Using Optics to Measure Through Transparent Part

S3D_Adam · Jan 9, 2017

I occasionally get asked to measure depth through transparent parts, such as glass or plastic. For instance, someone may want to know the depth of a surface that lies on the opposite side of a 1mm thick piece of glass up to the top of the glass surface.

Does anyone know if it is possible to do this optically? Can the focus finder on an OMM be used to do it? Would a laser interferometer work? From what I've read, it sounds like it should work as long as the camera or laser is perpendicular to the glass. If not, the light will refract and the distance will be inaccurate. Is this correct?

Gordon B. Clarke · Jan 9, 2017

AWY said:
I occasionally get asked to measure depth through transparent parts, such as glass or plastic. For instance, someone may want to know the depth of a surface that lies on the opposite side of a 1mm thick piece of glass up to the top of the glass surface.

Does anyone know if it is possible to do this optically? Can the focus finder on an OMM be used to do it? Would a laser interferometer work? From what I've read, it sounds like it should work as long as the camera or laser is perpendicular to the glass. If not, the light will refract and the distance will be inaccurate. Is this correct?

And it isn't possible to remove the glass? I can't begin to imagine what it is you're trying to measure unless the glass is glued in position.

I think it might be helpful to know what accuracy and length/depth you want or need to achieve.

Maybe relevant

I've just been fined for speeding. 75 km/h in a 60 km zone. $350.

PeteM · Jan 9, 2017

There are optical depth micrometers that might work for this, after calibration.

The way they work is with a fairly high power and very shallow depth of field microscope. Depth of field may be on the order of .001" In normal use you focus on one surface (e.g. a gouge in a part) and then on another (e.g. the normal surface) and read out the difference directly. It's reasonably accurate.

You might have to mess around a bit to get the most accurate measurement through various transparent materials; but it should be possilble.

CalG · Jan 9, 2017

Optical Path Length

AWY said:
I occasionally get asked to measure depth through transparent parts, such as glass or plastic. For instance, someone may want to know the depth of a surface that lies on the opposite side of a 1mm thick piece of glass up to the top of the glass surface.

Does anyone know if it is possible to do this optically? Can the focus finder on an OMM be used to do it? Would a laser interferometer work? From what I've read, it sounds like it should work as long as the camera or laser is perpendicular to the glass. If not, the light will refract and the distance will be inaccurate. Is this correct?

Measurement of distance though a transmitting medium is a product of linear distance and the medium's REFRACTIVE INDEX. Regardless of angle. (though any angle will change the path length and MAY change the refractive index.

By convention, Air has a refractive index of 1 (with vacuum very near that). Glass has a refractive index of 1.52 (or more) Many plastics have a refractive index higher than float glass. Some approaching 2. Water has a refractive index less than glass.

Optical (interferometric) measurements could be made, If the "window" were well characterized, or the measurement device could be zeroed with only the window in place.

jCandlish · Jan 9, 2017

Snell's Law is the killer here.

You need a way to make the change in refraction cancel out. Either by including a feature in the covering window that can be easily optically measured for thickness (angular displacement) or by measuring the difference between the reflection and refraction at the boundry of the covering window and the object beneath it.

You might be able to pick up a peak difference bewteen reflection and refraction by choosing an ideal monochromatic light and filling the gap that you want to measure with a known fluid or gas.

Adapt the principle of the Fabry–Perot interferometer. Your cover glass and the material between it and the gap you want to measure becomes your Etalon.

All of the ways that I can imagine to do this measurement require well structured light.

Good Luck.

tomato coupe · Jan 10, 2017

CalG said:
Measurement of distance though a transmitting medium is a product of linear distance and the medium's REFRACTIVE INDEX. Regardless of angle. (though any angle will change the path length and MAY change the refractive index.

By convention, Air has a refractive index of 1 (with vacuum very near that). Glass has a refractive index of 1.52 (or more) Many plastics have a refractive index higher than float glass. Some approaching 2. Water has a refractive index less than glass.

Optical (interferometric) measurements could be made, If the "window" were well characterized, or the measurement device could be zeroed with only the window in place.

I think you have a typo. The bold section should read:

"Vacuum has a refractive index of 1 (with air very near that)."

