Line scan lights

How to choose a line scan light

A line scan camera builds its image one row of pixels at a time while the web or part moves past. It needs light only on a narrow stripe, but that stripe must be very bright, because each line is exposed for microseconds, and very even along its length, because any dip shows up as a streak through every image. A line scan light has a long, narrow emitting window, often with a lens that focuses the LEDs into a thin, intense line.

1. Length: cover the scan line plus margin

The lit length must be longer than the field of view the camera sees across the web. Brightness falls off over the last few centimetres at each end, so add margin on both sides - roughly 10% of the field width, or more when the light sits far from the part. Lengths here run from short units of under 100 mm for narrow parts and lab setups to bars of 2 m and more for full-width web inspection; several ranges come in fixed steps (for example 75 mm or 150 mm increments), so you can match the web width closely.

2. Brightness: work back from the line rate

The exposure time of a line scan camera is at most the line period: at 50 kHz a line has 20 µs. Use the line rate calculator to get the line rate from web speed and pixel size, then pick a light that gives enough signal in that time. Focused (condensing) versions put the LED output into a narrow line and give the highest brightness at their focal distance; diffuse versions spread the light and tolerate height changes better. Electrical power in this category runs from a few watts for mini lights to several hundred watts for long high-power bars, and the highest-power models are fan or water cooled.

3. Lighting geometry

• Bright field (specular): light and camera at equal and opposite angles to the surface. Shiny surfaces look bright; scratches and dents look dark.
• Dark field (oblique): light at a low angle and camera looking straight down. A flat surface stays dark and raised or broken features scatter light into the lens.
• Coaxial line light: the camera looks through a half-mirror along the light axis. For flat, mirror-like webs such as foil, glass and wafers - the line scan version of a coaxial light.
• Tunnel, double-angle and multi-angle: light from two or more directions to fill in shadows on embossed, curved or textured material.
• Transmitted: light under the web shining up into the camera, for holes, pinholes, coating gaps and contamination in film, nonwovens and glass.

4. Colour and wavelength

White suits colour line scan cameras. On mono cameras a single colour gives more contrast: red for general use and long LED life, blue for fine surface detail on metal, green as a compromise, UV to excite fluorescent inks, adhesives and coatings, infrared to see through some dyes and printing. A few models emit in the SWIR band for SWIR line scan cameras.

5. Driving, strobing and cooling

Line scan lights here run on 24 V DC or, for many of the longer and high-power bars, 48 V DC, so check which before choosing a light controller. The controller must supply the full power of the bar with margin; long bars may have two cables. Some ranges include an inline driver with analog dimming and overcurrent strobe from a PNP or NPN trigger. Fan- and water-cooled versions need airflow or a water supply.

What this category covers

Browse white line scan lights or infrared line scan lights.

Typical applications

• Surface defects on steel, aluminium and copper strip, foil and battery electrode coatings.
• Print inspection on labels, packaging film and printed sheet.
• Glass, display panels and film: scratches, bubbles, inclusions, using bright field or transmitted light.
• Nonwovens, paper and textiles: holes, thin spots and contamination with a light under the web.
• Cylindrical parts rotated under the camera to unroll the surface into one flat image.

Common mistakes

• Choosing the lit length equal to the field of view: the image ends go dark and the edge of the web is missed.
• Mounting a focused light at the wrong distance. The line is only thin and bright near its focal distance; a few millimetres off and brightness drops sharply.
• Sizing brightness for an area scan exposure. A line scan exposure is microseconds, not milliseconds.
• Ignoring flicker from PWM dimming that is not synchronised to the camera, which shows as bands across the image. Use a constant-current or strobed controller synchronised to the line trigger.
• Not running flat-field correction in the camera after the light is installed, so residual non-uniformity stays in every image.

Questions

Can I use a bar light instead of a line scan light?
For slow lines and short fields, a bar light can work. For high line rates the narrow, focused output of a line scan light puts much more of its output on the scan line from the same power.

Focused or diffuse?
Focused for the most brightness on flat material at a fixed distance. Diffuse when the surface is shiny and wrinkled, or the material height changes, since a diffuse line is less sensitive to distance and angle.

How far from the web should the light sit?
Use the recommended working distance printed on the product page. Where stated, recommended distances in this category run from about 10-30 mm for compact units to 10-500 mm for large high-power bars.

What else does a line scan station need?
A line scan camera, a line scan lens that covers the sensor length, an encoder trigger and a controller. Configure the system to match them.