we use Vogel’s Rule we need to find the spherical equivalent of the lens.  The spherical equivalent is the average of the powers in the two major meridians.

Spherical Equivalent

*Please note that the spherical equivalent for a sphere is the sphere.

Vogel’s Rule

Plus Lenses

Minus Lenses

Example:

What is the base curve needed for a +6.00 DS?

• We know it is a plus powered lens
• We know if is a spherical

• The base curve needed for a + 6.00 DS lens is a +12.00 Base.

Example:

• What is the base curve needed for a -2.00 -1.00 X 90?
• We know we have a minus lens
• We know we have cylinder, so we need to find the spherical equivalent

Now that we know how to find a base curve of the lens, we need to know what diameter of lens to pull. We are going to use the minimum blank size (MBS) formula, this formula gives us the minimum blank needed for the particular frame that was chosen.  In order to find the MBS we need to know the decentration.  Decentration is moving the lens so the OC is placed over the patient’s eye.  Although this is done on the finishing side of the lab we need to know the decentration to find the MBS.

Decentratioin

MBS

Example:

What is the minimum blank needed for the following:

A= 50

B = 40

ED = 50

DBL = 16

PD = 54

The image above shows a frame with the same frame dimentions as in the example above and a 70 mm lens blank super imposed over the lens.  The OC of the lens is decentered in 6 mm (decentration per lens).  If we measure from the optical center of the lens to the furthest point of the frame, we will get a 31 mm radius.  Since a lens blank is round we double 31 mm and we get 62 mm (the two millimeters in the MBS formula is for any error that may occur).  If you use the numbers above you will see that the MBS needed is a 62 plus the  2 mm for error.

Please note surfacing labs will use a smaller lens blanks than what the minimum calls for.  They will grind the OC off center in order for the lens to make cut-out.  If this is done then the minimum blank needed is the ED plus two millimeters, no need to account for decentration since it was ground in.  Please note that this formula is only 100% accurate if the ED lies on the 180° line.

Putting it all together

Example:

What is the minimum blank and base curve needed for the following:

A = 56                        Rx

B = 40                        OD -2.00 -1.00 X 45

ED = 58                     OS +2.00 -2.00 X 135

DBL  = 15

PD = 65

The right eye needs to be put on a 4.75 base curve and the left eye needs to be put on a 7.00 base curve and we need at least a 66 mm blank.