Eye care providers (ECPs) need to order lenses with prescribed prism. In lower amounts, this is a normal request. When the need arises for larger amounts of prism (approaching 4.00 Δ to 6.00 Δ), ECPs know that they are reaching the limit of many lens tolerances.
As a result, ECPs want to know the maximum surfaced prism tolerance of each lens style they recommend, whether it’s a progressive addition lens (PAL) or a single vision (SV) lens.
Download this PDF for a condensed version you can refer to as needed.
At HOYA, we do not offer a maximum surfaced prism amount for each of our lens styles. However, we do offer a range of maximum surfaced prisms as a broad reference for all of our lenses.
So, if an ECP cannot get the maximum surfaced prism amount, how can they prescribe the right lens style? It all comes down to understanding that maximum surfaced prism is not lens style dependent. It is the result of a complex formula with multiple variables.
Understanding the range of maximum prescribed prism
Each manufacturer has a formula that determines the maximum amount of prescribed prism that can be surfaced into a spectacle lens.
The maximum amount of prescribed prism that can be ordered can only be determined once the lens order is placed and the formula calculated to determine whether the result is over the surfaced prism maximum.
In general, HOYA lenses can accommodate 4.00 Δ to 6.00 Δ of prescribed prism. This is a generous amount of prism and will likely handle the majority of the prescribed prism prescriptions that most ECPs encounter.
Now that you understand the range of the maximum amount of prescribed prism that can be surfaced into a lens, let’s focus on the prism surfacing formula — this will increase your ability to design lenses to meet your patients’ needs.
Breaking down the prism surfacing formula
There are two major variables in the prism surfacing formula
- Thinning prism
- Decentration prism
- Prescribed prism
- Your desired substrate influences the outcome
Total prism is the largest contributor and should always be considered when designing lenses for your patients with prescribed prism. Total prism in lens production is more than just the prescribed prism. You have to consider prescribed, decentration, and thinning prisms.
Prescribed prism: You cannot manipulate the prescribed prism, as this is what the patient needs in their new lenses to have the best vision possible. However, you can influence the decentration and thinning prism by using proper frame fitting techniques.
Decentration prism: A direct result of decentering the lens during the blocking phase1 of the surfacing process. To decrease decentration prism, match the Frame PD2 to the patient’s binocular PD as closely as possible. (It is never recommended to order lenses with prescribed prism using a binocular PD. However, for this exercise in the fitting process, the binocular PD is used.) This will place the patient’s pupils near the center of each lens horizontally and will decrease the need for decentration during the blocking phase of the surfacing process.
Since we are decreasing the decentration for blocking, we have effectively decreased the resulting decentration prism. If this step in the frame fitting is followed properly, you can realistically increase the amount of prism that can be surfaced into your lens of choice.
Prism thinning: often employed to make progressive lenses more aesthetically pleasing. It is the process of adding yoked base down prism to make the thickness of the bottom of a progressive lens match the thickness of the top of that lens as closely as possible.
It is possible to order lenses without thinning prism, though it’s not recommended without proper fitting from the dispenser. A better approach is to minimize the amount of prism thinning needed with proper frame fitting. Consider the distance from fitting cross to both the bottom and the top of the frame.
Put it in perspective
Here’s an example.
If the minimum fitting height of your desired progressive lens choice is 14mm, the frame you choose for the patient should have no larger than a 16mm fitting height and roughly 10mm to 12mm of distance to the top of the frame.
14mm to 16mm fitting height will provide sufficient reading area without the worry of cutting off the near portion of the progressive corridor and 10mm to 12mm of distance above the fitting cross will provide sufficient distance viewing area. Taking care to ensure that the distance from fitting cross to both the top, and bottom of the frame are as equal as possible decreases the amount of thinning prism needed. This produces a highly functional progressive lens and increases the amount of prism that can be surfaced into the lens you are designing.
Index of refraction plays a major role. For instance, a prescription ordered with 4.75 Δ of prescribed prism is within the 4.00 Δ to 6.00 Δ mentioned earlier. If this order were to be placed in 1.50 plastic or 1.53 Phoenix, the job will likely be rejected, even if we follow the proper fitting suggestions listed above. However, if ordered in 1.67 hi-index, and the proper fitting recommendations were followed, the likelihood of this job being successfully completed are extremely high.
Design the perfect pair of lenses
By understanding the maximum surfaced prism range, the factors that affect that maximum, and how you can take steps to positively influence the outcome with proper fitting techniques, you will become more confident about designing that perfect pair of lenses the next time a prescription lands in your hands with prescribed prism.
1 Blocking is the process used to attach the lens to the surfacing machine. The optical center (OC) in single vision lenses and fitting point in progressive lenses is aligned to the axis of the surfacing machine. If the OC or fitting point is not the geometric center of the lens blank, it must be decentered for proper alignment.
2 Frame PD is the horizontal distance from the center of one lens to the center of the other in regard to the desired frame you are designing the lens for. You can easily calculate this by adding the ‘A’ box measurement to the distance between lenses (DBL) measurement. For instance, a frame stamped 47-18-140 has an ‘A’ box measurement of 47mm and a DBL of 18. 47mm + 18mm = 65mm. The frame PD of this frame is 65mm.