Backlights

How to choose a machine vision backlight

A backlight sits behind the part and shines toward the camera. The part shows as a dark silhouette on an even, bright field, so the edge position depends on geometry, not on surface colour, texture or print. That makes backlighting the first choice for gauging, hole and gap checks, presence checks and for finding particles or bubbles in clear material. Work through these points in order.

1. Size: cover the field of view at the backlight, plus a margin

Every edge you measure must fall on uniform light. With a standard (entocentric) lens the viewing cone widens behind the part, so the area the camera sees at the backlight plane is larger than the field of view at the part. Size the backlight to that larger area and add 10-20 % on each axis: brightness falls off toward the frame of any backlight. For long webs, sheets or extrusions, a narrow strip backlight under a line of interest is often enough.

2. Colour and wavelength

In a silhouette the colour does not change the contrast much, but it changes edge sharpness, ambient rejection and what the light passes through:

• Blue (455-470 nm): the shortest visible wavelength gives the least diffraction blur at the edge, the usual choice for precise measurement with a mono camera.
• Red (625 or 660 nm): good mono-sensor response and easy to isolate from room light with a red bandpass filter.
• Infrared (730, 850 or 940 nm): passes through many dyed plastics, inks and thin papers that block visible light, and is invisible to operators. Needs a mono camera or one without an IR-cut filter.
• UV (365-405 nm): many clear plastics and coatings absorb UV, so a transparent film that disappears on a visible backlight shows up dark.
• White: for colour cameras, for example checking the colour of a liquid in a clear container.

3. Construction: standard, edge-lit, collimated or with a centre hole

• Standard (direct-lit): LEDs behind a diffuser. The most output per area; housings are commonly 18-30 mm high.
• Edge-lit: LEDs at the edge of a light-guide plate. About 7-11.5 mm thick where stated, for tight machines, with less output than a direct-lit panel of the same size.
• Collimated: near-parallel light. Paired with a telecentric lens it gives the sharpest edges on thick or round parts. See collimated lights.
• With centre hole: a round or square opening (20-80 mm where stated) so the camera can look through the panel and use it as a flat front light, or so a second light can shine through.

Some panels take a polarizing film or a light-control (parallel) film to narrow the emission angle.

4. Drive: constant, dimmable or strobed

Panels without a driver need an external light controller. Panels with an in-line driver take 24 V DC directly and dim with an analog voltage; versions with a PNP trigger input can be pulsed above their continuous rating (overcurrent strobe) to freeze moving parts. Work out the exposure the line allows with the motion blur calculator, then check that the light's shortest pulse and strobe frequency fit it.

5. Environment and mounting

IP50 suits dry cabinets; wash-down lines need IP67, IP68 or IP69K. Check the mounting (T-slot channel, tapped holes, nut grooves), the cable exit and length, and whether the connector is flying leads or M12.

What the range covers

Filter by colour: white, red, blue, infrared and UV backlights.

Typical applications

• Diameter, length and profile of turned, stamped or extruded parts, and wire or rod diameter.
• Hole count, position and size in sheet metal, gaskets and PCBs.
• Connector pin presence and straightness seen from the side.
• Fill level, bubbles and particles in clear bottles, vials and film.
• Edge and gap tracking of webs, labels and cut sheets.

Common mistakes

• Sizing the backlight to the part instead of to the field of view at the backlight plane, so the corners go dark.
• Running the panel too bright or the exposure too long. Light spills around the edge and the silhouette shrinks: the measured size then depends on intensity. Fix the brightness and exposure before calibrating.
• Measuring thick parts with a diffuse panel and a standard lens. The camera sees the side wall, not the edge; use a collimated backlight and a telecentric lens.
• Placing the panel inside the depth of field, so dust and scratches on the diffuser appear in the image. Move it further back.
• Pairing an infrared backlight with a colour camera that has an IR-cut filter.

Questions

Diffuse or collimated backlight?
Diffuse panels are tolerant of alignment and suit presence, hole and fill checks on thin parts. Collimated panels give sharper, more repeatable edges on thick, round or reflective-edged parts, and are normally paired with a telecentric lens.

How far behind the part should the backlight sit?
Far enough that its surface is out of focus, so diffuser texture and dust blur away, and close enough that the panel still covers the viewing cone. Then size the panel for that distance.

Can a backlight inspect transparent parts?
Yes. Bubbles, particles, cracks and edges in glass or clear plastic bend or scatter the light and show as dark marks. For clear film that a visible backlight cannot show, try UV.

What else do I need?
A lens and camera that resolve the edge and, for strobing or several lights, a controller. Build the system to match them, or see the lighting guide to compare backlights with other light types.