Setting exposure for a moving part, and deciding whether to pulse the light

In short

Divide the blur you accept, in pixels, by speed times sampling (pixels per millimetre on the part). Under continuous light, the resulting time limits the exposure, and the camera must be able to set it. Under a short synchronized pulse that supplies nearly all the light, it limits the pulse, not the exposure. The whole pulse must fall while the rows you need are exposing, and ambient light must leave no trail. Only test images of the code region at real speed and light prove the image is good enough.

Sampling first, then the blur you accept

Motion blur is how far the part's image travels, in pixels, while the picture forms. Sampling is resolution divided by field of view, both along the direction of motion. Do not use the sensor's pixel pitch: it describes the sensor, not the part. The Camera and lens calculator reference gives the derivation.

Our calculator allows 1 px of blur for detection and presence checks and 0.5 px for measurement. These are our defaults, not a standard, though a one-pixel rule of thumb (Basler) is similar. Measurement finds edges to a fraction of a pixel, and blur can shift them. Tighten the allowance when a shifted edge would give a wrong result, not just a retry. Halving it halves the time limit.

Which time counts: exposure or pulse

Exposure time is how long the sensor collects light for one image. Under continuous light the image smears for that whole time. If the limit is below the minimum exposure in the camera datasheet, change the geometry or the speed.

A strobe is a short light pulse synchronized with the camera. It freezes motion with light, not with the shutter. The part keeps moving through the whole exposure. But if the pulse supplies nearly all the light, the image records only the travel during the pulse. Use the pulse duration in the blur calculation.

The whole pulse must fall inside the time when all the rows you need are exposing. This is the flash window in Basler's terms. Ambient light must be too weak to leave a visible trail.

A global shutter exposes all pixels over one common interval, so the window is simply the exposure. A rolling shutter exposes rows at different times. Some models offer Global Reset, which starts all rows together but may still read them out one by one. For a rolling shutter, take the timing from the model documentation.

What a shorter time costs

A shorter time means less light, so you may need a wider aperture or more gain.

The aperture is the lens opening, and a smaller f-number means a wider opening. A wider opening reduces depth of field, the range of part heights that stay acceptably sharp. It can also bring aberrations, which soften the image, and vignetting, which darkens the corners.

Gain amplifies signal and noise after the sensor, brightening the image without cleaning it.

A pulse concentrates the light into the short interval instead of spreading it over the cycle. Pulse timing differs by product, so send us the part numbers of the light and controller. We will confirm their pulse width, delay and duty cycle from the vendor document.

Worked example: a code on a conveyor

The camera is the MV-CA050-12UM: 2448 × 2048 px with square 3.45 µm pixels (datasheet). The rest are assumptions of this example. Parts travel along the image width at 1200 mm/s, and the object area in the picture is 160 × 120 mm. The code module, its smallest bar or cell, is 0.30 mm, and the target is 5 px per module. We accept 1 px of blur, with the lens at f/4 and a working distance of 500 mm.

The calculation measures that distance from the lens principal plane, an optical reference plane, not from the camera body. Check the lens documentation for the matching distance on the machine.

Figures below come from our calculator unless marked otherwise. With this sensor's square pixels, a frame 160 mm wide is 133.9 mm tall, not 120 mm. The 120 mm object height fits with room to spare. Filling that width from 500 mm takes a 25.1 mm focal length, so the calculator picks a standard 25 mm lens.

Rounding the focal length down widens the field and costs a little sampling, but the time limit barely moves:

In the installed field, the 0.30 mm module spans 4.58 px, short of the 5 px target. Reaching 5 px would take a 0.33 mm feature.

Under continuous light, 54.6 µs lies inside the camera's Standard exposure range, listed below. That is a range check, nothing more. With a pulsed light, keep the pulse at 54.6 µs or shorter.

Calculate for your case

Calculate for your case

The link carries the supported conditions; blurPx, fNumber are not part of the page and are available through the tools API.

The prepared link opens the calculator with this example's inputs except two: the 1 px blur allowance and the f/4 aperture. For those, or values of your own, use the optics route of our tools API and write down what you assumed.

For the selected camera and lens pair, the page shows the example's figures, rounded: 160.5 × 134.2 mm, 15.3 px/mm and 55 µs.

The line "At the minimum exposure time of 50 µs, calculated motion blur is 1.00 px" answers a different question. Its 50 µs come from the task's requirement, 0.30 mm at 5 px or 16.7 px/mm (calculated), not from the selected pair. "Provides 4.58 px instead of 5" is the same shortfall the example found. So the page ends with "No complete pair; send the application to an engineer".

Equipment that fits this example

The MV-CA050-12UM is the example's camera, a global-shutter CMOS model in 2/3" format. Its Standard exposure range is 15 µs to 10 s, and its UltraShort range is 1 to 14 µs (datasheet).

The MV-CT0MU0060/R is the rolling-shutter case. Its exposure runs from 8 µs to 1 s, and its exposure modes include Rolling and Global Reset (datasheet). Confirm its row timing before using it with a pulsed light. For both cameras, check the mount, interface and cable, and whether the lens image circle covers the sensor.

Common mistakes

• Treating the sensor's 3.45 µm pixel pitch (datasheet) as the size of one pixel on the part gives 2.9 µs instead of 54.6 µs (calculated). That is about 19 times too short and can make the image needlessly dark. Calculate sampling as resolution divided by field of view.
• Shortening the pulse while ambient light stays on gives a sharp image with a faint smeared ghost across it. Shield the scene from ambient light, or shorten the exposure.
• Firing a pulse without checking polarity, delay and placement leaves part of it outside the window. The frame comes back dark or unevenly lit.

When this rule breaks

The calculation assumes constant speed. It leaves out acceleration, vibration, motion out of the object plane and defocus. It also ignores the decoder's own tolerance and rolling-shutter skew, where rows taken at different moments make the part look slanted. A part that jitters on a worn belt can fail at a limit the arithmetic approves.

A line-scan camera builds the image one line at a time, like an office scanner. The line rate you need is speed times sampling along the direction of motion. The usable exposure is limited by the line period (the time per line), the duty cycle and any camera maximum, whichever is tightest. Time-delay integration (TDI) adds up the signal of several sensor lines and changes the effective exposure. The number of physical lines does not tell you by how much.

Next step

Choose continuous or pulsed light, and write down the blur allowance and aperture you assumed. Test the code region at the real speed and light. Check that its elements stay separate, and look for clipped highlights (burnt-out whites) and noise. Then measure the read rate across the operating range the line requires.

Related

• Deciding whether a rolling shutter is good enough for your moving part
• Line-scan image stretched or compressed: how to find the cause
• Why a code does not read, and what to check first

How this material was prepared

Prepared by MVisionPro from the stated sources and the MVisionPro calculation engine. A physical test is claimed only when the material says so explicitly. Read the editorial method.

Editorial status: verified. Content updated 2026-09-23.