Optical Epoxy with Refractive Index of 1.0008

[Jaer57]

Does anyone know if an optical epoxy exists with a refractive index as close to air as possible? I'm not even sure an epoxy like this exists, but maybe someone out there knows better. Thanks!

 

[IRstuff]

Most of the time, such things are used to glue glass together, so you should want something that matches glass, not air.

TTFN

FAQ731-376

 

[Jaer57]

Thanks, but we're trying to replace a small open air gap with epoxy in order to hold two materials together, but we still need to maintain a refractive index of air for the optical system to work unchanged.

 

[IRstuff]

Well, usually such small gaps are extremely sensitive to errors in that gap. A glue is not something that promotes accurate spacing, what with shrinkage, moisture absorption, etc. A mechanical spacer is reliable and repeatable.

TTFN

FAQ731-376

 

[Jaer57]

Very true. This is for an experiment with a controlled environment, though, so hopefully shrinkage, temperature, and moisture will be controlled and measured.

 

[IRstuff]

I guess, but why not just glue them to a spacer ring?

TTFN

FAQ731-376

 

[lumenharold]

Perhaps you can control temperature and moisture in the environment and shrinkage after the adhesive is set but the best you'll do is to try to anticipate the shrinkage during the cure. How much it shrinks will depend in the thickness and total volume of the glue among other things. I would bet that getting all the air bubbles out and maintaining the glue level would be a challenge also. Most of the thick adhesives we've used are intended for edge cure and are not very clear. The clear adhesives are thin and runny. Depends on how you define "clear" I suppose.

I would assume that if the system is sensitive to the small gap then you will need to fixture the lenses anyway which gets back to IRStuff's question of why not use a spacer?

If you use an adhesive (or urethane, etc) and you know the refractive index can you adjust your data? In other words just verify the error you expect is the error you get? Is this a single wavelength application; are you trying to image something, etc? That could make difference also.

Harold
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[DanStro]

How large is the gap?

 

[jedward]

The index of air is ~1.00029
I boldly assert that there are no solid adhesives with the very low index you seek. Only some gases will match.

Controlling the thickness of any adhesive is dificult and VERY difficult to use if you must control layers to be less than 0.0002 inches, even with thin epoxies that will flow between clamped glass surfaces or spaces that might be cosidered. The capillary action of the fluid can generate significant forces to separate the parts.

 

[DanStro]

My thoughts are that if the gap is very small then putting them into optical contact may be the answer.

 

[damienmorton]

The lowest optical index solids I have found are Teflon AF with an index on 1.29.

You wont find a solid with an index close to air.