The lens decides how sharply a defect is imaged; the lighting decides whether the defect has any contrast. Something with no contrast cannot be seen by any premium lens or by any AI. This is where the most effective skill in visual inspection lies.
The numerical selection of the geometry (coaxial / ring / dome / dark field / backlight) is covered by the "Illumination design" section of optics/guides/optics_imaging.md (illumination_design / defect_contrast / lighting_sweep compute the contrast from first principles). This document is teaching material on what lies outside that —— the practice of equipment and the shop floor.
optics_imaging.md)| Defect / purpose | Lighting that works | Reasoning |
| --- | --- | --- |
| Unevenness and engraving on mirror surfaces | Coaxial | Returns the specular reflection straight to the camera and darkens only the tilted faces |
| General surfaces | High-angle ring | General purpose. Try this first |
| Rough defects (chips, pits, fine scratches) and foreign matter | Low-angle ring (dark field) | Picks up only the scattered component. Flat areas are dark and only rough spots light up |
| Glossy curved surfaces, pigment colour | Dome | Illuminates from all directions to remove specular unevenness |
| Outline, dimensions, holes | Backlight | Sees only the shape. Not disturbed by surface patterns |
| Directional texture | Bar (oblique) | Emphasizes relief with shadows |
Caution: "Scratches light up in dark field" applies to scratches with rough flanks. A surface that is merely smoothly tilted (e.g. 10°) does not light up in dark field —— it only specularly reflects the low-angle light away from the camera (defect_contrast shows this in numbers).
• Shorter wavelengths favour fine detail. The diffraction limit is proportional to λ (Airy radius = 1.22 λ N). With the same lens, blue (450 nm) can resolve roughly 3 tenths finer than red (630 nm). If you are after fine defects, use a short wavelength.
• Infrared passes through. Many resins transmit near infrared, so you can see contents through bags or resin covers, or look inside adhesive layers. The silicon sensor itself is sensitive up to about 1100 nm (the band gap of Si), so up to that point an ordinary camera can capture it.
• With UV-excited fluorescence you can see "things invisible in the visible range" —— uneven adhesive application, oil, residual foreign matter. What glows is the object itself, not a reflection, so the exposure becomes orders of magnitude longer.
• Create contrast with colour. Shining light of the complementary colour of the target makes that part dark. Capturing green foreign matter under red lighting makes it sink to black. Monochromatic lighting + a monochrome camera is often better than a colour camera in both sensitivity and resolution.
• ★ A narrow-band LED + band-pass filter is the standard countermeasure against ambient light. Attaching a filter to the lens that passes only the lighting wavelength removes most of the fluorescent light and sunlight. Before building a light-shielding cover, this is worth trying first.
• Remove glare with crossed polarisers. Put polarisers on the light-source side and the receiving side and cross them. Specular reflection from metal or transparent layers preserves polarisation well, so it disappears. On the other hand, diffuse reflection, which comes out after scattering inside, has lost its polarisation, so it remains. → Used, for example, to remove the glare of the clear coat on a painted surface and see the base layer.
• Stress birefringence makes the internal stress of transparent bodies (glass, resin) visible. Residual stress in moulded parts, inspection of tempered glass.
• The price is less light (1/4 or less with 2 polarisers). Consider it together with the drive modes of §4.
| Mode | Light output | Where to use |
| --- | --- | --- |
| Continuous | As rated | Stationary objects, simple setups. Brightness drifts until the heat reaches steady state (§6) |
| Strobe (pulsed) | About the same as continuous | Freezes motion blur. Flashes briefly in sync with the exposure |
| Overdrive | Greatly increased (around 10 times the rating, depending on conditions) | Solves insufficient light, fast transport and darkness caused by polarisers all at once |
Key points of overdrive (primary sources are the lighting makers' application notes):
• The pulse width can be reduced from a few ms down to 1 µs, and the duty cycle to below 1%.
• Because of heat constraints, a duty cycle of about 10% is considered the upper limit.
• Because the duty cycle is low, heating of the LED junction actually decreases, which favours lifetime. It also gives design room to make the heat sink smaller or unnecessary.
• Trigger synchronization with the camera exposure is mandatory. If it slips, "occasionally dark images" get mixed in, and it is hard to notice that the lighting is the cause (if these get into the training data, quality drops).
• If the PWM dimming frequency and the line rate produce a beat, horizontal stripes appear in the image. The further you dim, the lower the duty cycle and the more visible they become. The rule is dim with constant current, not PWM.
• Ambient light is the same. 100 / 120 Hz from mains power, several tens of kHz from inverter fluorescent lamps.
• Order of countermeasures: ① shield the light ② remove it with a band-pass filter (§2) ③ make the exposure time an integer multiple of the flicker period ④ switch to constant-current dimming.
• LED luminous flux degrades over time. Even when driven at constant current, the light output falls as the junction temperature rises.
• That is why you measure at thermal steady state. Right after power-on it is bright, and it settles after several tens of minutes. An exposure value decided right after start-up is too dark in steady state.
• Equipment that holds its decision threshold in absolute luminance should be calibrated periodically with a reference plate (white diffuse plate), or use a power supply with light-output feedback.
• The same applies when making synthetic data —— if the brightness of the real hardware drifts, put that range of variation into inspection_dataset(intensity_jitter=...).
