In vision, the camera is the part everybody shops for, but the light is what makes or breaks the inspection. The same scratch can be invisible under one light and glowing like a neon sign under another. So instead of starting with the light, start with what you're trying to see.
"I need to measure it, or check holes and edges."
Backlight. Put the light behind the part, facing the camera, and you get a crisp black silhouette on a white background. Perfect for outer dimensions, hole presence, gap checks and edge positions. It won't show you anything on the surface, though, because you're only seeing the outline.
"It's shiny and curved, and the glare is killing me."
Dome light, sometimes called a cloudy-day light. It surrounds the part with soft, even light from every direction, so there's no single bright reflection to blind the camera. Great for reading print on cans, foil packs, blister packs, and anything that looks like a fun-house mirror under a regular light.
"I need to find scratches, dents, or raised or engraved text."
Low-angle light, also called darkfield. The light comes in nearly flat, skimming across the surface. A smooth surface stays dark, but anything that sticks up or digs in catches the light and shows up bright. Scratches, embossed codes, stamped characters and surface dents pop right out.
"It's flat and shiny, like a laser mark on metal."
Coaxial light. It shines down along the same line the camera looks through, using a little beam splitter. Flat, mirror-like surfaces come back bright and even, and anything that breaks up the surface, like a scratch, a laser-etched mark or a smudge, shows up dark against it.
"It's a plain matte part and I just need to see features."
A regular ring light or bar light pointed at the part (bright field) is often all you need. It's simple and cheap. Just watch out for hot spots the moment anything shiny shows up.
"I need to know how tall something is, or see a bead of glue in 3D."
That's where laser line or structured light comes in. It projects a line or pattern, and the camera reads how it bends over the part to work out height. Handy for gap and flush, bead inspection, or anything where the problem is shape, not color.
Color is a sneaky good tool, too. Light a part with the same color as a feature and that feature washes out. Light it with the opposite color and the feature goes dark and pops. Say you need to check blue print on a white label: red light makes the blue print go nearly black. Infrared can help you ignore color differences and some inks altogether, and UV light makes fluorescent glues, inks and marks glow so they're easy to spot.
A couple of extra tricks. Polarizers on the light and the lens cut glare on plastics and glossy surfaces. A filter on the lens that matches your light's color, plus a shroud around the inspection area, keeps sunlight and overhead lights from messing with your image when someone opens a door.
If you're not sure which light to use, try several on real parts before committing. Swapping lights on a bench for an hour beats fighting the wrong light on the line for a month.