Machine Vision Light Source Colour: Wavelength Selection and Photolithography Yellow Light

POMEAS optical applications team · Reviewed by Tiger Xie · Published 2026-10-14

Quick answer. Light source colour is not chosen by matching the colour of the part. Three questions decide it: do you want the target to get brighter or darker in the image, how do target and background have to differ, and which wavelengths does the material reflect, absorb or transmit. A monochrome camera does not see red or blue — it sees a grey-level difference, and the wavelength is simply the tool that creates it. Every POMEAS wavelength quoted below is taken from published product specifications.

1. Decide first whether the target should get brighter or darker

With a monochrome camera the sensor does not record red, green or blue; it records a grey level for each pixel. So before choosing a colour, answer one simple question: do you want the inspection target to be brighter or darker in the image?

• To make a red character brighter, test red light first.
• To make a red character darker, test blue or cyan-direction light.
• To preserve the true colour of the product, test white light first.
• To reduce the influence of some visible printed patterns, test infrared.

Once target and background form a stable grey-level difference, identification, positioning and defect judgement all become easier. The three questions above are worth more than a long optical specification list at the beginning of a project.

2. Same-colour light and complementary light

When light of the same colour as the target falls on it, the target generally reflects more of that band and appears at a higher grey level on a monochrome camera. When light from the complementary direction is used, reflection of that band is weaker and the target tends to read darker.

Target colour	To make the target brighter	To make the target darker
Red	Test red light first	Test blue or cyan-direction light
Green	Test green light first	Test red or magenta-direction light
Blue	Test blue light first	Test yellow or red-direction light

This is only an initial rule. Ink, metal, plastic, coatings and matte surfaces reflect different wavelengths differently, and two surfaces that look the same colour to the eye can end up at very different grey levels. When the colour relationship is complicated, an image of the part plus its inspection position resolves the question faster than a full parameter sheet.

3. Choosing among the common light source colours

Colour	Common use	What to watch
White	Colour identification, labels, packaging, general viewing	Natural colour rendering, but not necessarily the highest grey-level contrast
Red	Character reading, some films, profiles, ambient-light rejection	Does not mean it transmits through every film or material
Blue	Fine scratches, surface texture, very small features	Incident angle is often more important than the colour change
Green	Edges, contrast work, some reflective materials	Has to be considered against camera response in that band
Yellow	Special materials, photosensitive processes, specific colour contrast	Cannot be judged from the perceived yellow appearance alone
Infrared	Reducing some visible pattern interference, viewing material differences	Does not penetrate every plastic, ink or coating

White light: preserving true colour

For judging product colour, labels, packaging or colour print content, a colour camera with white light is the usual combination. But if the target and background are different colours yet very close in luminance, white light may not be the highest-contrast option.

Red light: characters and some film projects

Red light transmits and scatters differently from blue in some materials, which is why it is commonly used for characters, profiles, features under some protective films, and with a red filter to reduce ambient-light interference. Whether it transmits through a film still depends on the film material, colour, thickness and surface coating.

Blue light: some fine surface features

Blue light has a shorter wavelength and can scatter more readily on some rough surfaces and fine structures, which may help reveal scratches, texture and very small defects. Not every scratch suits blue light; in many cases changing the illumination angle is more effective than changing the colour.

Infrared is not a universal penetrating light

Infrared can reduce the influence of some visible colour and print patterns, but the transmittance of different materials varies widely between infrared bands. A near-infrared source is also not the same thing as a longer-wavelength infrared imaging system.

The published POMEAS wavelength set across the light source families is:

Colour	Peak wavelength or colour temperature
Red	620–630 nm, and 645–660 nm
Green	525 nm
Blue	470 nm
White	6500 K (colour temperature)
Infrared	850–950 nm
Ultraviolet	365 / 395 nm

Ultraviolet is published as a family that spans ring, bar, area, coaxial and point formats at 365 nm and 395 nm, and is documented for applications where a material contains a fluorescent compound, such as UV adhesive overflow inspection, anti-counterfeit code reading and UV curing. Infrared is published across the same families at 850 nm and 950 nm for projects where penetration matters more than visible-light performance.

