What Makes Lighting Technology Crucial in Industrial Machine Vision?
Machine Vision illumination is a fundamental component for creating contrast for efficient image processing. Without light there is no way for a camera to “see” the features to be inspected. Using LED illumination in the correct way will create black on white contrast that allows an image to be processed with a high throughput. The brightness of the lighting is also critical for Machine Vision. With a higher intensity of light, the image processing will be more robust and repeatable.
More power enables to …
• … reduce the exposure time, thus eliminating motion blur and limiting the impact of ambient light.
• … close the aperture which results in a bigger depth of field.
• … reduce the exposure time, thus eliminating motion blur and limiting the impact of ambient light.
• … close the aperture which results in a bigger depth of field.
What Is Light?
Illumination technology emits light in several wavelengths of varying intensity. The radiation emitted from the sun comes in a huge range of wavelengths from ultraviolet light through to infrared. Machine Vision illumination comes in a range of wavelengths mostly in the visible spectrum.
For example, a red light illumination can have its peak at about 630 nm, as can be seen in the chart. The red light emitted from an LED has a wide spectrum where the intensity across the spectrum falls off from the peak.
The human eye is particularly sensitive to color variations. Two products with the same color may appear differently to the eyes of the user. However, a strict bin sorting in the selection of LED’s used in wenglor illumination products ensures that the maximum variation of the peak wavelength does not exceed 10 nm. The image chip of a camera or a barcode scanner has different sensitivities to different wavelength ranges specified in the respective operating instructions. For optimal brightness conditions, both the image chip sensitivity and the type of light must be matched to each other. External blocking filters can be used to avoid extraneous light. In some cameras, a filter is already installed.
The human eye is particularly sensitive to color variations. Two products with the same color may appear differently to the eyes of the user. However, a strict bin sorting in the selection of LED’s used in wenglor illumination products ensures that the maximum variation of the peak wavelength does not exceed 10 nm. The image chip of a camera or a barcode scanner has different sensitivities to different wavelength ranges specified in the respective operating instructions. For optimal brightness conditions, both the image chip sensitivity and the type of light must be matched to each other. External blocking filters can be used to avoid extraneous light. In some cameras, a filter is already installed.
How to Choose the Right Illumination Color?
Color Machine Vision allows the detection of color differences similar to the human eye. These differences cannot be recognized with the 256-level gray scale of monochrome cameras. Processing times therefore increase slightly when using color cameras due to the large amount of information.
In combination with monochrome cameras, however, the use of colored light sources is crucial to define the desired gray scale values of specific features to create contrast. Thus, for example, red and neighbouring green objects appear in different shades of gray in the captured image. However, good contrast can be achieved when the background color is chosen in the complementary color of the illuminated object. This increases the inspection stability of the Machine Vision system.
In combination with monochrome cameras, however, the use of colored light sources is crucial to define the desired gray scale values of specific features to create contrast. Thus, for example, red and neighbouring green objects appear in different shades of gray in the captured image. However, good contrast can be achieved when the background color is chosen in the complementary color of the illuminated object. This increases the inspection stability of the Machine Vision system.
Visible light is characterized by the fact that its radiation is within the wavelengths from approximately 380 nm (violet) to approximately 780 nm (red). Using a monochrome camera in combination with red light, for example, a red part will appear white for the camera while a blue part will appear black.
Color Camera with White Light
Monochrome Camera with Red Light
Monochrome Camera with Blue Light
Monochrome Camera with Green Light
Infrared light is invisible light with a wavelength exceeding approximately 780 nm. This can be useful to avoid operators being exposed to powerful light. Infrared light works best for plastic parts and is not suited for metal parts. However, testing is necessary when illuminating black objects.
In the example, plastic objects of different colors are inspected. When illuminated with infrared light, the objects appear white in the captured image as the light goes through the material and is not reflected by it. An infrared backlight can even illuminate objects through a non-transparent conveyor belt.
In the example, plastic objects of different colors are inspected. When illuminated with infrared light, the objects appear white in the captured image as the light goes through the material and is not reflected by it. An infrared backlight can even illuminate objects through a non-transparent conveyor belt.
Color Camera with White Light
Monochrome Camera with White Light
Monochrome Camera with Infrared Light
Ultraviolet light (UV) is invisible light with wavelengths below approximately 380 nm. Certain materials react with UV light when illuminated and emit visible light, which in turn can be detected by a camera (fluorescence). It is therefore necessary to place a bandpass filter on the lens of the camera to make sure that only the object’s light generated by fluorescence hits the image chip. The UV light that generates the fluorescence is not permitted to pass through.
