A line scan camera can only be as fast as its light allows. Engineers who move from area cameras are often surprised that a light which looks dazzling on the bench gives a dark, noisy image at line speed. The reason is simple arithmetic: each line is exposed for microseconds, and the light has to fill that window. This guide works through the light budget, then covers the decisions that follow from it: focused or diffuse, how long, which geometry, cooling and the controller.
Why line scan needs so much light
The exposure of a line scan camera can never be longer than one line period. At 10,000 lines per second that is 100 µs; at 40,000 lines per second it is 25 µs. An area camera inspecting the same part with a strobed light might expose for 1 to 2 ms.
The image brightness depends on irradiance on the part multiplied by exposure time (and on the aperture, pixel size and sensor sensitivity). With everything else equal, cutting the exposure by 20 times needs 20 times the irradiance to reach the same grey level.
A worked example
An area camera with 3.45 µm pixels images a printed web at 2 ms exposure and reaches a good signal. The same web on a line scan camera with 7 µm pixels runs at 12,500 lines per second, so the longest possible exposure is 80 µs.
- Exposure ratio: 2,000 / 80 = 25 times less time.
- Pixel area ratio: (7 / 3.45)² = 4.1 times more area per pixel, which collects 4.1 times more light at the same irradiance and aperture.
- Net: 25 / 4.1 ≈ 6 times the irradiance on the web for the same grey level.
That factor of 6 is before any margin. If the line will later run 30 percent faster, or you want to stop the lens down for depth of field, the factor grows. This is the reason line scan lights concentrate their output into a narrow stripe, run at high power and often carry fans.
To see where your own line rate comes from, the line rate how-to walks through field of view, pixel size and speed.
Focused or diffuse line light
A line scan camera only needs light on one row of the part: a strip one pixel tall, typically 0.05 to 0.2 mm. Everything the light puts outside that strip is wasted. Two designs deal with that differently:
| Focused line light | Diffuse line light | |
|---|---|---|
| Stripe on the part | Narrow, bright, set by a lens or reflector | Wider and softer |
| Peak irradiance | High at the design working distance | Lower, but changes slowly with distance |
| Height tolerance | Small: the stripe widens and dims off focus | Large: parts of varying height stay lit |
| Alignment | Camera line must sit inside the stripe | Forgiving |
| Typical use | Fast webs, dark materials, dark field | Shiny or curved surfaces, varying heights |
A focused light used at the wrong distance can lose most of its advantage. Measure the working distance on the machine and check it against the light's recommended range before ordering.
Here is a fan-cooled, high-brightness light offered in standard, high-luminance and high-uniformity types:
- Size
- 400 mm +
- Voltage
- 24 V
Its data sheet recommends the high-uniformity type for bright field and the high-luminance type for dark field, which matches the trade-off in the table.
How long the light must be
Short lights are a common mistake. The irradiance at a point on the part comes from the whole length of the light, not just the LEDs directly above it. Near the ends of the light, the contribution from one side is missing, so the edges of the field go dark.
For a diffuse emitter (no focusing optics), the falloff at the end can be estimated from the geometry. With the light ending a distance L beyond the edge of the field of view at working distance WD:
- L = 0 (light exactly as long as the field): the edge gets about 50 percent of the centre irradiance.
- L = 0.5 × WD: about 78 percent.
- L = 1 × WD: about 91 percent.
- L = 2 × WD: about 98 percent.
So for a 600 mm field at 100 mm working distance, a light of 600 + 2 × 100 = 800 mm keeps the edges near 90 percent. Flat-field correction in the camera can take out the remaining few percent, but it cannot recover signal that never arrived: the edge pixels still carry more noise.
Irradiance also changes with distance. Under a long diffuse line, it falls roughly in proportion to 1 / distance, not 1 / distance² as for a spot. Doubling the working distance halves the light, which is gentler than with a point source, but still significant.
Lighting geometry for line scan
The angle between light, part and camera decides what the image shows, exactly as with area cameras:
- Bright field (specular). Light and camera at equal and opposite angles to the surface. Shiny surfaces look bright; scratches, dents and coating gaps scatter light away and look dark.
