Machine Vision Light Controller: Channels, Dimming, Strobe and Trigger

Lighting specifications — watts, wavelength, size — describe what a light can do when it is driven correctly. On the station, what it actually does is decided by the controller sitting between the power supply and the LED array. Constant-current drive, channel count, dimming resolution and trigger response are controller specifications, and they are the ones that determine whether the image on the first part matches the image on the ten-thousandth.

All values below are published POMEAS figures for the controller range. This is a selection method, not a compatibility claim: no statement is made that any POMEAS controller replaces a specific third-party unit, and no third-party specification is quoted.

Why constant current, not constant voltage

An LED's forward voltage drops as it heats up. Drive it from a constant-voltage source and the current rises as it warms, which raises the output, which raises the temperature further. On an inspection station that shows up as a gradual brightness drift through a shift — and brightness drift moves an edge-detection threshold.

A constant-current controller holds the current and lets the voltage vary, which is why machine-vision lighting controllers are specified as current sources. POMEAS publishes constant-current control on both the PMS-GE244PRT four-channel controller and the GE series (GE5-4PRT), with constant-current accuracy specified at 5 % on the GE series.

Channel count and the current budget

Each light needs a channel, and each channel has a current limit. POMEAS PMS-GE244PRT is published with:

The GE series adds high-speed optocoupler-isolated triggering, selectable high- or low-level triggering, front-panel or RS-232 remote operation, and automatic overload protection.

The budgeting step that gets skipped: add up the per-channel current the lights actually need and compare it against both the per-channel limit and the controller's total. Lighting power figures are useful here — the POMEAS GHN shadow-free ring series is published at 8 W (red) / 13 W (white and blue) for GHN80, 11 / 16 W for GHN116, 14.4 / 20.2 W for GHN132, 16 / 26 W for GHN166 and 26 / 31 W for GHN196, all at 24 V. A 31 W light at 24 V draws about 1.3 A, which already exceeds one 1000 mA channel — so a large ring either runs on a higher-current channel or gets its own controller.

Dimming resolution: 256 levels is a repeatability feature

The published 0–255 (256-level) dimming on both controller families is not about fine aesthetics. It is what lets an illumination setting be recorded and reproduced: "channel 2 at 168" is a specification that can be written into a work instruction, where "about two-thirds brightness" cannot. RS-232 control extends that — the value can be set by the host, stored per recipe, and restored on changeover.

For multi-SKU lines this is the practical reason to buy a controllable controller rather than a manual one: the illumination becomes part of the recipe.

Strobe and trigger: what the numbers mean

Two figures matter, and they are different things:

- Response time (200 µs) — how quickly the light reacts to a trigger edge. This sets the timing margin between the trigger and the camera's exposure window. - Maximum trigger frequency (1 kHz) — how often it can be triggered. At 1 kHz the period is 1 ms, so a station triggering faster than that needs a different approach, typically continuous illumination with the camera gating the exposure instead.

Strobing is used to get more light into a short exposure without heating the LED, and to freeze motion. It only works if the flash and the exposure window overlap — which is a wiring and timing question, not a brightness question. Optocoupler isolation (GE series) is what keeps electrical noise on the trigger line from producing spurious flashes.

The exception that cannot be wired like the rest

Most of the POMEAS lighting range runs at 24 V DC. The telecentric parallel backlights do not. POMEAS publishes, for VP-GLTD (30–265 mm beam diameter, working distance 45–500 mm by model) and VP-GLTDL (35–265 mm), a forward voltage of 3.5 V and a forward current of 350 mA, and states explicitly that these units need a matched constant-current driver or light controller and must not be wired as ordinary 24 V machine-vision lights.

This is the single most common wiring fault in a telecentric measurement build, and it destroys the collimation advantage the backlight was bought for. Check the forward voltage and current on the light's own datasheet before choosing the drive — the series voltage on the catalogue cover is not the drive voltage of every product in it.

Cable length is a specification, not an accessory

POMEAS publishes a 0.5 m standard cable with 1 m, 3 m and 5 m extension options across the lighting range, and the GHN documentation notes that longer extensions require checking controller output and connection stability. Voltage drop over a long cable reduces the current actually delivered; with constant-current drive the controller compensates up to its compliance limit, but beyond that the light dims.

Specify cable length at the design stage rather than discovering it at installation.

Practical selection order

1. Count the lights and how many need independent control → channel count. 2. Sum per-channel current from the light's wattage and voltage → per-channel and total budget. 3. Decide whether the setting has to be reproducible and host-settable → RS-232 and 256-level dimming. 4. Check trigger requirements: level (5–24 V, high/low selectable), isolation, frequency against 1 kHz, and 200 µs response against the exposure window. 5. Check the drive mode of every light in the bill of materials — especially telecentric backlights.

What this does not cover

It does not cover safety, earthing, or the enclosure requirements for high-intensity or UV sources, which are installation decisions. It also does not cover the camera-side exposure and gain, which interact with strobe timing but are set independently.

FAQ

Why constant current instead of constant voltage?

An LED's forward voltage falls as it warms; constant voltage then raises current and output, producing brightness drift through a shift. Constant current holds output stable.

How many channels do I need?

One per independently controlled light. PMS-GE244PRT is published with four independent channels at 0–1000 mA each.

What does 256-level dimming give me?

A reproducible setting. "Channel 2 at 168" can be written into a work instruction and set remotely over RS-232.

What is the trigger response time?

200 µs on PMS-GE244PRT. It sets the timing margin between trigger and camera exposure.

What is the maximum trigger frequency?

1 kHz on PMS-GE244PRT. Faster stations generally run continuous illumination with the camera gating exposure.

Can every POMEAS light be wired to 24 V?

No. VP-GLTD and VP-GLTDL telecentric parallel backlights are published at 3.5 V forward voltage and 350 mA forward current and need a matched constant-current driver.

Next Step

Send the light models and quantities, the per-channel power, whether the setting has to be host-settable and stored per recipe, and the trigger source and frequency, and POMEAS will return the controller configuration with the channel allocation and current budget.