Ambient light rejection specification for vision acceptance tests

A vision cell that never specified ambient light passes on the integrator's bench and fails the first week on a lit production floor. The fix belongs in the equipment specification, before anything is built, and it is short: a number for the strobe-to-ambient signal ratio, a timing clause that pins exposure and pulse width, and a buy-off that repeats the pass/fail decision under every lighting state your plant can produce. This checklist gives you the clauses to copy, the arithmetic behind the 4:1 margin, and the lighting-state matrix that makes acceptance hard to argue with. It is written for the engineer who signs off integrator equipment and wants the rejection argument settled in the specification, not in a warranty dispute.

The number that makes the specification enforceable

Engineered illumination creates the signal; ambient light adds a noise term on top of it. That framing does the whole job: once the specification states the ratio between the two, ambient rejection stops being an opinion. The same fight runs one bench over in reflective sensing, where IR tachometers hold their ambient light rejection with optical filters instead of timing — shared physics, different countermeasure.

Define both terms at one frozen camera setting, in the region of interest the inspection actually uses:

• Ambient signal A — the ROI mean gray value with the engineered light off and plant lighting at its worst: high-bays on, inspector lamp on.
• Strobe signal S — the ROI mean of the strobed frame minus A, same exposure, same gain.

We write S ≥ 4 × A into every strobed station specification. At 4:1 the ambient term is at most 25 % of the signal, so a 2× relamp or a seasonal sun shift still leaves ambient at or below half the strobe term, and the decibel form is 20 × log10(4) = 12.04 dB — round it to 12 dB on the drawing. Below about 2:1, the ambient's own spatial variation starts modulating measurements frame to frame, and no software step recovers that cleanly. Pixel values are amplitudes, so the convention here is 20·log10 of the ratio; keep one convention when you compare margins with someone else's.

Why the strobe carries the margin

Ambient charge accumulates for the whole exposure; strobe charge arrives inside the pulse. At fixed illuminance, the ratio at the sensor is the product of peak strobe radiance and pulse width, divided by the product of ambient irradiance and exposure time. That single comparison explains the standard countermeasures — high-power strobing with short pulses, and physical enclosures — and it exposes the third lever nobody invoices for: exposure time. A 200 µs pulse at high peak current beats a 10 ms ambient term by the ratio of those products, and only a strobe-rated controller delivers that peak current; a continuous-duty driver cannot. Strobe capability is common — 127 of the catalog's 573 light controllers list a strobe feature — but pulse power and channel count still get sized to the light, not the other way around.

The LS-CTL-0070 digital control box provides 4 channels and 200 W with an RS232 link and a strobe feature, for a multi-light station driven from the camera exposure output.

Constant-current drive is the other half of the clause: regulated current holds pulse energy stable shot to shot, so a voltage sag or an aging supply moves the ambient term, not your signal.

Worked example: the 4:1 strobe margin on a 10 ms exposure

A buy-off record for a stamp inspection on a lit assembly line: monochrome 8-bit camera, ROI over the character zone, exposure frozen at 10 ms, gain frozen at 1×, strobe at rated current with a 200 µs pulse. Your station replaces the gray values; the grid and the arithmetic stay identical.

Rule in words: subtract the light-off mean from the strobed mean to isolate S, divide by A, convert with 20·log10.

• S = 148 - 24 = 124 DN
• S / A = 124 / 24 = 5.17
• margin = 20 × log10(5.17) = 14.3 dB

Decision: 14.3 dB clears the 12.04 dB requirement with 2.2 dB of headroom, so the station passes the ambient clause. Sign it.

Now the what-if the clause exists for: the plant relamps and ambient doubles. A′ = 48 DN at the same 10 ms exposure; the strobe term does not move.

• S / A = 124 / 48 = 2.58
• margin = 20 × log10(2.58) = 8.2 dB → below 12 dB, the station fails.

The repair is the free lever: shorten exposure from 10 ms to 2.5 ms. Ambient charge scales with exposure, so A falls to 48 × (2.5 / 10) = 12 DN, while S stays 124 DN because the 200 µs pulse still fits inside the window.

• S / A = 124 / 12 = 10.33
• margin = 20 × log10(10.33) = 20.3 dB → passes again, with motion blur cut 4× as a side effect.

Raising strobe current works too, until the light's duty-cycle ceiling stops you; the overdrive, duty-cycle and timing arithmetic shows where that ceiling sits. Reach for exposure first — it costs nothing and moves ambient by a factor of four.

Clauses to copy into the equipment specification

Paste these as specification lines, each with its reason, so a bidder argues with physics instead of with you. If the whole cell is still on paper, the system components buyer's guide shows where these clauses sit in the rest of the stack.

