Visible White Light
a. Square Bar Light
The square bar light consists of a combination of 4 high-intensity LED arrays to fit into a rectangular flat-board structure. Each array’s angle can be adjusted for different height applications allowing versatility.
Below images showcase three different angles of illumination in terms of the array’s lighting focal point producing different characteristics.
From the images, direct illumination is aimed towards the object directly with no angle, and high-angled illumination is aimed at an angle of around 60-degree parallel from the object. Both showcase very similar traits in terms of imaging, but with the high angle, the object receives a greater concentration of light resulting in more reflection than direct illumination. No components are hindered of light from top-view and there is greater clarity with brighter printings and a greater focus on the components.
For low-angled illumination as the projection of light is at a low degree from the side of about 20-degree, the details of the individual components are not generally well-lit with blind spots. However, the printings are considerably brighter and the side details of the components are more announced. This technique is best to perform edge detection, embossing printing and damaging check on surfaces and is ideal for slightly uneven surfaces and scratches. With a 360-degree illumination, reflective objects from an angle can be illuminated without producing reflections.
b. Dome Light
Dome light provides indirect illumination from an inner conical or square surfaces, which allows a workpiece to be uniformly illuminated from all angles, resulting in excellent illumination of the inspection object without creating any unwanted glare, hotspots or shadows. This illumination is suitable for applications involving inspection of reflective surfaces and contents, ideal for objects with glossy, non-flat surfaces, foil-wrapped objects or mixed reflectivity samples. Often, the smaller models are cylindrical in shape with the bigger models differing.
As shown above, the illumination level is constant throughout the details of the object with no reflection and blind spots. The disadvantage for dome light is that the sizing for it is fixed resulting in limited flexibility in the range of object usage. It also requires close proximity to object and it can get physically bulky for the bigger models.
Some applications of dome light are:
- Inspection of cracks, chips, stains, and marks2
- Solder and substrate inspection
- Inspection of printing on shiny metal or plastic packaging
c. Backlight Illumination
The backlight is a high intensity, uniform and collimated illumination that produces a very uniform surface light emission profile. It generates instant contrast as it creates dark silhouettes against a bright background, highlighting the object’s shape and outline distinctly.
As we can see above, the holes and outlines of the board are unmistakably shown.
Some applications of back lighting are:
- Detection of presence and absence of holes and gaps
- Detection of part placement or orientation
- Sizing measurement
- Shape recognition
- Edge detection
- Lead frame inspection
d. Coaxial Light
Coaxial light is a uniform light that runs perpendicular to the camera axis across the entire field of view. It functions by having light projected from its light source being reflected by 45˚ half mirror to the object providing a light parallel with the camera.
It is suitable for applications that involve the inspection of objects with transparent, translucent and reflective surfaces, as only the desired specular reflected light from the object is captured by the camera, thus producing a clear and bright image without reflection or glare. To be effective it requires close proximity to the object with lighting distance to be as short as possible.
As shown in the image above, components are generally well-lit with some components being evidently shown as compared to previous images, such as the screw heads and the electromagnetic coil. The disadvantage with coaxial light is that the working distance between the object and lighting are extremely close with the object required to be smaller than the illumination area restricting the flexibility of usage. This technique is ideal for plain reflective objects with little to no profile or any surfaces.
Some applications of coaxial light are:
- Surface inspection and alignment
- Wafer and metal surface inspection
- Inspection of film, LCD, glass, and pattern
- Enhancing angle differences, textured, or topographic features on relatively flat objects.
e. Polarized Light
Polarized light is produced when the polarization filter is placed in front of illumination. These filters are capable of avoiding reflections on images such as glass, liquid and reflective materials which usually and unavoidably are produced when the light diffuser is not used. Reflections may appear on dark and glossy materials as white spots that conceal inspection characteristics resulting in an unsuccessful inspection.
From the images above, we can see that through polarization that the metal parts appeared darker than usual. However, all the reflections from components are removed revealing the details which were covered before polarization. The disadvantage of this is that non-flat objects like cylindrical, curved surfaces or crinkled foils can hardly be illuminated in a homogenous way after polarization due to its natural characteristics.
For more information and products regarding the polarized filter, please click here.
f. Infrared (IR) Illumination
The infrared light operates from the wavelength range of approximately 780nm to 1000nm. As a result, it is capable of penetrating deep into materials and generate fewer surface reflections than visible lights, sometimes possible to look through an object. As Infrared light is not visible to the human eye, it is great for applications where the visible light would be uncomfortable. IR can help reduce colors, glare, and reflections for certain applications requiring imaging filters. Certain defects and flaw detection can be identified with IR where visible light is unable to.
From the image above, two types of bar lights (normal white light and infrared light) are utilized to demonstrate the difference in imaging results. When a piece of receipt paper is placed over both bar lights, the visible white bar light is unable to penetrate through the material, whereas the infrared counterpart is able to, producing clear imaging of the printing on the receipt paper.
Apart from infrared light being able to penetrate surfaces, it can also be used to diminish color effects on surfaces and reduce the effect of ambient illumination under artificial or indoor lighting.
Some applications of infrared light are:
- Inspection for fill level of packaging
- Reduction of color effect in monochromatic images
- Background removal to isolate characters for OCR
The use of colored light sources is essential in industrial machine vision. The LED illumination is available in different colors and emitted in small bands of wavelength, suppression of ambient lights not within the selected LED wavelength is achievable through color filters. The result is an image with the characteristics of the object still preserved with good contrast. Adding on, the selected color of light is able to alter the contrast of colors in monochromatic images, increasing or decreasing the difference in color effects. In industrial machine vision, both effects are combined. Colored illumination can be used in order to darken the complementary object color and to brighten up the same object color by additive and subtractive color mixing. Those with similar luminance values of grey can better be differentiated with red and green resulting in different shades of grey. In this way, it can be a substitution for color cameras by displaying contrasts in coloring.
This is demonstrated by having these four images above captured with a color camera. Under white light, the colors behave normally but when certain colors of light are projected, the object with similar color will be highlighted with a significant difference in brightness.
For more information and products regarding illumination, please click here.