· VisionSelect Team · Guides · 5 min read
Machine Vision Lighting Guide: Types, Selection, and the China Supply Advantage
Lighting decides more of your image quality than the camera. A practical guide to the six lighting geometries, selection by defect type, the specs that matter — and why China now leads this category.
Ask any vision engineer what ruined more inspections — cameras or lighting — and lighting wins. The camera gets the datasheet attention; the light decides the image. This guide covers the six lighting geometries, how to choose by defect type, and why the Chinese supply base is now the default in this category.
The six lighting geometries
| Geometry | How it works | Best for |
|---|---|---|
| Backlight | Illuminates from behind the object | Silhouettes, dimensions, edge position, presence |
| Dark field | Light at a low angle; flat surfaces reflect away from the lens | Scratches, dents, embossing, cracks, edge chips |
| Bright field | Diffuse light on-axis; flat surfaces reflect into the lens | Presence/absence, label presence, general shape |
| Coaxial | Light through a beamsplitter along the optical axis | Flat reflective surfaces, scratches on metal, wafer marks |
| Dome / diffuse | Bounced off a curved surface for omnidirectional soft light | Curved, shiny or complex surfaces (bottle caps, machined parts) |
| Structured light | Projects a known pattern (lines, grids) for 3D reconstruction | Height, flatness, volume, 3D profiles |
The geometry question — not the brightness question — is the one that decides whether your defect is visible at all.
Selection by defect type
- Surface scratches on metal → dark field or coaxial. The scratch scatters light toward the lens while the flat surface does not.
- Dimensional measurement → backlight. Clean silhouettes give sub-pixel edges with the right lens.
- Presence of a component or label → bright field or dome. You are looking for “is it there”, not texture.
- Text or codes on curved surfaces → dome or multi-directional. A single bar light leaves a shadow band on the opposite side.
- Height, flatness or volume → structured light (laser or projected pattern) with a matching camera.
- Glossy black plastic, glass → low-angle dark field with controlled polarization; coaxial if the defect is on the surface plane.
The specs that actually matter
- Wavelength. Red (625 nm) is the economical default; white for colour-critical work; blue (470 nm) for fine resolution on shiny surfaces; UV for special applications; IR to suppress ambient colour. Match the wavelength to your sensor’s QE and your part’s response.
- Uniformity. A light with 20% brightness variation across the field creates phantom “defects” downstream. Ask for the uniformity map, not the “uniform” claim.
- Strobe vs continuous. Strobed LED light delivers far higher peak brightness for short exposures — and less heat. Continuous-on is simpler but thermally limited.
- Brightness control & thermal behaviour. Overdriven lights dim as they heat. A 30-minute soak test at your duty cycle is the only honest check.
- Polarization. A polarizer on the light and a crossed analyzer on the lens kills specular glare — the difference between seeing a surface and seeing a mirror.
Why China leads this category
Machine vision lighting is one of the clearest examples of Chinese supply-chain depth:
- Scale. Leading domestic suppliers such as OPT (奥普特) build 45 standard lighting series with nearly 1,000 standard models, plus over 30,000 documented custom designs — with custom lights delivered in as fast as three working days.
- Range. Bar, ring, backlight, coaxial, dome, point, line and structured light all come from the same catalogue, matched to the same camera line — no multi-vendor integration headache.
- Cost. For equivalent geometry and power, domestic lighting typically lands 30–50% below imported equivalents — while the premium tier (Japanese suppliers such as CCS) still defines the top of the market for specialised wavelengths.
- Speed. Custom geometry (a specific bar angle, a specific dome curvature) is a routine request, not a project.
That combination is why domestic brands now dominate lighting for Chinese vision systems — and why overseas buyers increasingly source lighting from China while keeping premium brands only for exotic wavelengths.
Verification points before you buy
Lighting is the easiest component to overspecify and the hardest to blame when it fails. Our verification plan covers:
- Uniformity map at your working distance and field size — measured, not asserted.
- Illuminance at the part, with your intended geometry, using a calibrated meter.
- Spectral output at your working wavelength (critical for UV and narrowband work).
- Thermal drift over a 30-minute soak at your strobe duty cycle.
- Batch consistency — colour temperature and brightness spread across a sample batch, not a single unit.
- Integration check — controller behaviour, strobe sync with your camera trigger, warm-up time.
The five most common mistakes
- Buying brightness instead of geometry. More lumens cannot fix a wrong angle.
- Specifying the light before the lens and camera. The three are one system; specify them together.
- Ignoring the environment. Ambient light, window light and nearby station lighting all land in your image — shield or overpower them.
- Skipping the uniformity check. The cheapest light in the batch is the one that will generate false rejects.
- Treating lighting as a commodity line item. It is the highest-leverage component in your BOM.
The short version
Lighting geometry decides your image; the Chinese supply base decides your options and your budget. The winning combination is a domestic lighting line, specified as part of the imaging chain, and verified with measured uniformity and thermal data before the purchase order.
That is exactly how we build shortlists: geometry first, brand second, verification always — in our Selection Sprint and Sample Verification.
