Multi wavelength light: sequence colours to get more from one camera

Choosing one light colour is a compromise when a line runs products of different colours, or when the defect you need shows in one colour and the feature you measure shows in another. A multi-wavelength light carries several LED colours in one housing, each switched separately, so the station can change its contrast in software. Fired in sequence, the channels give several images of one part from one camera. This guide covers both uses, the arithmetic of timing, and the calibration that makes the images comparable.

What a multi-wavelength light is

The LEDs of each colour are wired as a separate channel, so a multi-channel controller can light any one of them, any combination, or step through them. Typical sets are red, green, blue and white, sometimes with infrared or UV, in the same geometries as single-colour lights: backlights, bar lights, ring lights and domes. For example, a backlight with red, green, blue and white channels:

And a bar light for low-angle and directional lighting from the same family:

The basic rule of colour contrast still applies: a feature looks bright under light of its own colour and dark under its complementary colour. Choosing that colour is covered in monochrome colour sorting under changing ambient light. A multi-wavelength light lets you make that choice per product instead of once per machine.

Use 1: recipe switching

On a line that runs red, blue and white caps, a single colour of light gives poor contrast for at least one of them. With a multi-wavelength light, the recipe for each product selects the channel that gives the best contrast, and the PLC or vision software sends the command at changeover. No light swap, no re-alignment, and the camera, lens and mounting stay the same.

During commissioning, image each product under every channel and note the grey levels of the feature and its background. The best channel is the one with the largest difference between the two relative to the image noise, not simply the brightest image. Store the channel, its intensity and the camera exposure together in the product recipe, so that one changeover command sets all three.

Use 2: sequential capture

Fire the channels one after another with one camera frame each, and you get several images of the same part, each under a different colour. Three things become possible.

Full-resolution colour from a mono camera

A colour camera has a Bayer filter: each pixel sees only red, green or blue, and two of the three values at every pixel are interpolated from neighbours. The filter also absorbs roughly two thirds of white light. A monochrome camera under sequential red, green and blue light measures all three colours at every pixel, with no interpolation and no filter loss. Colour edges stay sharp, which matters for print registration and small colour defects.

Ratio images

Dividing one channel's image by another's cancels effects that are the same at all wavelengths: uneven illumination, curvature, shading from the part's shape. What remains is the colour difference itself:

On a curved, matte part, a stain that is hard to threshold in any single image often stands out clearly in the ratio, because the curvature shading divides out.

Multispectral checks

With more channels, each pixel gets a short spectrum. This ring light covers eight wavelengths from UV to near-infrared:

Its channels run from violet at 400 nm to near-infrared at 856 nm. Eight points per pixel are enough to separate inks, coatings, ripeness grades or contaminants that look the same in RGB, without the cost and data load of a hyperspectral camera. A common route is to find the useful wavelengths with a hyperspectral camera in the lab, then run them on a multi-wavelength light in production; see how hyperspectral imaging works.

The timing cost

Each channel needs its own camera frame, so a sequence takes:

A worked example

A mono camera runs at 100 frames per second with 0.1 mm pixels on the part.

1. Three channels (R, G, B): frames at 0, 10 and 20 ms, a sequence of 20 ms between first and last.
2. At a conveyor speed of 0.1 m/s, the part moves 0.1 × 20 = 2 mm between the first and last image: 20 pixels. The colour images do not line up.
3. Options: stop the part for the sequence; raise the frame rate with a smaller region of interest (at 1,000 frames per second the shift drops to 2 pixels); or register the images in software using part features.
4. With eight channels at 50 frames per second, one sequence takes 160 ms, so the station can handle at most about six parts per second.

Sequencing suits indexed lines, robot-presented parts and stop-and-go stations. For continuous fast motion, a colour camera, or one carefully chosen channel, is usually better.

Controllers and synchronisation

A sequencing controller steps through its channels on each trigger and tells the camera when to expose, so light and image stay in step without software timing. This eight-channel unit lights CH1 to CH8 in turn and outputs a synchronized camera signal:

It also pairs with zone-sequencing lights, where the channels are directions rather than colours. Lighting a part from several directions in turn gives a set of images for shape-from-shading and surface defect checks:

For line scan cameras, alternating two channels on alternate lines is the equivalent technique; it is described in the strobe and overdrive guide.

Balancing the channels

Each channel differs in LED output, and the sensor's sensitivity differs by wavelength. Unbalanced channels make ratio images and colour reconstruction wrong. Calibrate once per recipe:

1. Place a white reference target in the field.
2. For each channel, adjust the controller level or the camera exposure until the reference reads the same grey level, for example 200 of 255.
3. Capture a dark frame with all channels off.
4. Store the levels, exposures and dark frame with the recipe, and repeat when lights are replaced or after a warm-up check shows drift.

When a single colour is enough

If one colour gives clear contrast for every product on the line, a single-colour light is simpler, brighter for the same size and needs no sequence. Multi-wavelength lights earn their place when products change colour, when one image cannot show everything, or when you need colour or spectral information at full resolution from a monochrome camera.

Next step

Browse multi-wavelength and sequencing lights in the special lights, backlights and bar lights ranges, pick a multi-channel unit from the light controllers, or build the station in the configurator. More lighting articles are on the lighting topic page.