Shadowless lights

How to choose a shadowless light

A shadowless light lights the part from a broad, diffuse surface around the camera axis, with a viewing hole in the middle for the lens. Because light arrives from many directions at once, small bumps, edges and surface texture throw no hard shadows, and glare spots on shiny parts spread out into an even background. The category covers flat dome lights, square and round shadowless panels with a centre hole, low-angle and high-angle shadowless rings, arch-shaped units for cylinders and striped units. Choose by surface, size, distance and viewport.

1. Shadowless, dome or coaxial?

All three fight glare. A dome light gives the most even light on strongly curved or crumpled shiny parts, but it is tall and has to sit close to the part. A coaxial light is best for flat mirror-like surfaces. A shadowless or flat dome light sits between them: much thinner than a dome, it lights moderately curved, textured and slightly shiny parts evenly and leaves room in the machine. Our article on dome lights for shiny, curved and wrinkled parts shows where a full dome is still needed.

2. Size: the lit area must cover the field of view

Diffuse light is only even over the central part of the emitting surface. Choose a light whose emitting area is clearly larger than the field of view - for a flat dome, the lit area should be at least the field size plus a margin on every side. Square models range from about 45 x 45 mm to 342 x 246 mm of emitting surface, round ones from about 26 mm to 400 mm outside diameter, and one flat dome range is built to order from 80.4 mm to 690 mm of lit width.

3. Working distance and angle

Shadowless lights work at short distances. Where stated, the recommended working distance is 5-50 mm or 10-100 mm for most models and 100-300 mm for a few larger ones. Low-angle versions put more light at grazing incidence and bring out scratches, embossing and raised print; high-angle versions are closer to a flat dome and give a flatter, more uniform image. If the part height varies, test at both ends of the range.

4. Viewport (camera hole)

The lens looks through the hole in the middle. The hole must be larger than the lens's field cone at that height, or the corners of the image are cut off. The hole also shows as a dark spot on very shiny parts, so keep it as small as the lens allows. One flat dome range offers viewport diameters from 6 mm to 75 mm in steps; other models have a fixed inner diameter, from 10 mm on small units up to 300 mm on the largest rings.

5. Colour, power and connection

Standard colours are white, red, green, blue and infrared, with 455 nm blue, 505 nm cyan, 590 nm amber, 660 nm red, 730 nm and 940 nm infrared also offered in one range. Red, blue and white are the usual choices for mono cameras; white for colour cameras. The lights run on 24 V DC, with power from under 1 W on small units to well over 100 W on the largest flat domes. Most connect to a constant-current or strobe light controller; some flat domes have a driver built into the cable with analog dimming and a PNP trigger for strobing. Several flat dome models are sealed to IP67 or IP68 for wet and dusty areas.

What this category covers

Browse white shadowless lights or infrared shadowless lights.

Typical applications

• Reading print, date codes and laser marks on slightly curved or textured packaging.
• Checking labels, blister packs and foil lids where glare hides the print under ordinary front light.
• Inspecting PCBs and electronic assemblies for missing or misplaced parts without component shadows.
• Surface checks on machined, cast or moulded parts with a matte or brushed finish.
• Reading direct-marked 2D codes on metal parts.
• Caps, lids and other round parts under a ring shadowless light; cylinders under an arch-shaped one.

Common mistakes

• Picking a light the same size as the field of view: the edges of the image are darker than the centre.
• A viewport that is too small for the lens, so the hole vignettes the image.
• Mounting too far from the part. Diffuse light falls off quickly with distance, and the light stops being shadowless once the part no longer "sees" the whole emitting surface.
• Expecting it to replace a coaxial light on mirror-flat parts. On polished flat surfaces the camera hole reflects as a dark spot in the image.
• Running at constant full power when the part moves. Strobe it from the camera trigger instead; the image is sharper and the LEDs last longer.

Questions

What is the difference between a shadowless light and a ring light?
A ring light has a narrow ring of LEDs, often with a lens, and lights from one angle, so it can throw shadows and produce ring-shaped glare. A shadowless light spreads the light over a wide diffuse surface for an even, low-contrast image of the surface.

Is a flat dome the same as a dome light?
It gives a similar diffuse, on-axis light from a thin panel, which saves height and allows longer working distances. On strongly curved mirror-like parts a full dome is still more even.

Which colour should I start with?
Red for general inspection on mono cameras, blue for fine texture and print on metal, white for colour cameras, infrared to suppress coloured print and see through some films.

Not sure which light fits the part?
Start with the lighting guide, or configure a system with the light, lens, camera and controller together.