By Clifford W. Brooks OD
Knowing Lens Surfacing covers either conception and medical program of ophthalmic lens surfacing from begin to end. either scholars and educators will locate the textual content effortless to learn and good geared up: lens surfacing concepts are defined, established via basically conveyed examples, and summarized in step by step tables
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Extra resources for Understanding Lens Surfacing
The lens is a left lens. Power = + 3 . 5 0 D Frame A = 53 mm DBL = 19 mm Wearer's PD = 64 mm Determining Decentration Prism for Spherocylinders With glass lenses there is no overriding advantage for blocking on center, even when decentration of the major reference point (MRP) is required. Sim- 46 / UNDERSTANDING LENS SURFACING NOSE > LEFT LENS CONVEX SIDE UP Figure 3-19 Can you tell if this on-center blocked lens is a plus or a minus lens just by the prism base direction? ply blocking at the desired MRP location will usually work well and requires no calculation for prism.
In many cases it is still the method of choice. The other method is termed on-center blocking. With the advent of the lens computer, this method is now the most frequently used method. Why Surface a Single-Vision Lens? It makes sense to use a finished lens instead of surfacing a semifinished lens whenever possible. Yet there are times when surfacing a single-vision lens is either preferable or necessary. Here are a few. 1. Either the sphere or the cylinder power of the needed lens may be greater than is normally stocked in uncut lenses.
See Figure 3-18. Draw an arrowhead to show the direction of the prism's apex. ) See Figure 3-19. The arrowhead is drawn on the front of the lens so that it is not ground a w a y during surfacing. OPTICAL CENTER BLANK GEOMETRIC CENTER Figure 3-16 How a lens looks on the block after having been surfaced. ) Mark a reverse " L " in the upper half of the lens on the front surface. The job number is marked in reverse writing in the lower half. o ^ o «J n ' i i Mill I I 1 1 1M i l l r\ ^j 5 u 1U 35 28 25 Figure 3-17 Standard positioning of a single-vision lens that is to be marked for on-center blocking.