S3D_Adam · Jan 11, 2017

Thanks for the input, everyone! It sounds like I need to do some more reading on optical subjects!

CalG, are you saying that if I were to measure distance through a piece of glass (constant refractive index = 1.52) with an OMM, the measurement would be larger than it should be?

CarbideBob · Jan 11, 2017

What is the surface to be measured made out of?
Say a steel part with a glass top can be checked with a structured light or laser line projected on a angle.
Center the projection and measure the z with the slides position, solve the angle.
Measuring the glass itself is harder. Not sure if you a are measuring a part through glass or measuring a glass part.
If measuring a part how would you measure Z heights with no glass?
Bob

billzweig · Jan 13, 2017

If the surface under the glass is metallic, one could calibrate an inductive proximity sensor. What is needed is a sensor that provides an output according to distance, not an on/off device.
Inductive sensor will provide an accurate reading to about 30mm and is not influenced by the glass or plastic.

jdgoguen · Jan 15, 2017

If a microscope with a calibrated z-axis is focused on the top of the glass and then on bottom through the glass layer, the thickness can be calculated as the distance the objective was moved multiplied by the refractive index. This is an old technique for measuring refractive index when the thickness of the glass can be readily determined. It gives better accuracy with monochromatic light.

Jon

CalG · Jan 15, 2017

AWY said:
Thanks for the input, everyone! It sounds like I need to do some more reading on optical subjects!

CalG, are you saying that if I were to measure distance through a piece of glass (constant refractive index = 1.52) with an OMM, the measurement would be larger than it should be?

Yes. The "distance" light travels is "longer", that is, it takes more time to travel through the material thickness than it would take to travel the same distance through vacuum or air.

S3D_Adam · Jan 20, 2017

Thanks CalG and jdgoguen, that is exactly what I wanted to know!

Billzweig, unfortunately I'm measuring glass through more layers of glass, so the inductive sensor won't work. It is an interesting idea that I had never new about before, though!

AA+ · Jan 21, 2017

The index of refraction of a transparent object makes the observed distance through it shorter by 1/(index-of-refraction)

Best wishes --- Allen

CalG · Jan 21, 2017

further reading
Optical path length - Wikipedia

specfab · Jan 21, 2017

Regarding the possibility of using laser interferometer for measuring to the far side of thin glass: This is a classic limitation of laser interferometry; as the coherence length of a laser-based interferometer is quite long typically (unless special design has been employed to limit that length) the closer two surfaces are to each other, the more difficult it is to separate the fringe signatures from each surface, assuming the two surfaces are generally identical in characteristics.

Discostu · Jan 25, 2017

I used to use one of these to check we had the right bullet proof glass installed on site. Air gaps show up on it too.
Merlin Lazer Glass Measurement Gauge - Glass Testing & Measurement - Merlin Lazer Ltd

hopefully will work for your job.

deadlykitten · Jan 26, 2017

AWY said:
I occasionally get asked to measure depth through transparent parts, such as glass or plastic. For instance, someone may want to know the depth of a surface that lies on the opposite side of a 1mm thick piece of glass up to the top of the glass surface

put some water inside one part ...
put something strong inside another one ...
mix
drink

cactusbastard · Jan 27, 2017

Gordon B. Clarke said:
Maybe relevant I've just been fined for speeding. 75 km/h in a 60 km zone. $350.

You're lucky they didn't notice you were posting on your phone at the same time!

tomato coupe · Jan 27, 2017

specfab said:
Regarding the possibility of using laser interferometer for measuring to the far side of thin glass: This is a classic limitation of laser interferometry; as the coherence length of a laser-based interferometer is quite long typically (unless special design has been employed to limit that length) the closer two surfaces are to each other, the more difficult it is to separate the fringe signatures from each surface, assuming the two surfaces are generally identical in characteristics.

The coherence length of a laser can't ever be "too long" for a measurement, it can only be "too short" for a measurement. Longer coherence length is always a good thing -- it means the laser is more monochromatic.

On edit: In re-reading your post, I realized you were referring to the "coherence length of the interferometer", not the coherence length of the laser. Unfortunately, that term has no meaning. What property of the interferometer are you referring to?

Using Optics to Measure Through Transparent Part

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