Which families are published in which colours

The colour set is not identical across every product family. The published coverage is:

Family	Published colours	Published electrical data
Ring light (GH series)	Red / white / blue on standard part numbers; infrared and ultraviolet variants also published	24 V; 0.4 W to 41.8 W depending on model
Shadowless ring light (GHN series)	Red / white / blue	24 V; 8 W to 31 W
Coaxial light	Red / green / blue / white	Controller VP-DPI2404-4-RS232T
Bar light (GL series)	Red / white / blue; infrared variants published	24 V
Flat / parallel-flat area light (GBT series)	Red / white / blue	24 V; 0.8 W to 29 W
Point light (GP series)	Red / white / blue	5 V; 1 W and 3 W
Telecentric parallel backlight (VP-GLTD / VP-GLTDL)	Red / green / blue / white	3.5 V; 350 mA
Ultraviolet family	Ultraviolet, 365 / 395 nm, across ring, bar, area, coaxial and point formats	24 V (point format 5 V); 1.7 W to 6.2 W
Infrared family	Infrared, 850 / 950 nm, across the same families	24 V (point format 5 V)

A colour that is published for one family is not automatically available in another, so a wavelength requirement has to be checked against the family you intend to mount, not against the brand as a whole.

4. Matching a colour camera or a monochrome camera to the light

When true colour has to be preserved, a colour camera and white light are the normal pairing. When the objective is only to read characters, detect defects, extract profiles or separate different colour regions, a monochrome camera with monochromatic light more often produces a stable grey-level difference.

Camera type	Common pairing	Primary objective
Colour camera	White light	Preserve true colour information as far as possible
Monochrome camera	Red, blue, green or another monochromatic light	Actively increase the grey-level difference between target and background

After changing the light source colour, exposure, gain and aperture have to be re-set. The previous camera parameters cannot simply be carried over, because the camera's spectral response and the effective irradiance both change.

5. Case: why photolithography asks for yellow light

A photolithography customer once stated the requirement directly: we need yellow light, and it must not contain blue.

That requirement cannot be read as "change the white light to yellow". Photoresist is a photosensitive material, and some resists respond not only to ultraviolet but also to short-wavelength visible light. If the source contains short-wavelength energy that the process forbids, the resist or another photosensitive material can be affected even though the light looks yellow to the eye.

The sensitive range is not identical between different resists, materials and processes, so a fixed wavelength cannot be assigned from the words "photolithography industry" alone. What has to be established is the specific photosensitive material, its sensitive band, the forbidden range defined by the process and the permitted exposure dose.

6. Why light that looks yellow can still contain blue

Warm-white or yellowish LEDs

Some warm-white LEDs use a blue emitter chip to excite a phosphor. The light looks warm or yellowish to the eye, but the complete spectrum can still contain a distinct blue peak.

A mixture of red and green

Red and green combined also look yellow to the human eye, but that is not a single yellow band, so the complete spectrum still has to be checked against the process requirement.

Direct-emitting yellow or amber LEDs

A direct yellow or amber LED usually has a more concentrated spectrum, but the centre wavelength, the spectral bandwidth and the presence of energy at the short-wavelength end still have to be confirmed against the customer's forbidden range.

Looking yellow is not the same as containing no blue. When a requirement like this arrives, the useful first step is to establish whether the customer is restricting the perceived colour, the centre wavelength, or the whole short-wavelength range — because those three lead to different specifications and different verification methods.

7. Turning "no blue light" into an acceptance requirement

Writing "No Blue Light" on a drawing is intuitive but very difficult to use as a production and acceptance standard. A workable requirement normally specifies:

• the forbidden wavelength range;
• the maximum permitted energy within the forbidden band;
• the centre or dominant wavelength of the source;
• the spectral bandwidth;
• the actual illumination distance;
• single and cumulative exposure time;
• whether a full spectral test report on the finished source is required.

