What do I need to take better microscope images for optics inspection?

[Hootis Tigglebits]

I have a cheap digital microscope with a C-mount lens.  95% of the time it's used for reworking PCBs, but sometimes my employer has me inspect small lenses for imperfections.  We have an older stereo microscope (the lens on it says Nikon 5x and the eyepieces say 10x/20 and it has a fiber optic ring light.) that can easily see all kinds of imperfections which my digital one does not show.  I know I need different lighting (see attached images), but what kind?  I believe I also need a different power lens (my current one says it's 130X Magnification) because we have an older stereo microscope (the lens on it says Nikon 5x and the eyepieces say 10x/20) that can easily see all kinds of imperfections which my digital one does not show.  How do I know what type of lens I need?

What about this 8.3mp scope?  https://amscope.com/products/c-hhd-aarm/?variant=/43603360809135

[jfiresto]

What diameter are the lenses? What imperfections are you trying to spot and what size(s)?

[Hootis Tigglebits]

I don’t have the exact measurements but the diameter of the lenses are about 6mm and the imperfections we look for are scratches which are thinner than a human hair and cannot be seen under a magnifying glass. 

[PlainName]

I think you want a single source light (that is, not a multi-directional ring light but more towards the laser end) which you can shine at different angles across the lens. A pure white light, such as from the Nikon, will be refracted by the scratch and make it easier to spot. LED ring lights tend to not have decent light in that respect.

Got a hazy memory that a polarizer can be used - the scratch twists the light so using two sheets of polarizer, with the lens in the middle, can make the light viewed through it dark, except where the scratch lets it shine through.

[jfiresto]

I don’t have the exact measurements but the diameter of the lenses are about 6mm and the imperfections we look for are scratches which are thinner than a human hair and cannot be seen under a magnifying glass.

I can not determine the required resolution of your inspection system from what you have written. Is that proprietary information?

I expect a stereo microscope would trump a monocular, video microscope to distinguish contaminants from scratches.

I am with PlainName that using the right lighting will be a big win. I think transmitted or reflected dark field illumination would be the bee's knees for inspecting lenses. This page includes some LED (reflected) dark field ring lights. Originally, they were all fiber optic.