# Thread: help calculating setting value

1. ## help calculating setting value

hello,

i need some help how to calculate setting value and actual value when surfacing a lens

2. Any more specifics?

3. The "Setting Value" is the desired height of a lens' back surface above the reception chuck on a Loh (or derivative) generator after generating. So, it's the height of the blocking ring, plus the "generator thickness" of the lens (that's the thickness of the lens at the surface block center, including any compensations for tape and stock removal), less the sag of the front of the lens in the blocking ring.

The "Actual Value" is the height of the back surface of a blocked lens above the reception chuck before generating. So, substitute "blank thickness" for "generator thickness" above.

4. hr=Ringh High
h1=Sag High 1
h2= Sag High 2
d=diameter
dm=Center Thickness
E=Setting Value
dr=edge Thickness

E = hr-h1+dm

h1=r1 - √r1^2- (d/2)^2
h2=r2 - √r2^2- (d/2)^2

dm= h1+dr-h2

dr= dm+h2-h1
h1= dm+h2-dr
h2= h1+dr-dm

5. thank you all for the help

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