Tunnel lights for industrial machine vision

Tunnel light or vault light?

Classic tunnel lights guide the light around the object under test through a tunnel-shaped, diffusing geometry. They are therefore particularly suitable for elongated and cylindrical parts such as shafts, tubes, bars or rotationally symmetric components.

Vault lights combine properties of dome and bar lights. The light reaches the object over a large area and diffusely, while the camera looks at the inspection zone through a viewing window. Depending on the version, this viewing window can be linear or round.

Such lights can be useful both with area scan cameras and in line scan camera applications.

Typical advantages

Tunnel lights are used above all when even illumination matters more than strongly directional light. Typical advantages are:

• even illumination of curved and cylindrical surfaces
• reduced hard shadows
• fewer disturbing direct reflections
• large continuous illuminated areas
• good conditions for surface and completeness inspection
• depending on the model, use with area scan or line scan cameras
• various light colours and wavelengths available

How strongly reflections are actually reduced always depends on material, surface structure, camera angle and workpiece geometry as well. A tunnel light therefore does not eliminate reflections in principle, but creates more favourable and more reproducible lighting conditions in many applications.

Typical applications

Tunnel lights are used among other things for:

• surface inspection of shafts and cylindrical metal parts
• detection of scratches, cracks and flaws
• inspection of tubes, bars and rotationally symmetric components
• inspection of packaging and printed surfaces
• checking glossy or reflective workpieces
• presence and completeness checks
• AOI applications
• applications with area scan or line scan cameras

Tunnel type

The tunnel type essentially determines how the light is guided around the object under test. Classic tunnel lights are particularly suitable for elongated parts and defined transport geometries. Vault lights have a more flexible design and, depending on the model, can be used for area scan or line scan camera applications.

Illuminated area

Width and length of the illuminated area indicate which part of the object can be lit. These values should not be confused with the outer housing dimensions. For the optical selection, the illuminated area stated by the manufacturer is far more meaningful than the housing size alone.

Viewing window

With vault lights, the viewing window can be linear or round depending on the version. The geometry has to match the camera system, lens and field of view in use. If the manufacturer does not publish a clear specification for a particular model number, this feature should not be estimated from model names or drawings.

Recommended working distance

The working distance describes the distance between light and object intended by the manufacturer. Within the recommended range, the intended light distribution can usually be achieved most reliably. Working distance, illuminated area and camera field of view should therefore always be considered together.

Light color and wavelength

The suitable light colour depends on material, surface and the desired contrast. White light suits many general inspection tasks and coloured objects. Red light can strengthen certain contrasts and is frequently used with monochrome cameras. Green and blue light can offer advantages with certain materials and surface structures. Infrared light can be of interest when visible colour structures matter less or material differences become more visible in the near infrared. With monochromatic lights, the specific wavelength should be considered in addition to the colour.

Operating mode

Depending on the model, continuous light, pulsed operation or both are possible. Continuous light suits continuous inspection processes. A specified pulsed or strobe operation can be useful with fast movements and short exposure times. The manufacturer limits for pulse duration, duty cycle and electrical load that apply to the model should always be observed.

Which tunnel light suits my application?

For a first selection you should above all compare these points:

• tunnel type and lighting geometry
• required width and length of the illuminated area
• camera system and, where applicable, viewing window
• recommended working distance
• light colour respectively wavelength
• required operating mode

Electrical and mechanical details such as supply voltage, connection or housing dimensions become relevant only afterwards.

Since manufacturers name technical features differently in part, and do publish all values for every model, we only adopt clearly documented specifications. Unpublished dimensions or working distances are never estimated from model numbers or drawings. This way the available tunnel and vault lights can be compared cleanly on a technical basis, without filling gaps in manufacturer data artificially.