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Subject:
Lighting for surface defect detection *PIC*

John K Jordan
>>>One problem that seems to be creeping up on me is my eyesight. Having to work harder to compensate for it. Been upgrading the lighting. Any suggestions? It seems a really strong point source would be good for detecting those last scratches. Actually, I think the sun is best, but then there is that "night" thing.

I can see scratches best with small light sources rather than broad, diffuse lighting. Indirect light is the worst and a wide array of overhead fluorescent lights is the next worst. The diffuse light "fills in" scratches with light making them hard to see or invisible. Bright lighting coming from near the eyes, such as over the shoulders, is also horrible.

This first hit home with almost 50 years ago when exploring caves. When I used a headlamp I simply couldn't detect unevenness on the cave floor and I stumbled too much - the bumps and crevasses disappeared. A hand-held light created shadows on the floor making it far easier to walk.

Far better are several small diameter task lights positioned so light is at an angle across the work. Small lights are sometimes referred to as "point sources." I find this especially useful on the lathe and work with multiple task lights on gooseneck or positionable supports. This shows some at my primary lathe:

Overall these lights make it easy to work but coming from different angles not only the scratches but the compound curves are easier to see and judge. If working on a bench or in the middle of the room, I might use a bright flashlight held so the light glances across the surface to check for scratches and other defects.

Another thing I do is wipe a bit of naptha on a surface which can highlight sanding defects. Naptha dries extremely quickly and doesn't raise the grain or seem to affect anything else such as finishes. I keep some handy in a squeeze bottle.

JKJ

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