# Thread: Aspheric lens can measure by lens measure to determine the len's index????

1. ## Aspheric lens can measure by lens measure to determine the len's index????

do the aspheric lens can measure by lens measure determine the len's index , how it measure??????????????

2. Lens index can be calculated roughly with standard lens makers formula, assuming curves font and back are measured within the normally spherical center, at a maximum center of 3 to 4 mm diameter. curve measurement will need to be as accuarte as the index you are to determine.

3. ## what?

can some1 explain to me wah exactly is a lens measure, like wah does the formula mean?

4. Originally Posted by scared_1st_year
can some1 explain to me wah exactly is a lens measure, like wah does the formula mean?

5. Originally Posted by scared_1st_year
can some1 explain to me wah exactly is a lens measure, like wah does the formula mean?

Any one week old optician knows what a lens clock is and what it is used for.

Second:cnet baby:
Aspheric lens can measure by lens measure to determine the len's index????
do the aspheric lens can measure by lens measure determine the len's index , how it measure??????????????

Again anyone who has been in the business should know this!

6. Originally Posted by Newyorkoptician
Aspheric lens can measure by lens measure to determine the len's index???? do the aspheric lens can measure by lens measure determine the len's index , how it measure??????????????

Again anyone who has been in the business should know this!

Can you?

7. Harry:

I been in this business since 1958 and have never heard this instrument called a lens measure. Always a lens clock.

Chip

8. Originally Posted by HarryChiling
Can you?

A lens clock is calibrated for crown glass which is index of refraction 1.523. If you measure both sides of the lens and adding the surface powers together will give you the approximate power of the whole lens. If your readings are way off this could explain that it maybe a different refractive index so in a sense you can use a lens clock to find the index. I didn't say it was perfect and I might have come across a little gruff and I apologize for that but as you can see you can get an approximation for an index other than crown.

9. You can determine the index with a lens clock.
The formula:
$n_{t} = \frac{1 + f_{t}(n_{tm} - 1)}{F_{tm}}$

nt is the index you are looking for
ft is the power read from the lensmeter
ntm is the index of the clock (1.53)
Ftm is the sum of the surface powers from the clock

It becomes more difficult to do it with an aspheric lens because you cannot accuratley determine the front curve if it is ashperic.
You would need to reverse the back vertex formula to solve for D1
$D_{1} = \frac{(b_v - D_2)}{1 + (\frac{t}{n})b_v - (\frac{t}{n})D_2}$

D1 is the front curve
bv is the back vertex power
D2 is the back curve
t thickness in meters
n index of lens

10. Originally Posted by lensgrinder
You can determine the index with a lens clock.
The formula:
$n_{t} = \frac{1 + f_{t}(n_{tm} - 1)}{F_{tm}}$

nt is the index you are looking for
ft is the power read from the lensmeter
ntm is the index of the clock (1.53)
Ftm is the sum of the surface powers from the clock

It becomes more difficult to do it with an aspheric lens because you cannot accuratley determine the front curve if it is ashperic.
You would need to reverse the back vertex formula to solve for D1
$D_{1} = \frac{(b_v - D_2)}{1 + (\frac{t}{n})b_v - (\frac{t}{n})D_2}$

D1 is the front curve
bv is the back vertex power
D2 is the back curve
t thickness in meters
n index of lens
You beat me to it :hammer:

11. Originally Posted by chip anderson
Harry:

I been in this business since 1958 and have never heard this instrument called a lens measure. Always a lens clock.

Chip
I have heard it referred to both ways. My Franel clock says Lens Measure on it. My Hilco at home does as well.

See a picture here:

www.hilco.com

part# 20/030/0000 (search for this#)

12. Originally Posted by lensgrinder
You can determine the index with a lens clock.
The formula:
$n_{t} = \frac{1 + f_{t}(n_{tm} - 1)}{F_{tm}}$

nt is the index you are looking for
ft is the power read from the lensmeter
ntm is the index of the clock (1.53)
Ftm is the sum of the surface powers from the clock

It becomes more difficult to do it with an aspheric lens because you cannot accuratley determine the front curve if it is ashperic.
You would need to reverse the back vertex formula to solve for D1
$D_{1} = \frac{(b_v - D_2)}{1 + (\frac{t}{n})b_v - (\frac{t}{n})D_2}$

D1 is the front curve
bv is the back vertex power
D2 is the back curve
t thickness in meters
n index of lens

I'll second that you beat me too. I was trying to remember the complete formula could not remember the whole thing. But to the question above from Harry Yes I can.

13. Originally Posted by Newyorkoptician
A lens clock is calibrated for crown glass which is index of refraction 1.523. If you measure both sides of the lens and adding the surface powers together will give you the approximate power of the whole lens. If your readings are way off this could explain that it maybe a different refractive index so in a sense you can use a lens clock to find the index. I didn't say it was perfect and I might have come across a little gruff and I apologize for that but as you can see you can get an approximation for an index other than crown.
Actually you can see from the formulas below that the lens clock calibration is for n= 1.53, and NOT crown glass n=1.523. This is because
in the late 1800's the only available glass had an index = 1.53. Later, when crown glass was invented with n=1.523, the lens manufacurers did not want to change all their laps and they determined that if they stayed with the n=1.53 laps, the new crown glass lenses would be out of power by only 1/16th diopter (0.06), which was within tolerance.
Now lens clocks are also available that are specifically calibrated for lenses with n=1.49 and n=1.60. But NOT for n=1.523 crown glass. :cheers:

14. On a toric lens, it would be necessary to find the meridians of greatest and least powers to determine the approx lens power and index.

15. Originally Posted by tmorse
Actually you can see from the formulas below that the lens clock calibration is for n= 1.53, and NOT crown glass n=1.523. This is because
in the late 1800's the only available glass had an index = 1.53. Later, when crown glass was invented with n=1.523, the lens manufacurers did not want to change all their laps and they determined that if they stayed with the n=1.53 laps, the new crown glass lenses would be out of power by only 1/16th diopter (0.06), which was within tolerance.
Now lens clocks are also available that are specifically calibrated for lenses with n=1.49 and n=1.60. But NOT for n=1.523 crown glass. :cheers:

And if you read what I wrote I said it would be an approximate power and index not exact but I appreciate you educating me. We can all learn.

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