BOX Measurements

Box Measurement Definitions and Additional Verbiage to help with following formulas.

A = Width of the Lens

B = Height of the Lens

ED = Effective Diameter (longest diameter of lens)

DBL = Distance between Lenses

GC = Geometric Center of lens

DBC/GCD (Distance between GC/ Geometric Center Distance) = Distance between centers of each lens (frame PD)

Datum Line = Line that runs horizontally through the GC of the lens. Also could be called the “mounting line”

OC = Optical Center of Lens (that point on a lens through which a passing ray is not deviated). Provides best VA when line of sight is lined up with the OC. Does not take into account prescribed prism.

MRP = Major Reference Point. (The OC and MRP are the same if there is no prescribed prism).

PRP = Prism reference point or point at which the prescribed prism is located.

DRP = Distance Reference Point (should be slightly higher than the fitting cross (based on progressive lens design)

NRP = Near Reference Point, reference point for best visual acuity for near vision.

FRAME PD

The frame and the patient both have their own PD measurements. The frame PD is calculated by adding the A measurement to the Bridge Measurement.

Total Frame PD = A + DBL

To get each eye separately, divide your answer by TWO.

Frame PD = (A + DBL)/2

Another way to calculate the FRAME PD is to find the distance between the GC of each lens.

Decentration

Decentration is the difference between the FRAME PD and the PATIENT PD. When we hear decentration we are usually speaking of a lens being decentered horizontally. For vertical decentration you would take HALF of the B measurement and subtract either the OC or the SEG height. These measurements tell us how far away the OC of the lens is from the center of the frames. It is important to know this because the thickness of a lens is greatly affected by the amount of decentration.

DEC = FRAME PD - PATIENT PD

Vert DEC = OC HT - (B/2) <This is the distance of the Patient’s OC Height from the Datum Line)

For example if the patient’s PD is 61 and the frame PD is 65 the deccentration would be

DEC = 65-61 = 4 mm decentered nasally. (nasally because the patient’s pd is more narrow than the frame PD)

Example for vertical decentration, if the OC is 48 and the SEG HT is 22 then

(48/2) - 22 = 24 - 22 = 2 mm decentered below the datum line.

Minimum Blank Size (MBS)

What we are trying to find out here is what is the smallest lens that can be used that can fit into the frame that was selected. This is important because many lens designs may not have a size available that fit the patient’s frame choice.

MBS = (GCD - PD) + ED

For example if the frame we choice has a frame PD of 75, the patient’s PD is 65 and the ED is 50 then the MBS would be calculated as

MBS = (75-65) + 50 = 10 + 50 = 60 mm

Nominal Lens Formula

The nominal lens formula gives us the total power of the lens by adding the power of the front surface(s) to the power of the back surface(s).

Power = Front Surface Power + Back Surface Power or Dn = D1 + D2

Base Curve = The curve from which all other curves are measured. The Base Curve is on the front of the lens and is POSITIVE (+) in power.

*The more shallow curve on the backside of a lens is the called the “toric curve”.

• The steeper curve is called the “cross curve”

Vogel’s Rule

Used To determine base curve’s

• + Powers: Spherical Equivalent +6.00 D

• - Powers: 1/2 Spherical Equivalent +6.00 D

& Tscherning