Thermal expansion/contraction of lens

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    Lens Optics Thermal
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Discussion Overview

The discussion revolves around measuring the thermal expansion and contraction of optical lenses, specifically the SF56A lens, under varying temperature conditions ranging from -40°C to +70°C. Participants explore methods for accurately measuring this thermal strain while considering the challenges associated with bonding strain gauges to the lens surface.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant suggests using a strain gauge to measure thermal strain but expresses concerns about the difficulties of bonding the gauge to the lens without damaging it.
  • Another participant proposes using a tiny mirror or reflection off the lens surface combined with optical magnification to detect surface movement, offering a non-contact alternative to strain gauges.
  • A participant questions the necessity of measuring the coefficient of thermal expansion (CTE) given that the material properties are known, raising the issue of potential lot-to-lot variability.
  • Concerns are raised about the thermal expansion of the strain gauge itself and the bonding material, suggesting that a non-contact measurement method may be more suitable.
  • One participant highlights the need to measure the expansion/contraction of the lens specifically to prevent cracking when housed in an aluminum assembly and inquires about the feasibility of interferometric methods for this application.

Areas of Agreement / Disagreement

Participants express differing opinions on the appropriateness of using strain gauges, with some advocating for non-contact methods instead. The discussion remains unresolved regarding the best approach to measure thermal strain effectively.

Contextual Notes

Participants note limitations related to the bonding process of strain gauges, the potential for damage to the lens, and the sensitivity of different measurement methods, but do not resolve these issues.

deepak.midri
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Application:
to measure the thermal strain of optical lens. Due to temperature variation the lens surface is expanding / contracting in radial direction with two extreme operating temperature. (-40C to +70C )
Optical lens is SF56A (CTE - 7.9*e-6 per Kelvin)

What to do:
I need to measure this expansion/contraction of lens. I think I can do that by strain gauge. But I was looking for some earlier references on internet as bonding on gauge is very tricky. Even a small scratch on lens can deteriorate its performance. So I need to very careful while bonding gauge on it.

Questions:
1. Anyone have previous exp/suggestion on how to bond the gauge & its placement.
2. Any references on internet.
 
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Hi deepak.midri. I see you're new here, so http://imageshack.com/a/img515/4884/welcomesign.gif

I can't offer advice on strain gauges. But if you could fix to the lens a tiny mirror (or simply use a reflection off the already polished surface), you might be able to use optical magnification to determine movement of the surface. Arrange it so that a laser beam bounced at an angle off the surface will make a wide sweep across the wall of the room for a tiny movement of the mirror.
 
Last edited by a moderator:
deepak.midri said:
Application:
to measure the thermal strain of optical lens. Due to temperature variation the lens surface is expanding / contracting in radial direction with two extreme operating temperature. (-40C to +70C )
Optical lens is SF56A (CTE - 7.9*e-6 per Kelvin)

What to do:
I need to measure this expansion/contraction of lens. I think I can do that by strain gauge. But I was looking for some earlier references on internet as bonding on gauge is very tricky. Even a small scratch on lens can deteriorate its performance. So I need to very careful while bonding gauge on it.

Questions:
1. Anyone have previous exp/suggestion on how to bond the gauge & its placement.
2. Any references on internet.

I'm a little confused. First, since you know what the glass is and have a CTE, why do you need to measure the CTE? Are you concerned about lot-to-lot variability? Second, why do you have to use a strain gauge? I can think of a few alternate methods (for example, I expect an interferometric measurement to be much more sensitive).
 
I agree using a strain gauge is a bad idea. How are you going to allow for the thermal expansion of the gauge itself, and whatever material you use to bond it to the lens?

A non-contact measurement method seems more appropriate.
 
Hi,

I need to measure the expansion/contraction of lens only. Since sometimes lens develop crack as it is holded in a Aluminium assembly. So when both expand/contract lens develop a crack in it.
I need to measure at what expansion/contraction of lens/housing this happen.

1. I am facing trouble regarding strain gauge method. Since pasting is problem. moreover it may develop a crack.

2. What about interferometric method. Is there any special method for such type of application
 

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