Without UV Light
With UV Light
What Is the Effect Using a Polarization Filter?
A polarization filter is an accessory for lights and cameras to improve contrast by reducing glare and unwanted reflections. It is mainly used for applications including shiny or transparent materials. The advantages of polarization can be achieved through the use of a linear polarizing film on the light combined with a polarizing filter placed on the camera.
Without Polarizing Filter
With Polarizing Filter
What Is UV Light?
Ultraviolet (UV) light is electromagnetic radiation with wavelengths in the spectral range of 280 to 400 nm, which is invisible to the human eye. When UV illumination is combined with a luminescence substance (luminophore), the emitted light of the luminophore is visible. This process is called fluorescence, where light is emitted in response to excitation by UV light.
Fluorescent substances exposed to UV radiation absorb them and emit them as visible, longer-wave radiation, typically in the colors blue (470 nm) or green (525 nm). This emitted light can then be used for visual inspection.
Fluorescent substances exposed to UV radiation absorb them and emit them as visible, longer-wave radiation, typically in the colors blue (470 nm) or green (525 nm). This emitted light can then be used for visual inspection.
How Is UV Light Used?
Ultraviolet light has a wide range of applications in medical, industrial and commercial areas. It is used for disinfection, hygiene control as well as the detection of security inks and fluorescent, counterfeit-proof packaging features. UV fluorescent security markings are used to fight counterfeiting and authenticate brands through 2D codes, barcodes, alphanumeric codes, images and graphics. These markings ensure the authenticity of products and support traceability in the supply chain. The fluorescence of UV markings depends on the type of ink used. Some inks fluoresce over a broad spectrum of UV wavelengths, while others reflect on more specific wavelengths.
What Happens When Light Hits a Surface?
When installing a Machine Vision system, it must be considered how the light photons react when they strike a surface. It is important to map out how the light will reach the camera. The light can react in five common ways: reflection, transmission, absorption, fluorescence and diffusion. However, a large number of effects appear simultaneously.
How Does Light React to Different Surface Materials?
Light Reflection on a Shiny Surface
When light irradiates shiny surfaces, most of the light photons reflect in the same incident angle. However, even a highly polished mirror reflects approximately 95% of the incident light.
Light Behavior on a Surface Inbetween Shiny and Matte
When light irradiates surfaces that are neither highly polished nor extremely rough most of the light photons scatter in many different angles. Despite the scattering, most of the light intensity will follow the reflected incident angle.
Light Diffusion on Matte Surface
When light irradiates surfaces that are very rough or matte the light photons will scatter. In theory, an ideal diffuse surface reflects the light in all possible directions. In this case the highest light intensity follows the normal (90°) to the surface.
How Do Different Shapes of Surfaces Reflect Light?
The form of the inspected object is decisive for the way the light reaches the camera to create contrast. In the diagrams, the light output is simplified and the illuminated surface is assumed to be a perfect mirror. The inspected indents are representative of a dot peen marked code but simplified down to a single groove.
The dotted blue lines show the light reflection which reaches the camera and is seen in the image as white. The dotted gray lines show that the light does not reach the camera, this creates an absence of light in the field of view.
The dotted blue lines show the light reflection which reaches the camera and is seen in the image as white. The dotted gray lines show that the light does not reach the camera, this creates an absence of light in the field of view.
Feature on a Flat Surface
By placing the camera in the reflected incident angle of the illumination, most of the light photons reach the camera. Indents cause an absence of light that shows the surface feature with good contrast.
Feature on a Curved Surface
On a curved surface, most of the light photons do not reach the camera. Due to this, applications with a curved surface normally require a larger light or a light that comes from many directions around the object.
Where Do I Need to Place my Illumination Relative to the Camera?
Extended Illumination Variants for Spot Lights
In combination with matching lenses, the LSL spot light enables the implementation of two further specialist illumination solutions that meet different lighting and image quality requirements.
Backlight with Telecentricity
A special backlighting method can be achieved with telecentric lenses. In combination with the LSLx003 spot light, the telecentric lens becomes a powerful telecentric backlight. Two telecentric lenses are aligned with each other and the light from the backlight is positioned precisely along the optical axis of the camera.
A typical application example is the inspection and measurement of the neck of objects such as ampoules or preforms.
This illumination method is mainly used for precise measurements of round or transparent objects, as well as for position and presence checks or surface inspections. Particularly with light-transmitting materials, such as glass or transparent plastic, the parallel light changes its direction through the telecentric lens when it hits the object. Since the camera’s telecentric lens receives only parallel light, the deflected light does not enter the camera, meaning that only the silhouette of the transparent test object is visible.
