Master the Color: A Simple Guide to Wavelength in Machine Vision

When setting up a machine vision system, one of the most common questions we get is: "Why can’t I just use a standard white light?" While white light works for many things, using a specific color (wavelength) is often the "secret sauce" that makes a difficult inspection possible. By choosing the right color, you can make hidden defects appear instantly.

Why Color Matters for Your Camera

Industrial cameras usually "see" in black and white (monochrome). They don't care about the color; they care about contrast. Contrast is the difference between light and dark. If your software can't tell the difference between a part and its background, the inspection will fail.

By using specific wavelengths, we "force" the camera to see exactly what we want.

The Two Golden Rules of Contrast

To keep it simple, you only need to remember two rules when choosing a light color:

1. The "Opposite" Rule (To create Dark Contrast)

If you want a colored feature to look black on your screen, use a light color from the opposite side of the color wheel.

• Best for: Reading barcodes, OCR (text), and finding dark spots or defects.

• Example: Use a Green or Blue light to make a Red label look dark and sharp.

2. The "Same Color" Rule (To create Bright Contrast)

If you want a colored feature to look white or "disappear," use a light that is the same color as the part.

• Best for: Seeing through colored plastic, or hiding background patterns so you can focus on something else.

• Example: Use a Red light on a Red part to make the surface look bright and even.

Common Colors and When to Use Them

•  Red (~630 nm) Typically used for general inspection. Offers stable performance and high sensor sensitivity, working perfectly on plastics, paper, and matte surfaces.
• Blue (~470 nm) Shorter wavelength scatters on the surface rather than penetrating it. This helps highlight fine surface details, making it the best choice for detecting scratches or micro-defects on shiny, reflective, or metallic parts like copper.
• Green (525 nm) Commonly used to maximize contrast by darkening warm-colored features. It is ideal for inspecting red or orange components in printing, packaging, and OCR applications.
• Infrared (850 nm) Invisible to the human eye but highly effective for reducing visual noise. It can "see through" certain inks and plastics to check internal features or fill levels without interference from surface printing.
• White (5500K) The universal choice for multi-colored applications.
  • With color cameras: Enables accurate color verification and sorting.
  • With monochrome cameras: Provides a balanced, natural contrast across different materials and diverse part sets.

The Reality: Why We Always Test

Even though physics gives us a great starting point, every application is unique. A "Red" plastic part from one manufacturer might have a glossy finish, while another has a matte finish. They will react differently to the same light.

How to Find the Perfect Light for Your Part:

1. Start with the Rule: Use the Color Wheel to pick the 2 or 3 most likely colors.

A simplified guide for wavelength selection.
Choose the opposite color on the wheel to maximize contrast or the same color to eliminate surface features.

2. Test the Angles: Sometimes it isn't the color that is the problem, but the angle. A Bar Light at 10° (Darkfield) will show very different results than a Ring Light at 90° (Brightfield).

3. Adjust the LWD: Moving the light closer or further away (Light Working Distance) changes the intensity and the "spread" of the light.

4. Validate: Once you find the light that makes the defect look the sharpest, you have found your solution.

Technical Application: Inspecting Copper Plates

To demonstrate the power of wavelength selection, let’s look at a common industrial challenge: Inspecting Copper Plates.

Copper naturally reflects wavelengths in the Red and Orange spectrum. If you use a standard White or Red light for this application, the copper surface will reflect the light directly back into the lens, causing "washout." This hides surface defects, scratches, and textured markings like serial numbers.

When Blue light hits a copper substrate, the material absorbs the blue wavelengths rather than reflecting them. On a monochrome sensor, this causes the copper background to appear dark (high contrast), while surface imperfections, edges, and laser-etched codes "pop" in sharp detail.

Wavelength Impact: Under Red light (625nm), the copper substrate reflects light, washing out surface details.
By switching to Blue light (465nm), the material absorbs the wavelength, darkening the background to reveal sharp text on the surface.

Key Takeaway

There’s no single “best” light color — performance is determined by material, surface, and system setup.
In many cases, lighting type plays an equally important role as wavelength — a topic we’ll dive into throughout this series.