Basics of machine vision lighting selection
The desired inspection result cannot be obtained unless the brightness or the light information of the inspection target is accurately captured by the camera, even if the image processing software and the camera are excellent. Therefore, it is necessary to select the appropriate lighting for the purpose.
It is indispensable to illuminate foreign objects to be detected such as dirt, characters and marks, measurement parts, etc. become clear as an image. After the illumination light hits the object, it is reflected in various forms depending on the surface condition and shape, and as a result, the object to be recognized is not reflected in the information (image) of the light entering the camera, or the illumination light emitting unit itself. In order to capture the object accurately, it is important to illuminate it with stable illumination depending on the object and purpose.
No lighting is applicable to all objects. It is necessary to select the most suitable method and equipment from various light sources and lighting methods according to various lighting conditions.
To achieve objectives
The first important thing to consider when selecting lighting is what to do with it.
Image processing applications fall into four main categories: visual inspection, OCR, dimensional measurement and alignment.
There are three major factors that go into making image processing a stable imaging application:
Color and Contrast
A color that is in the opposite position to the work color is called a complementary color and has the effect of making that color stand out.
The color next to it is called a similar color and has the effect of absorbing (blending in) the color. Use those effects to choose a complementary color if you want the background or unwanted colors to appear black, and a similar color if you want them to be white.
Also, due to the characteristics of the wavelength, the shorter the wavelength, the greater the scattering rate, so that minute scratches can be revealed.
White lighting will be often used for color cameras. Thus, the lighting color is selected based on the characteristics of the work and wavelength.
Relationship between work color and lighting color
Shape and Condition
Depending on the shape of the work, the light angle of the illumination may be constant and the regular reflected light of the illumination may enter the camera and cause halation on the inclined surface. If the surface condition is mirror-like, the illuminated part itself will be reflected. Depending on the shape and condition of the work, it is necessary to select the uniform and optimum illumination from the combination of coaxial-illumination method, direct-illumination method, indirect illumination method, transmitted illumination method and lens.
- Coaxial illumination
- Mirror surface such as IC pattern, LCD pattern mark, silicon wafer ID code, gold-plated mark (alignment)
A method of illumination perpendicular to the object from directly above along the optical axis of the lens. Primarily used on mirror-like objects (flat and highly reflective). For example, it is suitable for highlighting areas with slightly different reflectance, such as IC patterns, LCD electrodes, silicon wafer codes, and gold-plated marks. There are the following two methods depending on whether or not to pass the light through an optical lens.
Co-axial Spot light
The half-mirror and beam splitter are built into the lens barrel and are illuminated through an optical lens. This is also called telecentric lighting. It is only effective for objects with a high degree of reflection, such as mirrored surfaces, because it receives a positive reflection of light that is perpendicular to the object. Scratches and foreign objects will appear darker due to diffuse reflection and will contrast easily with the mirrored area. It is not suitable for objects that are not mirrored or for objects that are tilted and not made to be a direct reflection.
Spot light/High power spot light
External Co-axial Box Illumination
A method of illuminating an object vertically with a half mirror or beam splitter that does not pass through an optical lens and makes the illumination optical path the same as the optical axis of the lens. Because it does not pass through an optical lens, it can handle objects that are not mirror surfaces to some extent and can be used in combination with lenses other than telecentric lenses.
In that case, it is necessary to make the mirror size and aperture size larger than the telecentric lens.
External Co-axial Box Light
- Direct lighting
- Edges with many irregularities that are not mirror surfaces, surface scratches, cracks, etc.
It is a method of directly illuminating an object and is the most frequently used lighting method.
It is mainly used for detecting edges such as scratches and cracks on uneven surfaces of objects that are not mirror surfaces. Since it is applied directly to the target object, it is used when brightness is required. It is used in various shapes and angles depending on the shape of the workpiece, such as ring light, low angle light, horizontal light, bar light, surface light and line light.
A ring-shaped light (360° ) that prevents shadows from forming. Easy to clamp and easy to install, it is used in a wide range of applications such as surface scratch inspection and dirt check and 2D code reading. Various angles are available from flat type to horizontal to the work.
In the case of a mirrored workpiece, combination with a polarizing filter may prevent the light from being reflected on the workpiece, or depending on the installation distance, light may be omitted or unevenly distributed depending on the directionality of the lighting.
Ring/Flat/Low Angle/Low Angle Horizontal
Direct lighting system
A lighting system that emits light in the shape of or on the line.
It is used to illuminate only the necessary part of the line when it is recognized by a line sensor camera.
For area camera applications, its sharp and excellent linearity allows for recognition methods through emphasizing minute irregularities by illuminating from an irradiation angle close to the horizontal direction. Depending on the application, a condenser lens, diffusion plate, and polarizing filter may be used together.
Backlight/Direct type transmitted illumination
This lighting is used to capture the silhouette of an object to be inspected by placing the object between the lens and the illumination. It is mainly used for measuring the shape of an object and detecting scratches on transparent bodies and foreign matter. It provides a stable image because of the easy contrast. However, the luminescent surface is larger than the object and requires more space for the light to be installed on the rear side. To reduce the amount of light surrounding the lens, there is also a method that uses a condenser lens that matches the NA of the telecentric lens on the light receiving side.
- Indirect lighting system
- Objects with irregularities and heights, mirror-like objects with R, solder joints of printed circuit boards, etc.
In normal lighting systems, the directionality is constant. Therefore depending on the shape of the object, it may not be recognized due to unevenness in brightness created by strongly reflected area. In addition, there are problems with curved mirrored objects, such as the lighting emitting part being reflected in the object. To solve these problems, we use an indirect lighting system that irradiates in a variety of directions.
Dome/Shadowless Ring/Shadowless Low Angle
This is an indirect lighting system in which light is diffused and softly illuminated by passing through a nearly semispherical, dome-shaped reflector and a light guide plate that shines only the reflected light from below onto an object. As a result, the light hitting objects in various angles makes it possible to obtain an uniform illumination without uneven brightness even if it is flat or curved.
However, it is not suitable for high-magnification imaging or high-speed imaging because the illuminance lowers as it hits the reflector or passes through the light guide plate.
FOV and lighting installation distance
A telecentric lens does not have an angle of view even in the peripheral part of the FOV, so when adopting a direct light, the light size can be a little larger than the FOV. Since normal lenses have an angle of view, it is necessary to select the light size in consideration of the effect of the angle of view in the case of direct illumination or transmitted illumination. It is necessary to select a light with adequate consideration on the angle of view and the hollow.
Light source comparison
|Item||LED lighting||Halogen light source + optical fiber||Fluorescent lighting|
|Lifespan||About 30,000h||100w/about 1,000h 150w/about 50h||About 2,000 – 5,000h|
|Absolute illuminance||High brightness with quantity and power elements||High illuminance
(tens of thousands of Lux -)
|Bright but no light condensing|
|Illuminance control||0 – 100%
(depending on the power source, etc.)
|Lighting on/off||Very responsive||No (mechanical shutter type supported)||Impossible|
|Cable routing||Easy due to the small diameter||Depends on bend R||Depends on cable diameter|
|wavelength||Can be manufactured for each wavelength range||Visible range||250nm – visible|