Back light
The light sits behind the product, facing the camera. The product shows as a sharp dark outline. Use it for counts, outlines, gaps, holes, and fill level in clear or translucent bottles.
Guides
Lighting and optics for inspection
How the angle, spread, and color of the light decide what a camera can see, and which lens keeps a measurement steady.
Why the light comes first
A camera records only the light that reaches it. A scratch, a dent, or an embossed code shows up when the light is placed so that the fault sends light toward the lens differently from the good surface around it. Station lights are strobed and shielded, so the picture stays the same whatever the room lights are doing.
Four ways to place the light
Bright field
The light bounces off a flat surface straight into the lens. Smooth areas look bright, and print or a mark that breaks the surface looks dark. Use it for print on flat cartons and codes on labels.
Dark field
Low-angle light skims the surface. A flat surface reflects it away from the lens and looks dark, while scratches, chips, and raised or stamped characters catch it and glow. Use it for embossed lot codes and marks on glossy parts.
Diffuse
A dome or an on-axis light sends light from many directions at once, which removes glare and shadows. Use it on shiny or curved surfaces: foil lids, blister film, metal cans, and wrapped product.
Directed light, a narrow beam from one side, does the opposite of diffuse light. It casts shadows and makes texture stand out. Pick directed light when texture is the fault, and diffuse light when texture hides the fault.
Choosing the light by the fault
Fill level in a clear or translucent bottle
Light that usually shows itBack light
Why it worksLiquid and headspace pass different amounts of light.
Missing piece in a tray
Light that usually shows itBack light, or diffuse light from above
Why it worksEach piece shows as its own outline.
Embossed or laser-etched lot code
Light that usually shows itLow-angle dark field
Why it worksRaised and cut edges catch light that skims the surface.
Scratch or dent on a glossy part
Light that usually shows itDark field
Why it worksThe flat surface stays dark and the mark lights up.
Print on foil, film, or a curved can
Light that usually shows itDiffuse dome or on-axis light
Why it worksGlare and hot spots disappear, so the print reads evenly.
Wrong label or flavor color
Light that usually shows itWhite light with a color camera
Why it worksThe check needs the true color.
Faint print on colored packaging
Light that usually shows itColored light chosen to darken the print
Why it worksLight of the opposite color makes the print look dark. Light of the same color makes it fade.
Product seen through printed or tinted film
Light that usually shows itInfrared light
Why it worksSome inks and dyes let infrared pass. Test it on samples first, because infrared is invisible to the eye.
Glue, coating, or residue
Light that usually shows itUltraviolet light
Why it worksMaterials that fluoresce glow against a dark background.
Try the light on real good and bad samples before settling the design. A small change in angle can turn a clear fault into a faint one.
Lenses
01
Fixed focal length.
Most line checks use it. It focuses from close range out to far away, and its aperture trades light for depth of field. Objects farther away look smaller, so a part that rides higher or lower changes its apparent size.
02
Macro.
For a small view at close range, such as a printed code a few millimeters across or the tip of a needle.
03
Telecentric.
Keeps the part the same size across a range of heights and shows straight sides without perspective, which makes it the usual choice for gauging. The front of the lens has to be at least as wide as the view, so it is larger and costs more than a standard lens.
04
Zoom.
Rare on production lines. The extra ring can move, which shifts the calibration. A fixed lens locked in place keeps measurements repeatable.
To size a lens for a view, use the imaging calculator. For the rest of the system, see How machine vision works.