How do you define an optically sharp edge

[danap2005]

I have an aperture for a device that uses a laser to illuminate a part. The fellow who designed it didn't want an edge that caused excessive reflections so he noted it as "sharp" or "optically sharp". My difficulty is how do I define this on a drawing so that the machinest knows what is needed? Is there a way to define the edge in terms of surface texture? This part is intended to be low cost so the option of defining it in terms of some type of optical measurement does not solve the problem as it leaves the definition wanting.

Anybody have any idea's or suggestions?

 

[IRstuff]

We've had similar issues in the past, with no obvious solution.

Part of the issue is how sharp? We wound up using razor blades.

TTFN

 

[nbucska]

first requirement seems to be that any deviation from
the mathematical "line" should be << than lambda of the
used light and so should be its convolution with any
window which is narrower than lambda.


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[IRstuff]

In any case, optically sharp is EXTREMELY dfficult to achieve. A shallow incidence angle on any surface, not matter how short can achieve a specular reflection.

One thing we did not try before is a photolithographically defined aperture on a glass substrate, since we had other constraints. A chrome on glass aperture might work well, particularly with a relative thin chrome layer.

TTFN

 

[nbucska]

I would prefer an etched slit in a very thin foil
i.e. without a transparent substrate.

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[danap2005]

I want to thank all that responded. There were some ideas expressed that stimulated some of my own. Probably the use of the term "optically sharp" is a poor idea (one mans sharp is anothers dull). It may be usefull to define the hole as a asme y14.5 profile and add a note about the surface condition. I will have to mull on it.

Thanks again.

 

[IRstuff]

nbucska raises an interesting point.

An etched foil might work in some cases. However, most foils are really limited a few mils thickness, in order to achieve a certain degree of rigidity. That bit of thickness can result in some horrific glints with a shallow incidence angle.

We found in our example that even a tapered edge would still leave sufficient surface to get a monster glint. And that's one issue with a standard hole; your surface condition is irrelevant when it comes to glint conditions. The shallow angle can make the dullest paint look like a mirror (we know from sad experience).

TTFN

 

[nbucska]

Use a bimetal substrate ( e.g. Cu with a vacuum deposited thin Au plating).

Etch first a thin slit into the Au and then a wider one in the Cu base.


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032