Dear Ms. Specs in the City,
I would like to know if monocular PDs have to be the same for both eyes for a bifocal prescription and trifocal prescription.
Thank you for your excellent question! This is a conversation I have had many times with my optician colleagues and students. It falls within the protocol of priority: 1. Safety (Trivex or poly for children or monocular patients). 2. Optics—pairing the spectacle Rx with the appropriate frame selection, lens material and correct centration metrics (PD and seg. heights), and 3. Fashion/Cosmetics (patients/clients want to feel attractive in their eyewear).
If we follow standard centration practice for segmented multifocal lenses, you will take monocular distance PD measurements and monocular near PD measurements and provide to the lab. They will determine the bifocal inset (centration) by subtracting the near PD from the distance PD for each eye.
Distance PD, monocular: OD 30 mm and OS 28 mm with Near PD monocular: OD 29 mm and OS 27 mm
Right Eye Near Centration: OD 30 - 29 = 1 mm inset from distance centration point
Left Eye Near Centration: OS 28 - 27 = 1 mm inset from distance centration point
If the Mono PDs are the same, the segment placement will be mirrored horizontally between the left and right lenses, which may appear more balanced and aesthetically attractive. (Yes, Ms. Specs is highly aware of the irony of using attractive and segment in the same sentence!)
When the Mono PD values differ, you may be concerned that segment location differences in the lens will be apparent to either the patient or those looking at the patient. The difference is minor, and the chance of it being noticed is unlikely.
Should it come up, explain the importance of the precise centering of the lens in front of the eye to produce the best optics and visual comfort. This may be an opportunity to discuss additional options, like a back surface free-form bifocal which will provide the best of both worlds, better optics and cosmetics.
Below we illustrate the amount of induced prism in the distance as well as the near segment of the lens, due to centration errors caused by the use of a binocular PD measurement with an equal but incorrect amount of decentration.
Note: The horizontal prism ANSI Z80.1-2015 tolerance in a single vision or segmented lens is less than 0.67 ∆ diopters, or for powers over +/- 2.75D the interpupillary distance (PD) error can’t exceed 2.5 mm (both eyes).
Rx: +2.00 – 0.50 x 180 OU with a 2.00 add with PD: OD: 28.5 OS: 31.5
Prentices Rule: (measure horizontal prism at the 180)
∆ = (D x d)/10, where:
D = power at 180 degrees, the horizontal meridian
d = error in decentration in mm
10 = a constant, to convert from cm to mm
If we go with the binocular PD, 60 mm (30 mm per eye) there would result in 1.5 mm of error in decentration per lens both in the distance portion of the lens and in the near segment.
Prism at Distance Power OC: The distance power at 180 degrees is 2.00 D, multiplied by 1.5 = 3.00, divided by 10 = 0.3∆ diopters per eye. Although the distance power prism induced meets the 0.67∆ ANSI tolerance. Note: Had the distance power been higher than 2.75 D, the distance between the optical centers would have exceeded the 2.5 mm ANSI tolerance.
Prism at Near Segment Center: Once we add the +2.00 D of distance power to the +2.00 D of add power we have: 4.00 x 1.5 = 6.00 D/10 = 0.60 ∆ per eye, or 1.2 ∆ diopters total exceeding the prism tolerance in the near segment. (Note: If the distance Rx is for minus lenses [-2.00] then there is no power at the 180 when you add it to the +2.00 near addition power and therefore no prism.)
As much as we want symmetrical segment placement for cosmetics, we must consider the effect on lens optics and wearer comfort. When a patient/client wears glasses with unwanted prism, it adversely affects how they see and feel. The wearer may feel a “pulling effect” (straining the extrinsic muscles), or an all-around “uncomfortable visual experience” (asthenopia) where they may experience eye fatigue and headaches.
And as always, combining impeccable manners with optical expertise is a formula for success.
See Well and Be Well,
Ms. Specs in the City
Laurie Pierce, ABOM