A standard that classifies lamps and lamp systems into 4 risk groups.
| RG | Meaning | Guideline |
| --- | --- | --- |
| RG0 (exempt) | No hazard. No harm even when viewed for a long time | Continuous exposure up to ~10,000 s |
| RG1 (low risk) | No harm for a limited time | ~100 s |
| RG2 (moderate risk) | Does not normally lead to injury because of natural aversion responses such as the blink reflex, but is uncomfortable even briefly | ~0.25 s |
| RG3 (high risk) | Hazardous even briefly. Especially at close range | — |
• RG2 / RG3 require warning labels and precautions for use. Strong industrial lighting commonly falls into RG2 (e.g. the projector of the Zivid 2+ is RG2 under IEC 62471-5).
• With inspection equipment, work in which people look into the machine always occurs (setup, cleaning, troubleshooting). Check the RG at design time and make a structure in which no one is exposed in a looking-in posture.
• Take particular care with UV and blue (blue-light retinal hazard). UV fluorescence inspection presupposes a light-tight enclosure.
• Light-shielding covers, small openings, and matte black on the inside (black anodizing, flocked sheet).
• Stray light comes from "where you are not looking". A white floor, an operator's white coat, the monitor of the neighbouring machine. Look around the room from the camera position and look for bright things.
• If shielding is not enough, use the band-pass filter of §2. Making the lighting stronger so that ambient light becomes relatively negligible is another option, and that is where overdrive (§4) helps.
| Element | What exists now | What is missing |
| --- | --- | --- |
| Geometry | light_source / light_spec (ring / dome / bar / coaxial / backlight), illumination_design | — |
| Wavelength | light_spec(wavelength_nm, bandwidth_nm), light_wavelengths | Real spectral distributions, fluorescence (wavelength conversion from excitation → emission) |
| Polarisation | light_spec(polarization=), jones_* / mueller_* op | Rendering carried through polarisation from lighting → surface → receiver |
| Drive mode | None | Strobe / overdrive, synchronization slip with the exposure |
| Flicker | None | Beat stripes in line scan (a main failure mode of real hardware) |
| Ageing | Can be varied with inspection_dataset(intensity_jitter=) | Systematic drift due to heat (monotonic, not random) |
| Safety | None | Risk-group calculation |
| Rendering | illumdesign is a sum of point sources | No multiple reflection or interreflection, anisotropic BRDF not supported |
| Symptom | Suspect first | How to check |
| --- | --- | --- |
| The defect is not visible at all | The lighting geometry does not fit | Get candidates with illumination_design(surface, defect) and try them on real hardware from the top |
| Scratches are visible but the flat area also glares | Specular reflection is entering the camera | Put in crossed polarisers, or change the elevation angle (lighting_sweep) |
| A smoothly tilted defect does not light up in dark field | That is normal. Dark field works on roughness | Look at the regime of defect_contrast. If it is topographic, dark field is unsuitable |
| Horizontal stripes appear in the image (line scan) | Beat between PWM dimming and ambient light | Constant-current dimming, shielding, exposure an integer multiple of the period |
| Occasionally dark images are mixed in | Synchronization slip between strobe and camera exposure | Look at the trigger signal on an oscilloscope. Check whether it reproduces with continuous lighting |
| Decisions differ between morning and evening | Ambient light getting in | Does it reproduce with shielding? Compare with a band-pass filter inserted |
| The pass rate is high only right after start-up | The LED has not reached thermal steady state | For 60 min after power-on, image the reference plate every 10 min and record the luminance |
| Gradually drifts out of the threshold over several months | LED ageing | Look at the history of the reference plate's luminance. Include periodic calibration in operation |
| Light is insufficient and exposure cannot be extended | Upper limit of continuous lighting | Switch to a power supply that supports overdrive (duty cycle within 10%) |
| Operators say it is dazzling | Risk group | Check the RG and review the exposure at the looking-in position |
• Advanced Illumination — *The Mechanics of Strobe Overdriving LED Lights for Machine Vision Applications* (distinction between continuous / strobe / overdrive, lowering the duty cycle, reduced junction heating) — <https://advancedillumination.com/the-mechanics-of-strobe-overdriving-led-lights-for-machine-vision-applications/>
• Smart Vision Lights — *OverDrive Driver* (pulse width from a few ms to 1 µs, duty cycle below 1%, duty-cycle upper limit 10%, current on the order of 10 times the rating) — <https://machinevisiondirect.com/blogs/news/svl-overdrive>
• Opto Engineering — *LED pulsing and strobing* — <https://www.opto-e.com/basics/led-pulsing-and-strobing>
• Advanced Illumination — *IEC 62471 Photobiological Safety of Lamps and Lamp Systems* (classification RG0–RG3) — <https://www.advancedillumination.com/wp-content/uploads/2023/04/IEC-040119-Photobiological-Safety-of-Lamps-and-Lamp-Systems_042723.pdf>
• Luminus Devices — *Understanding IEC 62471 and IEC 62778 Eye Safety Risk Groups* —
<https://luminusdevices.zendesk.com/hc/en-us/articles/10532958752397-Safety-Understanding-IEC-62471-and-IEC-62778-Eye-Safety-Risk-Groups-visible-and-nonvisible-light-sources>