Ring lights

How to choose a machine vision ring light

A ring light mounts around the lens or just above the part and lights it from all sides at once, so shadows cancel and the light stays centred on the camera axis. It is the usual starting light for a new inspection. The choices that matter are the illumination angle, the working distance, the diameter and the colour.

1. Illumination angle

Ring light angles are given between the light ray and the horizontal plane of the part. The angle decides whether the part looks bright or dark:

• 60-90° (high angle, bright field): light comes down close to the camera axis. Matte surfaces, print and labels look evenly lit. On shiny parts the LEDs reflect as a ring of hot spots.
• 30-45° (medium angle): a compromise that lights the face of a part and still shows some relief.
• 0-15° (low angle, dark field): light skims across the surface. Flat areas stay dark and edges, scratches, engraving, embossed characters and chips light up.

Lensed rings narrow the beam (for example 14-32°) to throw more light at a longer distance; a diffuser plate, flat or cup-shaped where offered, softens the output on semi-gloss parts.

2. Working distance and diameter

The angle only works at the distance it was designed for. As a guide, one of the ranges here lists these working distances by angle:

Check the stated best working distance of the model you pick. For a bright-field ring, a larger diameter at the same distance gives a softer, more even light; for a low-angle ring, the inner diameter must be larger than the part so the light reaches its edges. If the ring mounts on the lens, the inner diameter must clear the lens body and its field of view.

3. Colour

• Red (625 or 660 nm): general use with mono cameras; a red filter on the lens blocks room light.
• Blue (455-470 nm): more contrast on metal surfaces and fine detail.
• Green: strong contrast for red or brown marks, such as solder or rust, on a mono camera.
• Infrared (730, 850 or 940 nm): hides printed colour, passes through some plastics, invisible to operators.
• UV (365-405 nm): makes fluorescent marks, glue and coatings visible.
• White: for colour cameras and colour checks.

4. Glare and shiny parts

A ring light on a flat shiny part produces a reflected ring of LEDs. Options: tilt to a lower angle so the reflection misses the lens, use a polarized ring with a polarizer on the lens, or change the light type. Flat mirror-like parts suit a coaxial light; curved or crinkled shiny parts suit a dome light.

5. Drive and connection

Most rings run on 24 V DC through a light controller. Rings with an internal or in-line driver dim with an analog voltage, and versions with a trigger input can be pulsed above their continuous rating to freeze moving parts. Check the connector (flying leads, M12 or a small 3-pin plug) and, for wet areas, an IP65 to IP69K rating.

What the range covers

Common sizes: 50 mm, 70 mm, 90 mm, 120 mm and 150 mm ring lights. By colour: white, red, infrared and UV ring lights.

Typical applications

• Print, label and date-code checks, and code reading on matte surfaces (high angle).
• Engraved or stamped characters, chipped edges and burrs (low angle).
• Presence and position of small parts, screws and components on PCBs.
• Colour and assembly checks with white light and a colour camera.

Common mistakes

• Choosing by diameter alone. A 90° ring and a 0° ring of the same size produce opposite images.
• Mounting a low-angle ring too high. Above its working distance it acts as a medium-angle light and the dark-field effect is lost.
• Using a high-angle ring on polished parts and fighting the reflected LED ring in software.
• Letting the inner diameter clip the lens's field of view, which shades the image corners.
• Running long exposures on moving parts instead of strobing; check the time with the motion blur calculator.

Questions

Ring light or dome light?
A ring light is directional and leaves texture visible; it suits matte parts and, at a low angle, surface relief. A dome light is diffuse from every direction and removes glare on curved shiny parts.

What does the angle number mean?
It is the angle between the light ray and the part plane. 90° shines straight down; 0° shines across the surface.

Do I need a diffuser?
On semi-gloss parts or at short distance a diffuser hides individual LED spots. On matte parts at a longer distance a clear or lensed ring gives more light.

How do I match it to the lens?
Start from the field of view and working distance set by the lens focal length, then configure the system to pick a ring that fits that distance.