Coaxial Illumination
Coaxial illumination is a special illumination method in which the light is coupled axially, i.e. in line with the optical axis of the camera. A semi-transparent mirror (beam splitter) scatters the light directly into the camera’s visual field and onto the object to be tested. The light reflected from the object surface passes through the beam splitter again and is returned to the camera, which thus only captures the directed and reflected light.
Applications include quality control of flat surfaces, detection of fine structures and inspection of shiny surfaces such as metal, wafers or image sensors.
This type of illumination is the only way to achieve shadow-free and even brightfield illumination across the entire field of view (FOV) of the lens.
How Is Homogeneous Illumination Created for the Camera?
Uniform illumination is an important prerequisite for reliable results in industrial image processing. In practice, however, the light distribution within a visual field is often not homogeneous. Particularly with bar lights, local intensity peaks, as well as brightness differences between central and peripheral areas, can occur.
In order to compensate for these effects, the light distribution is specifically adjusted for the curve effect. This allows the brightness perceived by the camera to be homogenized across the entire visual field.
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1
How Is Light Reflected from a Surface?
When light hits a surface, it is reflected according to a Lambertian distribution.
The intensity of the reflected light depends on the viewing angle.This means that not all areas of a surface appear equally bright.
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How Does a Traditional Illumination Solution Work?
With bar lights, the light consists of several individual light sources, each of which follows this distribution.
As a result, there often arises an uneven intensity distribution across the entire range.
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How Is the Illumination Detected by the Camera?
The camera records the light distribution according to the reflected intensity.
This can lead to a stronger weighting of central areas, resulting in so-called hotspots in the image.
These local overexposures cannot be detected with a luxmeter and can affect image evaluation.
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How Does the Curve Effect Ensure Even Illumination?
The curve effect adjusts the light intensity using angle changers in a targeted manner.
By controlling the light distribution, the brightness perceived by the camera can be balanced across the entire surface.
This creates an image with homogeneous illumination.
How Is the Curve Effect Implemented in Concrete Terms?
Different angle changers are combined along the illumination to create the curve effect. These are special light diffusers that are attached to the front of bar lights and specifically influence the beam angles. Depending on the design, flat, medium or wide illumination can be created.
A wider angle changer is used in the middle of the illumination than in the edge areas. This reduces the light intensity in the center, while illuminating the periphery more, creating an even distribution of brightness across the entire visual field.
The influence of the curve effect is clear in a direct comparison: The sample images were taken under identical conditions with no angle changer and while using three angle changers.
- In the first case, the illumination is operated with no angle changer, resulting in uneven illumination with intensity peaks in the center.
- In the second case, angle changers are used, with the middle angle changer having a larger beam angle. As a result, the light intensity is more diffused in the center and hotspots are reduced.
The curve effect can be used in applications that are illuminated in brightfield. A prerequisite is also a sufficient length of the bar light, as the effect can only be achieved from a length of 375 mm.
What Is the Effect Using Different Operating Modes?
Continuous Light
Continuous mode is when the light is turned on constantly or for a time much longer than the exposure time of the camera. As an example, the light may be switched on two seconds before image capture and then switched off two seconds after.
The most important advantage of continuous mode is the comfort for people in the environment.
The most important advantage of continuous mode is the comfort for people in the environment.
Strobe Light
Strobing the light means that the LEDs from the product will be switched on and off via an external signal, typically from a PLC/SPS or directly from the camera. When using a light in strobe mode, the light is turned on for the time the camera takes to acquire the image. This means the light is fully illuminated over the entire exposure time.
The main advantage of strobe mode is an increased LED life time due to less heat building up in the product. In some cases, especially for applications with large illumination installations, energy consumption can be reduced significantly.
In addition to the advantages of strobe mode listed above, the higher current enables a higher brightness which enables a lower exposure time and a reduced aperture for the camera.
The main advantage of strobe mode is an increased LED life time due to less heat building up in the product. In some cases, especially for applications with large illumination installations, energy consumption can be reduced significantly.
Strobe OverDrive Mode
Strobe OverDrive mode is a term used for LED lights that allow a higher current to be put through the LEDs with respect to a defined duty cycle. All wenglor OverDrive LED products contain an internal controller to enforce the duty cycle; this way, the product is protected.In addition to the advantages of strobe mode listed above, the higher current enables a higher brightness which enables a lower exposure time and a reduced aperture for the camera.
