Machine vision lighting guide: which light shows which defect

Why lighting decides what a camera can see

A defect can only be found if it sends the camera a different amount or colour of light than the good surface around it. That difference, contrast, is set mostly by the lighting, not by the camera or the software.

A smooth, shiny surface reflects light like a mirror: the light leaves at the same angle it arrived (HyperPhysics, reflection). If the light sits where that reflection enters the lens, the flat surface looks bright; this is bright-field lighting. If the light comes in at a low angle, the reflection misses the lens and the flat surface looks dark, while scratches, edges and dust scatter some light upwards and look bright; this is dark-field lighting (MicroscopyU, darkfield). Matte surfaces scatter light in all directions and depend less on the angle.

We choose lighting on your samples during the sample test, keeping the geometry that shows the defect consistently. The sample preparation guide lists what to send.

Six lighting geometries and what each reveals

Backlight

The light sits behind the part, facing the camera. Opaque parts appear as dark silhouettes, which makes outlines, holes, gaps, flash, short shots, presence and position easy to measure. It shows nothing of the top surface. Clear parts let the light through, so backlight also shows fill level, particles and bubbles in clear containers. You need access behind the part, and dust on the light shows up as a false defect.

Dome and other diffuse light

A dome surrounds the part with even light from nearly every direction, like an overcast sky. Reflections from shiny, curved or crinkled surfaces merge into an even background, so print, codes and marks can be read on cans, foil, blister packs and bottles. The dome must sit close to the part, the camera hole can show as a dark spot on flat mirror-like parts, and because diffuse light hides relief it also hides shallow scratches and dents.

Dark-field or low-angle light

Light arrives close to the surface from a ring or bars around the part. Flat areas stay dark; scratches, dents, embossed or engraved text, dot-peen codes, burrs and edges catch the light and glow. Height and tilt must be consistent, dust glows too, and a single bar shows features running across its light better than along it. See surface defect detection for how this applies to scratches, dents and moulding faults.

Coaxial light

A half-mirror, the beam splitter, sends light down along the camera's own axis. A flat, polished face square to the camera reflects it straight back and looks bright; scratches, pits and stains scatter it and look dark. Curved or tilted areas turn dark, and the beam splitter wastes much of the light.

Bright-field ring or bar light

A ring around the lens or bars above the part shine down steeply. It is the simplest, brightest option and suits matte parts: printed cartons, paper labels, presence and colour. On glossy parts it creates hot spots, reflections of the light itself; moving the light off the camera axis often moves them out of view (NI lighting guide).

Line light for line-scan cameras

A line-scan camera builds the image one line at a time as the product moves, for webs, film, sheet and rotating cylinders. It needs a narrow, bright line of light that is even along its whole length, because each line is exposed only briefly. Light and scan line must stay aligned, and an encoder keeps the image scale correct when speed changes.

Shiny, clear and curved parts

• Shiny and flat: coaxial light to see marks on the face, dark-field to see scratches and relief.
• Shiny and curved: a dome or other diffuse light, so the curve does not reflect a bright image of the light.
• Clear: backlight for fill level and outline; dark-field for cracks and chips. The camera sees through the part, so keep the background plain and still.
• Dark or black: these parts return little light; low-angle light brings out texture and relief.

Polarising filters help with glare. Light reflected from many surfaces is partly polarised (HyperPhysics, polarisation by reflection), so a polariser on the light and a crossed polariser on the lens can suppress glare while keeping surface detail. They cost light, and changing the geometry often works better (NI lighting guide).

Ambient light and shrouds

Daylight, overhead lights and neighbouring stations add light the recipe was not set up for, and it changes through the day. The NI guide describes three remedies: strobing the station light brightly with short exposures, enclosing the inspection area, and fitting a filter on the lens that passes only the station light (NI lighting guide). A shroud also shields operators from flashing light; design it with cleaning access in mind. The guide to reducing false rejects covers what happens when lighting drifts.

Colour of the light

With a monochrome camera, the colour of the light is a contrast tool. Light of the same colour as a feature makes it look bright; light of the opposite colour on the colour wheel makes it look dark (NI lighting guide). Red light, for example, makes red print fade and darkens blue or green print.

Beyond visible light, infrared passes through some plastics better than shorter wavelengths and can make colour differences in print fade, while blue light suits shallow features such as laser etching on black rubber. Ultraviolet makes fluorescent inks and dyes glow (NI lighting guide). For colour checks, use a colour camera with stable white light. Whether a colour helps is checked on your parts.

Strobing

A strobed light switches on only for the exposure, triggered with the camera. With a short exposure it freezes moving products and can outshine ambient light (NI lighting guide). Light and exposure timing must match, and pulse limits come from the light and controller datasheets.

Decision table

A starting point; the final choice is made on your samples.