DIY large aperture optical comparator

[Stormer]

Hi. In machine shops optical comparators have been used for a really long time to measure and inspect complex and usually small parts. But they are usually for small parts and they are usually really expensive machines. How can i do this for cheap for larger parts like for example for making form fitting foam inserts for cases? I know you can simply take a photo of the object against a uniform background but the trouble with that is perspective. Where closer objects gets larger, and objects further away get smaller. And this problem get worse the closer the camera is to the object being photographed, and it is not very practical to take photos really far away with telephoto lenses just to get rid of this effect. While a optical comparator collimates the light so there is no optical perspective problem.

Can this be done at a low cost with for example some Fresnell lenses and a LED light?

This is how it is traditionally done:

 

[Baluncore]

The optical comparator can be used where the item being measured is smaller than the optical collimation lenses employed. For articles that are too large for that, you must use a 2D measuring machine.

 

[Tom.G]

The optical comparator can be used where the item being measured is smaller than the optical collimation lenses employed. For articles that are too large for that, you must use a 2D measuring machine.

Or possibly add some optics between the object and the comparator to create a smaller image. However you still have the same perspective problem with the new optics vis-a-vis the workpiece.

In general, even comparators do best when measuring distances on a plane. I have seen installations where the comparator projection screen that you are viewing has the outline of the part drawn on it. This can compensate for the perspective problem of 3D parts.

And to decrease the system cost, you can use a video camera, a large display, and the camera lens can be whatever suits the application. And because no lenses are perfect, you then have to calibrate the system.

Cheers,
Tom

 

[Baluncore]

The magnification of an optical comparator increases the effective resolution of the human eye to about 1 μm. The optical comparator is then equivalent to a go/no-go gauge.
For the accurate measurement of large objects, stable temperatures are more important than magnification.