Strobe machine vision light: overdrive, duty cycle and timing

Motion blur limits most inspections of moving parts. The camera can shorten its exposure, but then the image gets darker, and turning up the gain adds noise. A strobed light solves both problems: it delivers a short, intense pulse exactly while the camera exposes, so the part is frozen and the image is bright. This guide shows how to size the pulse, how much overdrive the light can take, and how to time the pulse inside the exposure, with a worked example for a bottle line.

Step 1: how short the pulse must be

When the part moves during the exposure, its image smears over:

For measurement and fine defects, keep blur under half a pixel to one pixel. With a strobe and little ambient light, the effective exposure is the pulse width, so the formula gives the pulse length directly:

The motion blur calculator runs this for your speed and pixel size.

Step 2: overdrive and duty cycle

LEDs are rated for a continuous current that keeps the junction temperature safe. In a short pulse the junction does not have time to heat up, so the current can be raised well above the continuous rating. This is overdrive. Two limits apply:

• Peak current per pulse, set by the LEDs and the wiring, and by how long the pulse lasts. Longer pulses allow less overdrive.
• Average power, set by the light's ability to shed heat. It must stay at or below the continuous rating.

The average comes out somewhat higher than the formula, because the LED forward voltage rises with current, so power rises faster than current. For example, a 100 µs pulse at 100 Hz is a duty cycle of 0.01 (1 percent). At five times the continuous current, the average is a little over 5 × 0.01 = 0.05 of the continuous rating: no thermal problem. The same pulse at 2,000 Hz is 20 percent duty: five times the current would put the light at or above its full continuous rating, with no margin. Too much.

Light output does not rise in proportion to current. LED efficiency falls at high current, so five times the current gives noticeably less than five times the light. Plan with the brightness gain the light maker states for the light and controller pair. For example, this strobe light is specified for a gain of 5-14 times with its matching controller:

Step 3: choose the controller

A strobe controller needs a fast, repeatable response to the trigger, an adjustable pulse width and delay, and limits that protect the light. This overdrive controller sets pulse width and delay in microseconds:

Its data sheet lists a pulse width adjustable from 0 to 999 µs, a fixed lighting delay of 12 µs and external triggering up to 200 Hz, which covers most part-by-part inspections. For lights that only need switching in step with the camera without overdrive, a standard digital controller with a strobe mode does the job:

Constant-current controllers with an external trigger input suit lights where stable brightness matters more than peak boost:

Step 4: timing the pulse inside the exposure

The pulse must fall completely inside the camera's exposure. Each device adds a delay between the trigger and its action:

• Camera: exposure start delay after a hardware trigger, usually a few microseconds, and longer for software triggers.
• Controller: trigger-to-light delay, from a few microseconds to tens of microseconds, plus the LED rise time.
• Wiring and opto-couplers: a few microseconds more.

Two common wiring schemes:

1. One trigger, two receivers. A sensor or PLC output triggers both camera and controller. Set the camera exposure longer than the pulse by at least the controller's delay plus margin, so the pulse cannot spill past the end.
2. Camera drives the light. The camera's strobe or exposure-active output triggers the controller. The pulse then always starts after the exposure has begun; you only need to make the exposure longer than controller delay plus pulse width.

Verify the timing once with an oscilloscope on the camera output and the light current, or sweep the controller's delay setting and watch the image brightness: it stays flat while the pulse is fully inside the exposure and drops as it moves out.

A worked example: bottles at 1.5 m/s

Bottles pass at 1.5 m/s, ten per second. The camera sees a 400 mm field with 2,000 pixels, so 0.2 mm per pixel. The label inspection tolerates half a pixel of blur.

1. Allowed blur: 0.5 × 0.2 = 0.1 mm.
2. Maximum pulse: 0.1 / 1,500 = 67 µs.
3. In continuous mode at 67 µs exposure the test image reaches a grey level of 40; the target is about 180, so the light must be about 4.5 times brighter. Choose a light and controller pair whose stated brightness gain covers at least 4.5 times.
4. Duty cycle: 67 µs × 10 Hz = 0.07 percent. Even ten times the current is thermally trivial at this duty.
5. Timing: with a controller delay of 12 µs and a 67 µs pulse, set the camera exposure to about 100 µs, triggered by the same sensor. The pulse ends at about 79 µs, inside the exposure.
6. Ambient light: during a 100 µs exposure the room light contributes very little; the strobe dominates the image.

Strobing also beats ambient light

When the exposure is matched to the pulse, the ratio of signal to ambient light improves by the brightness gain of the strobe. A light that strobes five times brighter gives five times the margin over sunlight from a nearby window or the plant's lighting, without enclosing the station. For stronger ambient light, add a bandpass filter matched to the LED colour.

Strobing line scan lights

Line scan cameras take thousands of lines per second, so line lights usually run continuously. One special case uses strobing: alternating two lights on alternate lines, for example bright field on even lines and dark field on odd lines, so one camera captures two images in one pass. That needs a controller built to switch channels line by line:

See the line scan light guide for why line lights need so much power in the first place.

Checks before the line goes live

• Brightness stability. Grab 100 images of a static target and check the mean grey level varies by no more than a percent or two. Larger swings point to timing at the edge of the exposure.
• Worst-case frequency. Run the trigger at the fastest part rate the line can reach, including catch-up after a stop, and confirm the duty cycle still sits within limits.
• Heat. After an hour at full rate, the light housing should be no warmer than in continuous operation at its rating.
• Cable length. Long cables between controller and light slow the current rise in short pulses. Keep them as short as the layout allows.

Next step

Compare strobe and overdrive models in the light controllers range, see strobe-ready lights under special lights, or wire camera, light and controller together in the configurator. For trigger wiring on the camera side, see camera hardware trigger frame rate.