Lighting and drive

• Illumination shall be synchronized LED strobe lighting driven at constant current, triggered from the camera exposure output or the station PLC. Why: pulse energy that ignores mains sag is what makes the ratio reproducible.
• The settings record shall state the light model, rated strobe current, pulse width and maximum repetition rate. Why: an undersized drive forces long pulses that dilute the margin.

A multi-light station splits the pulse budget across channels; the LS-CTL-0180 constant-current controller provides 4 channels and 120 W for 24 V lights in an 89 x 147 mm housing.

Where the margin needs peak current rather than more channels, the drive shall be a high-power constant-current unit; the LS-CTL-0054 is rated 240 W per channel and 320 W total across 2 channels.

Timing

• Camera exposure and gain shall be fixed, with automatic exposure, gain and iris disabled and the fixed values recorded. Why: auto modes rescale the image as ambient changes and hide the drift this clause exists to expose — the brightness-drifts, turn every auto mode off article is the full argument.
• Strobe pulse width shall be shorter than the exposure, and trigger delay shall be measured so the pulse lands settled and centered inside the window. Why: a pulse outside the window photographs black — the black-frame timing fix is the usual hunt — and the dark-first-frame settling note covers the settling half of that delay budget.
• Pulse repetition rate and duty cycle shall stay inside the controller's rating at production rate. Why: duty cycle is the ceiling that ends the overdrive shortcut; the duty-cycle matching guide has the numbers.

Optics and shielding

• The station shall include a hood, shroud or enclosure blocking direct paths from plant lighting, windows and weld cells into the field of view, with matte-black interior surfaces. Why: spatial attenuation adds to temporal attenuation in decibels.
• Where the emitter wavelength permits, a band-pass filter matched to the lighting shall be fitted on the lens. Why: spectral rejection removes broadband ambient before it reaches the sensor.

Records and change control

• The settings record shall list exposure, gain, strobe current per channel, pulse width, trigger delay and ROI coordinates. Why: the margin becomes re-testable by anyone, which is the point of specifying it.
• Any change to plant lighting, glazing or adjacent equipment shall trigger a repeat of the buy-off matrix before production resumes. Why: the failure in your facility was a change nobody re-tested against.

The buy-off lighting-state matrix

Four states, one grid, no debate. Measure the ROI mean identically in each state — the ROI and reference method is the procedure — and record values, not impressions.

Run a known-good and a known-bad part through states 3 and 4; the decision must flip correctly with the plant lit. Thirty parts is our default sample: large enough to catch an intermittent trigger or a wandering pulse, small enough to run in half an hour. For a system already installed, the strobed ambient-rejection measurement converts this same grid into a before/after number. When optics and lighting are still being chosen, the lens and lighting selection test plan sequences this matrix after the optics trials, so buy-off re-runs numbers you already trust.

Checks before you sign

• Auto modes still off. Reopen the camera settings at the end of the buy-off and confirm exposure and gain match the record.
• Arithmetic redone by hand. Compute S, A, the ratio and the dB from the recorded gray values on paper. A spreadsheet nobody re-derived has passed 8 dB stations before.
• Current and duty inside ratings. Strobe current per channel at or below the light's rating; duty cycle inside the controller's rating at production rate.
• Repeatability. One stationary part, 30 repeat frames in state 3: the ROI mean holds within ±2 %. A margin that wanders points to timing or heat, not ambient.
• Record signed. The settings record is attached to the acceptance certificate, so the next lighting change restarts from numbers, not memories.

When strobing alone will not carry the margin

Two ambient sources outrun any duty-cycle-limited strobe: direct sun through glazing, and weld flash or other pulsed high-radiance sources nearby. If state 2 measures A above a quarter of your strobe term, the specification you need is an enclosure, not a brighter light — spatial attenuation is the only lever left that scales. Dock-door and outdoor installations live in this regime permanently; the outdoor image quality under changing light guide works the same margin from the camera side.

One more plant-side trap belongs in the risk register: high-bay LEDs dimmed by PWM put a flicker component into the ambient that beats against your exposure time, so a station can pass at buy-off and drift for months. The camera flicker diagnosis covers the beat-frequency arithmetic; the change-control clause above is what keeps it from becoming a warranty call.

Next step: shortlist a strobe-rated drive in the light controllers category, size channels and pulse power with the light-controller finder, then start the configurator at the controller step with your light list at hand. More illumination patterns sit on the lighting topic page. Send the lighting-state matrix with your request for quote and the buy-off becomes a formality instead of a negotiation.