Many of the lighting techniques commonly in use today have their origins in microscopy. Today, thanks to continual development over the years, they meet the stringent demands of industrial image processing. This is due in large part to the advent of the ISO-9000 standards, in which industrial image processing systems are used for quality assurance and documentation. Another area of application is in production, where yield and costs have been optimized through automation; for example, in the manufacture of chips for electronic equipment. The following outline is intended to provide assistance in selecting the best illumination for vision sensor applications.  

 

An image produced by a camera shows the light reflected from an object onto the camera's sensor chip. For subsequent image processing, it is important that the object shown in the image field can be reproduced reliably and with high contrast. Thus it is essential that the illumination be optimized for the reproduction of the object in question. Homogenous and constant lighting of the entire image field or object, independent of the surrounding area, is prerequisite for reproducible conclusions regarding position, dimensions or quality. The size and type of illumination are determined on the one hand by the shape and size of the reflective surface, and on the other hand by the distance and angle to the image field or object. Smooth surfaces are simple to illuminate, unlike recesses or indentations, for example, or cylindrical or spherical surfaces. The task at hand and the object's properties determine the characteristics of the lighting. At the same time, expectations regarding the service life of the light source are higher than ever. All in all, the wide range of lighting characteristics such as the properties of the light, the direction of the light source, and the properties of the illuminated field, yields a number of possibilities for combination.

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Lighting techniques

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The four principal types of lighting – reflected light, transmitted light, bright field and dark field – all result from the position of the camera and the lighting relative to the object. Reflected light and transmitted light can be used in a light field or a dark field, just depending on the object.

Reflected light

This type of illumination is used for the most part in microscopy and in machine vision applications. The light enters the lens of the microscope directly. Reflected light requires high-contrast objects on which the illuminated surfaces appear light in color on the image, while any unevenness appears dark.

 

Ring light

Ring light illumination is ideal for shadowless homogenous illumination of objects with matte surfaces, or surfaces that are not strongly reflective. Auxiliary components can be added to create diffuse, polarized lighting or a light that excites fluorescence.   

 

Directly reflected light

With this type of lighting, the object is illuminated by flexible or semi-rigid fiber optic cables. This method, too, is well suited for objects with matte surfaces, or surfaces with only weak reflection. Flexible fiber optic cables in various lengths are useful for illuminating areas otherwise difficult to access. ooo

 

Diffuse reflected light (dome)  
Diffuse reflected light is ideal for shadowless illumination of objects that reflect only weakly or not at all.  

 

Coaxial lighting

Coaxial lighting is required for objects with mirroring or strongly reflective surfaces. In a coaxial illumination system, or CIS, the light is produced by a diffuse light field and diverted to the object by a 50 percent transmissive mirror, so that the axis of illumination lies precisely on the optical axis of the camera. The advanced coaxial illumination system (ACIS) uses diffuse illumination as well.

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Line illumination

Line lights are mainly used as vertical illuminators, and occasionally for glancing light or transmitted light applications. Line illumination features good uniformity and homogeneity all the way to the line edges.

Dark field illumination

This method offers the considerable advantage that the object viewed is illuminated from the side in front of a dark background, so that only indirect light, reflected from the object, enters the camera. The result is an image of a brightly illuminated object on a black background. Optimum dark field illumination can make details visible that are otherwise difficult to distinguish. This is particularly true of smooth objects that have only very subtle differences between high and low points. Special ring lights, with a single row of dark fields that radiate approximately 85° to 90° to the optical axis, can highlight such structures extremely well.

Transmitted light illumination

Transmitted light or background lighting is excellent for measuring and monitoring contours. Background lighting can also be used as a diffuse vertical illuminator. Transmitted illumination is set up exactly opposite the optical system. The absorption of the test piece effects a homogenously bright, high-contrast image with near-binary properties.

To help ensure that bright field-transmitted light illumination as unaffected by contamination (dust) as possible, the lighting should be mounted at a distance from the test piece that is more than 3 times the depth of field. This ensures that even large particles on the light emission field are so far out of focus as to remain invisible.

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Polarized lighting
Unwanted reflections that occur with the bright field-reflected light method can be eliminated through the use of polarizing filters. The oscillation of unpolarized light on all oscillation planes is perpendicular to the propagation direction of the light. With a linear polarizer it can be filtered so that it only oscillates in the plane parallel to the transmission axis. With a second polarizing filter – the analyzer – placed directly in front of the camera, the polarized light can be absorbed completely by turning the analyzer so that its transmission axis is perpendicular to the first polarizer.

In practice, polarizing filters can be used to distinguish between direct and diffuse reflection, because polarization is maintained with direct reflections, while diffuse reflections change polarized to unpolarized light. Direct reflections are generally disruptive, because the intensity of the reflected light makes evaluation of the object difficult or even impossible.

Polarized light from direct reflection is absorbed by the analyzer, once it has been rotated 90°, while diffuse reflected light passes through the analyzer in the direction of the camera. Thus by rotating the analyzer between 0° and 90° you can control the amount of directly reflected light that passes through the filter. This lets you adapt the analyzer to prevailing lighting conditions. 

  

 

Are you looking for a solution that meets your own requirements? Just check the LINOS catalog, or contact our technical sales support team for a consultation..

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The author

Norbert Henze, is head of Product Management BU Catalog at LINOS in Germany.