Aspheric calculations!!

**amy88** · 05-05-2008, 04:12 AM

Hi..jus wondering if someone could help me on the following question:

A plano concave -10.00DS back vertex aspheric spectacle lens has a p2 value of -0.5. If the refractive index of the material is 1.5, and the centre thickness is 1.0mm, what is the lens diameter if the edge thickness is 6.0mm?

Thanks

**HarryChiling** · 05-05-2008, 10:47 AM

49.72mm, the equation your lookign for is:

y² = 2rx - px²

y = semi diamteter
r = radius at the apex
x = sagg
p = asphic value

**amy88** · 05-05-2008, 03:20 PM

Thanks for your reply...bt the answer in the book is actually 45.3mm!!!...dnt no how they gt that..and how can you work out the sagg if you dont no wat the lens diametr is?

**chip anderson** · 05-05-2008, 03:36 PM

Maybe dey usin Whitworth tools. Never could quite figure out what was actually the stated size on these things, but they did work on English bikes.

Chip

**HarryChiling** · 05-05-2008, 04:02 PM

Originally Posted by amy88

Thanks for your reply...bt the answer in the book is actually 45.3mm!!!...dnt no how they gt that..and how can you work out the sagg if you dont no wat the lens diametr is?

Same equation me being a dodo:

y2 = 2rx - px2

r = (1.5 - 1) / 10.00 = 50mm
x = 6.0mm - 1.0mm = 5.0mm (this is where I made a mistake used 6mm instead of subtracting the 1mm center thickness)
p = -0.50

y2 = 2(50)5 - (-0.50)(5)2
y2 = 500 - (-12.5)
y2 = 512.50
y = 22.64
diameter = 2y
diameter = 2(22.64)
diameter = 45.28mm

I make a lot of simple mistakes here and there so my work must be checked.:hammer:

**amy88** · 05-06-2008, 02:23 PM

Thank you soooo muchh....your a star!!!...lol....ive gt many more questions...so will keep it posted...lol..thanks agen...very much appreciated...

**rolandclaur** · 05-14-2008, 07:00 PM

Is there a specific name for that equation or what its generally used to calculate?

**lensgrinder** · 05-17-2008, 09:46 PM

Originally Posted by rolandclaur

Is there a specific name for that equation or what its generally used to calculate?

I do not know if the formula has a specific name. It is used to describe the surface of conic sections. For example a circle, an ellipse, a hyperbola.
The p-value or eccentricity is the amount that the surface deviates from a circle.
A value of 0 will result in a circle, a value of 1 will result in a parabola, a prolate ellipse (flatter surface) will have an eccentricity value of 0<p<1, and a oblate ellipse(steeper surface) will have an eccentricity of greater than 1.