Bar lights

How to choose a machine vision bar light

A bar light is a straight row of LEDs in a narrow housing. It is the most flexible light in a vision cell: one bar at a steep angle acts as a bright directional front light, one at a shallow angle grazes the surface to raise texture, two or four bars around the part make a square ring of any size, and a long bar lights a whole conveyor width. Choose by geometry first, then optics, colour and drive.

1. Length

The lit length should exceed the field of view along the bar by a margin of about 10-20 % at each end, because output falls off near the end caps. For several bars around a part, size each bar to the side of the field it lights. Long bars over a conveyor must cover the full belt width the camera sees, including part position tolerance.

2. Angle and distance: bright field or dark field

Mount angle decides what the camera sees:

• Steep (bright field): the bar sits near the camera axis. Flat, matte surfaces look evenly bright; good for print, labels and colour checks.
• Low angle (dark field): the bar sits low and to the side, 10-30° above the surface. Flat areas stay dark and scratches, embossing, dents and edges light up. Used for engraved codes, raised text and surface defects.

Working distance matters too: output per area drops with distance, and a bar placed far away lights a wider strip less evenly. Where a working range is stated it is typically 30-500 mm.

3. Beam angle: lensed or diffused

Many bars take a lens over the LEDs. A narrow beam (10-20°) puts more light on a strip far from the bar; a wide beam (36-55°) lights a broader area close to the bar. Some bars combine two beams (45° plus 15°). A diffused bar without lenses has a softer, more even output that reduces hot spots on semi-gloss parts. Pick the beam from the distance and the width of strip you need to light.

4. Colour

• Red (625 or 660 nm): the default for mono cameras; easy to separate from room light with a filter.
• Blue (455-470 nm): more contrast on metal and for fine scratches; sharper at high magnification.
• Green, cyan, amber: pick the colour that darkens the feature: light of the opposite colour makes a coloured mark look dark on a mono camera.
• Infrared (850 or 940 nm): suppresses printed colour and passes through some plastics; invisible to operators.
• UV (365-405 nm): makes fluorescent inks, glues and coatings glow.
• White: for colour cameras.

5. Drive, housing and wiring

Bars without a driver need a light controller. Bars with an internal or in-line driver run on 24 V DC, dim with an analog voltage and, with a PNP or NPN trigger input, can be pulsed above their continuous rating to freeze moving parts. Some bars can be daisy-chained so several share one cable run. For wet or wash-down areas choose IP65 to IP69K and, where offered, a stainless steel housing; IP50 is for dry cabinets. A few models are rated risk group 2 under IEC 62471: check the rating and shield operators from the direct beam.

What the range covers

Browse by colour: white, red, blue, green, infrared and UV bar lights.

Typical applications

• Engraved, stamped or laser-etched codes and characters, lit at a low angle.
• Scratches, dents and flatness defects on machined or painted surfaces.
• Label presence, position and print on boxes and bottles passing on a conveyor.
• Two or four bars as a large, adjustable ring light for parts too big for a ring light.
• Fluorescent glue bead and coating checks with UV bars.

Common mistakes

• Choosing a bar only as long as the part. The ends of the bar are dimmer, so the edges of the field come out darker.
• Mounting the bar where its mirror reflection lands in the lens on shiny parts. Move it so the specular reflection misses the camera, or use a polarized bar with a polarizer on the lens.
• Using a narrow beam at short distance: the light lands as a thin stripe instead of an even area.
• Expecting one bar to light a curved or crinkled shiny part evenly. That needs a dome light.
• Wiring a bar without a built-in driver to a plain power supply when its data sheet calls for a controller. Check the drive type on the product page before wiring.

Questions

Bar light or line scan light?
Bar lights light an area for area-scan cameras and are general purpose. Line scan lights focus a high-intensity line onto the narrow strip a line scan camera reads at high line rates.

How many bars do I need?
One for directional or low-angle light from one side. Two opposite bars balance shadows. Four bars around the part behave like a square ring light with adjustable angle.

Can I strobe a bar light?
Yes: bars with a trigger input strobe from the camera or sensor signal; others strobe through a strobing controller. Check the strobe frequency and shortest pulse against the line speed.

Where do I start?
Pick the light type in the lighting guide, then configure the system so camera, lens, light and controller fit together.