The final verification should be performed on the complete finished light source, not on the LED chip name or the chip supplier's isolated data. The diffuser plate, the lens, the protective glass, the filter, the drive current and the operating temperature can all change the output spectrum. A customer who only says "yellow light, no blue" is still a workable starting point — the requirement can be turned into a testable specification step by step.

8. An eight-step colour and wavelength selection procedure

• Confirm the camera type — monochrome or colour.
• Establish target and background colour — and why they are currently hard to separate.
• Decide brighter or darker — fix the image effect you want first.
• Judge the material — understand how it reflects, absorbs and transmits.
• Compare candidate colours — white, red, blue, green, yellow or infrared.
• Re-tune the parameters — filter, exposure, gain and aperture together.
• Confirm process constraints — photolithography and photosensitive material projects must define the forbidden band.
• Complete a real-sample validation — confirm the final option from raw images and the complete spectrum where required.

The customer does not have to complete all eight steps before contacting us. One image, one camera model or one sentence describing the inspection is enough to begin.

9. Common colour selection mistakes

• Assuming a red part must use red light.
• Assuming blue light is always better for scratch detection.
• Concluding that light contains no blue because it looks yellow.
• Assuming infrared penetrates all plastics and inks.
• Changing the light colour without re-adjusting exposure.
• Reading only the LED colour name without checking the complete spectrum.
• Testing the LED chip only, and not the complete finished light source.
• Ignoring the band response of the camera, lens and filter.

10. What to send when you are not sure

A complete technical file is not needed before asking. The smallest useful input is:

• an image of the part;
• the current captured effect;
• the camera or lens model;
• the original requirement as the customer stated it;
• a short note on which position has to be inspected.

Establishing the direction first, and adding the remaining information as the project develops, is faster than researching every wavelength and optical parameter up front. If you are unsure between red, blue, yellow and infrared, send one image or one sentence through Contact and we can start from the most likely direction.

Frequently Asked Questions

Does a red part always need red light?

No. To make a red region brighter on a monochrome image, test red light first; to make it darker, test blue or cyan-direction light. Which of the two you want depends on whether the target or the background should stand out.

Is blue light always better for scratch detection?

No. Blue light can help reveal some fine surface structures, but scratch direction, light source angle, surface roughness and lens imaging are equally important. In many projects, changing the illumination angle is more effective than changing the wavelength.

Does yellow light always mean there is no blue component?

No. Warm-white LEDs, phosphor-converted sources and mixed light can all look yellowish while the complete spectrum still contains blue or other short-wavelength content. The perceived colour is not a measurement of the spectrum.

Can infrared light penetrate all plastics?

No. Transmittance varies with the plastic type, the pigment, the thickness, the surface coating and the infrared band used. Real samples have to be tested.

What wavelength should I choose for photolithography yellow light?

There is no single wavelength that applies to every photoresist. The choice has to combine the specific resist, its sensitive band, the range the process forbids and the permitted exposure dose. It also has to be verified on the finished light source, not on the LED chip alone.

Should I use a colour camera or a monochrome camera with coloured light?

Use a colour camera with white light when true colour has to be preserved. Use a monochrome camera with monochromatic light when the objective is to increase the grey-level difference between target and background for characters, defects, profiles or region separation.

How do I turn a no-blue-light requirement into a testable specification?

Specify the forbidden wavelength range, the maximum permitted energy inside that band, the centre or dominant wavelength, the spectral bandwidth, the illumination distance, the exposure time and whether a full spectral test report on the finished source is required. Then verify the complete source, because diffusers, filters, drive current and temperature all shift the output spectrum.

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Talk to a POMEAS Engineer About Your Lighting Setup

If the part is in front of you and the image is not yet doing what you need, send the part image, the current capture, the camera or lens model, the region you need to inspect and the line speed. We can start from the illumination direction rather than from a product name.

Email: [email protected]  |  WhatsApp: +86 185 9810 2007

Please include: part material and surface condition, required field of view in mm, smallest defect in µm, working distance, available mounting space, whether strobe or polarisation is required, and the permitted exposure time at production speed.

Reviewed by Tiger Xie.