Industrial camera for machine vision

01

Read the EMVA 1288 datasheet, not the sensor name

EMVA 1288, the European Machine Vision Association's standard for characterising image sensors and cameras, defines the absolute sensitivity threshold µp.min as the mean number of photons needed for an input signal-to-noise ratio of 1, and the dynamic range as the saturation irradiation divided by that threshold, to be given as a ratio and in dB (20 log10 DR). It warns that saturation capacity must not be confused with full-well capacity: "It is normally lower than the full-well capacity, because the signal is clipped to the maximum digital value" before the pixel physically saturates. [1]

The standard's example summary datasheet (Release 4.0 General, values for illustration) lists an absolute sensitivity threshold of 39.6 photons, a saturation capacity of 51,548 photons, a dynamic range of 1303 (62.3 dB), a maximum SNR of 202 (46.1 dB), a total PRNU of 0.87%, a DSNU of 21.7 photons and a dark current of 5.1 ± 0.2 photons/s. DSNU is the pixel-to-pixel variation of the dark signal and PRNU the variation of sensitivity. [1]

EMVA states that anybody publishing data claimed to be EMVA 1288 compliant must also provide the full datasheet, and that a datasheet is only compliant if all mandatory measurements from at least one camera are reported. It also notes that the dark signal rises with exposure time because of thermally generated electrons. [1] Editorial note: compare cameras at the same operating point (gain, bit depth, exposure, wavelength) and ask for the full datasheet, not only the summary figures.

Give the supplier the part and the smallest defect or tolerance to resolve, field of view and working distance, line speed and parts per second, trigger source, lighting, colour or monochrome, wavelength, host interface and cable length, ambient temperature, protection rating and mounting space.

02

Check the frame rate and trigger against the line

Basler's camera documentation lists factors that limit frame rate: a frame-rate cap if set, assigned bandwidth (especially when more than one camera is connected to a computer), long exposure times, a large image ROI (Basler: "On most cameras, decreasing the image ROI height increases the frame rate"), sensor and transfer bit depth, sensor readout mode, and the Global Reset Release shutter mode, which prevents overlapped acquisition. Binning, where available, can raise frame rate. [2]

When triggering, Basler says to wait at least the reciprocal of the resulting frame rate between triggers; at 12.5 fps that is 1/12.5 = 0.08 s, and a trigger arriving earlier is ignored and raises a frame-start overtrigger event. It also warns against triggering faster than the host can transfer, store or process, or frames may be dropped. Trigger sources include an electrical input line, a signal carried over the data cable, an internal timer or counter, or a software command; a programmable trigger delay is set in µs. Triggering over CoaXPress gives a trigger accuracy below 0.5 µs on CXP-10 or CXP-12 and below 1 µs on CXP-3, CXP-5 or CXP-6. [3] [2]

Basler describes a global shutter as exposing all pixels at once, suited to fast-moving objects, and a rolling shutter as exposing rows one after another, with lower noise but possible distortion of very fast-moving objects; it recommends flash lighting to prevent that, and Global Reset Release mode requires flash lighting. [4]

Editorial calculation: a part moving at 0.5 m/s travels 0.5 × 0.0001 = 0.05 mm during a 100 µs exposure. With a 100 mm field of view across 2048 pixels, each pixel covers about 0.049 mm, so the blur is roughly one pixel. A 2448 × 2048 image at 8 bits and 23 fps is about 922 Mbit/s of image data before protocol overhead; compare it with the interface bandwidth the supplier states. Speed, exposure, field of view and image size are assumed examples.

03

GigE and HDMI cameras, vision kits and smart cameras

Four companies are shown below: Sensing and Pomeas list stand-alone industrial cameras, Swoer sells cameras with its feeders and vision inspection systems, and Future Robot lists smart cameras with its industrial PCs.

Listings range from GigE area-scan and line-scan cameras and Gigabit or HDMI industrial cameras sold with lenses and lighting to vision cameras supplied with feeding equipment and smart cameras from an industrial PC maker. Ask each for EMVA 1288 data for the sensor and operating point quoted.

04

Sensor, shutter, readout, trigger I/O, interface and housing

The sensor row is where EMVA 1288 data applies; the shutter, readout, trigger and interface rows follow the Basler documentation.

05

Resolution, frame rate, pixel size, interface and environment for the quotation

Field of view, line speed and host come from your inspection. Basler and EMVA 1288 notes in the middle column say what each value depends on.

06

Send the inspection, motion and host data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Inspection: [part; smallest feature; field of view; working distance]
Motion: [line speed; parts per second; trigger source]
Image: [mono or colour; wavelength; exposure; bit depth]
Lighting: [type; strobe or continuous]
Host: [interface; cable length; number of cameras]
Site: [temperature; IP rating; mounting]
Documents: [full EMVA 1288 datasheet; drawings]

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