- Dark field. Light at a low angle, camera looking straight down. Smooth surface stays dark; scratches, fibres and dust scatter light into the lens and look bright. Needs the most power, because only scattered light reaches the camera.
- Transmitted. Light under a transparent or translucent web (film, glass, nonwovens), camera above. Holes, gels, thin spots and inclusions show up in silhouette.
- Coaxial. Light reflected down the camera axis through a beam splitter, for flat, mirror-like surfaces such as foil, wafers and polished sheet.
- Dual angle. Two light rows aimed at the line from both sides, which removes shadows on embossed or textured surfaces.
A coaxial line light puts the beam splitter and the camera opening in one housing:
- Size
- 200-400 mm
- Voltage
- 24 V
And a dual-angle light for textured or embossed material:
- Size
- 400 mm +
- Voltage
- 48 V
Power, heat and cooling
High output means high power draw. As an example of scale, a 975 mm light in the range draws up to 403 W in blue or white:
- Size
- 400 mm +
- Voltage
- 24 V
- Connector
- WS20 4-pin
At 24 V, 403 W is about 17 A, which sets the cable cross-section, the controller and the power supply. Heat is the other half of the problem. LED output falls as the LED junction warms, and LED life shortens. Fan or forced cooling keeps a long, continuously lit bar stable through a shift.
Where space is tight or the light runs at partial brightness, a naturally cooled, compact bar can be enough:
- Size
- 200-400 mm
- Voltage
- 24 V
Practical checks:
- Keep fan inlets clear of dust from the web; a blocked fan defeats the cooling.
- Allow warm-up before calibrating flat-field correction; output changes during the first minutes.
- Mount the light on metal where you can: the frame helps carry the heat away.
Controllers for line lights
Line lights normally run continuously, with brightness set by the controller. The controller must supply the light's full power with margin, at the right voltage, and let the PLC or vision software change brightness for different products. A high-power, two-channel unit is a typical match for one or two long lights:
- Output
- Constant current, Constant voltage 48 V
- Channels
- 2 channels
- Control
- Analog
Constant-current drive holds the LED current steady, so the output only drifts with junction temperature rather than also with supply and cable voltage drops. That matters when the inspection compares grey levels across a long run; still allow a warm-up before calibrating. Size the controller for the sum of the lights' rated power plus about 20 percent, and check that its connector and voltage match the light. The light controllers range lists output power, channels and control type for each model.
Checklist before you order
- Line rate and exposure at the fastest speed you will run.
- Working distance on the machine, and whether it varies with part height.
- Field of view plus about one working distance at each end for the light length.
- Geometry: bright field, dark field, transmitted, coaxial or dual angle, tried on the real material.
- Colour: match the LED to the contrast you need and to the camera's sensitivity.
- Power, voltage, connector and cooling, matched to the controller.
Next step
Filter the line scan lights by length, voltage and colour, browse all machine vision lights, or build the camera, lens, light and controller together in the configurator.
Questions
How much longer than the field of view should a line light be?
For a diffuse emitter, about one working distance beyond each edge of the field keeps the edge irradiance near 90 percent of the centre; two working distances brings it near 98 percent. Focused lights with narrow beams fall off faster at the ends, so the same allowance is a safe starting point.
Should a line scan light be strobed?
Usually not. At tens of thousands of lines per second the light would have to switch on and off for every line, so line lights normally run continuously and the camera exposure sets the timing. Strobing makes sense for slow lines or when you alternate two lights on alternate lines.
Why does my line light look bright to the eye but the image is dark?
The eye integrates light over a fraction of a second; the camera may integrate for 50 microseconds. Check the exposure time, aperture and working distance first. A focused light used away from its design distance also spreads its stripe and loses most of its peak irradiance.
Do I need a fan-cooled line light?
Fan or forced cooling is worth it for long, high-power lights run continuously, because LED output and life drop as the LEDs heat up. Short or low-power lights, and lights run at partial brightness, often manage with natural cooling.