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Illumination Technology for Machine Vision

Lighting is essential for industrial image processing. Type, color and intensity of the illumination create contrast which defines what is visible in the captured image. Therefore only homogeneous lighting conditions result in a robust and repeatable image capture. Fluctuating light conditions such as daylight are to be avoided.

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.

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.

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.
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.

Color Camera with White Light

Monochrome Camera with White Light

Monochrome Camera with Infrared Light

Ultraviolet light (UV) is invisible light with a wavelength below approximately 380 nm. Certain materials react when being illuminated with UV light and emit a visible light which can be seen by the camera (fluorescence). ​​​​Thus, it is necessary to use a filter for the camera optics to make sure that only the visible light emitted from the object reaches the image chip. However, ambient light contains a high proportion of UV light which results in the need of an enclosure in order to avoid saturation.

Without Lighting

UV Lighting

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 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.

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?

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. 

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. 
 

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.

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