Basic Introduction to Light Sources
Light sources are a key component of vision inspection systems. For AIC-Lite split-type cameras, external light sources are typically required. Whether light sources are selected properly directly affects image contrast, feature visibility, and the stability of downstream positioning, inspection, and measurement.
In 2D positioning and image acquisition tasks, performance depends not only on the camera itself but also on the illumination method. A proper lighting scheme can highlight target features, suppress background interference, and improve inspection accuracy by enhancing contour contrast.
Importance of Light Sources in Vision Inspection
Common industrial light source types include area lights, bar lights, ring lights, coaxial lights, and dome lights. These light sources differ in illumination direction, coverage, uniformity, and reflection characteristics, so their applicable scenarios are also different.
In real applications, the core principle of light source selection is to highlight target features, suppress background interference, and form stable and distinct image contrast.
For example, in scenarios where edges, contours, or printed features must be recognized, prioritize lighting methods that enhance contrast. In scenarios with glare, complex surface textures, or strong background interference, improve imaging by adjusting illumination angle, light source type, or polarization method.
Light Color and Wavelength
Light is an electromagnetic wave. The wavelength range from 380 nm to 760 nm is generally regarded as visible light. Wavelengths below 400 nm are typically classified as ultraviolet light, and wavelengths above 780 nm are typically classified as infrared light.
An object appears in a certain color because it reflects light in specific wavelength bands while absorbing light in other bands. Therefore, in vision inspection, selecting an appropriate light color can enhance differences between the target and background.
When using light sources, you can use relationships between similar colors and complementary colors to enhance contrast or suppress interference. In general:
-
When an object mainly reflects light of a certain color, using a similar light color often provides higher reflected brightness.
-
When you need to suppress certain background information and highlight target features, try a light color that contrasts with the target color to improve image separability.
In addition to visible light, infrared and ultraviolet light are also valuable in specific scenarios.
-
Infrared light has strong penetration and can be used to inspect some covered, semi-transparent, or subsurface features.
-
Ultraviolet light can excite fluorescence in certain materials and is suitable for detecting special markings, coatings, or contaminants.
In addition, how objects reflect, absorb, and transmit light also affects image quality. For example, black objects usually strongly absorb many light colors and thus produce low image brightness. Transparent or semi-transparent materials may show both transmission and reflection and should be validated with backlight, coaxial light, or other schemes.
Polarization Characteristics of Light
Light is an electromagnetic wave, and its vibration direction has orientation relative to its propagation direction. By using this property, polarization technology can control the transmission of light in specific directions, thereby reducing stray reflections or surface glare.
In vision inspection, polarization is commonly used to suppress strong reflections from highly reflective surfaces such as metal, glass, and plastic films, so real defects or surface textures are easier to reveal.
A polarization system usually consists of a polarizer and an analyzer. By adjusting the mounting position and orientation of polarizing filters, unwanted reflected light can be suppressed while preserving image information more suitable for inspection.
For integrated cameras, a common approach is to switch lens hoods to achieve full-transmission, semi-polarized, or fully polarized configurations. For split-type systems, polarizing filters are typically installed on the lens side or light source side and then tuned based on on-site image results.
Recommendations for Using Light Sources
When building a vision inspection solution, prioritize validation in the following order:
-
Clarify the core features to inspect, such as edges, scratches, characters, hole positions, defects, or surface textures.
-
Determine whether the main challenge is insufficient contrast, surface glare, ambient light interference, or insufficient brightness.
-
Select an appropriate light source type based on feature type, and test different illumination directions and installation angles.
-
If color interference is obvious, further test light sources with different wavelengths or colors.
-
If obvious glare or specular reflection exists, prioritize validating a polarization solution.
-
After contrast is ensured, further fine-tune camera parameters such as exposure time and gain.
A proper lighting scheme can significantly reduce the difficulty of subsequent parameter tuning and improve consistency and stability of inspection results.