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Lighting is the most important aspect of any machine vision application. Without proper lighting it is impossible to obtain a proper image. Without a proper image no machine vision installation will be successful

Lighting is the one area of installing a vision system that usually requires some experimentation. It is virtually impossible to predict the exact lighting arrangement that will give the best results for a new application. This guide and the lighting techniques sections have been included to make this experimentation as efficient as possible. The main objective of lighting the object is to provide a vision system with a consistent image that clearly shows the features to be inspected.

 Light levels

 For fast moving production lines a fast shutter speed is likely to have been selected on a camera. When fast shutter speeds are used bright lighting is required, such as low-voltage spotlights, fluorescent backlights or fiber optic illuminators. For indexing or lower speed production lines, virtually all types of lighting can be used. 

It is not a good idea to use the general 'background' lighting and daylight for illumination. This is unlikely to provide consistent lighting levels. It is normal to make application lighting sufficiently bright so that variations in ambient light levels have little effect on the image.

 Low-voltage and fluorescent lamps have a limited life and will reduce in intensity with age. It is recommended that a regular lamp replacement program be implemented. As most lamps are inexpensive, replacing them half way through their rated life is a good approach.

 Installation

 With any lighting arrangement, some final adjustments will be required to light positions and angles. Make sure that this is carried out before deciding exactly how to mount the lights. It is also important to mount the lights rigidly and clamp them in position. Any movement in the lighting will change the image appearance and could effect the inspection being performed.

 Lighting fluctuation

 When using shutter speeds shorter than 20mS an effect known as "beating" occurs when using lighting that is modulated at a 60 Hz frequency. Normal tungsten filament bulbs, standard fluorescent lights and low-voltage lamps driven from non-regulated supplies all produce a heavily modulated output. The "beating" causes the average intensity of the image to fluctuate, making it almost impossible to set up a reliable inspection. Low-voltage bulbs with a DC supply, high frequency fluorescent lamps and LED lights all avoid this problem.

 Front lighting

 This is used for straightforward applications where features on the surface of the object are to be inspected. It is the simplest form of lighting, where one or more lights are shone onto the object from the same side as the camera. Typically the light(s) will be angled at about 45 degrees, with the camera directly above the object. These techniques are best used on surfaces which are not too reflective.

 Direct front lighting

 This is an easy technique that uses 1 or more lamps shining directly onto the object. Experiment with the number of lamps, their distance from the object and their angle relative to camera. In some cases it may be better to inspect the shadow cast by a feature than the feature itself. 

 This technique is used for all kinds of different objects where the presence or absence of a surface features require inspection such as assembly inspection or counting features. This technique is unlikely to work for reflective surfaces.

 Diffuse front lighting

 By adding diffusers to lights, contrast is reduced and some features become clearer. Experiment with the number of lamps, their distance from the object and their angle relative to camera. It is easy to compare Direct front lighting and Diffuse front lighting by removing the diffusers whilst looking at the live image. Note that the diffuser will significantly reduce the light intensity.

 Suitable lights for small objects are the IM-DSL1 low voltage spotlight fitted with IM-DDK1 diffuser kit. For larger objects the IM-FBL1 narrow backlight or the IM-FBL2 wide backlight can be used.  

 This technique is used where Direct front lighting produces an image with too much contrast and important features are hidden within dark or light areas in the image. 

 Ring lighting

 When inspecting circular objects it is often a good idea to use a circular light. This gives very even illumination around the object. Experiment by changing the height of the lamp. It will have a very great effect on the appearance of the object.

 Suitable lights for small objects are the VSE 50/28 ring light. For larger objects the VSE 90/60 ring light or the VSE 60/34 diffuse ring light can be used. This technique is used for looking at round objects. Ring lights also provide very even illumination when looking at small objects or small areas of larger objects.   

 Low angle lighting

 To highlight shallow surface features, low angle lighting is a useful technique. The angle between the light source and the surface to be inspected is kept small, so that the light is just glancing off the object. Raised textural features will appear bright against the dark background of the flat surfaces of the object. For this technique to operate reliably the objects must be presented at a constant angle relative to the light source.  

 Suitable lights for small objects include the IM-DSL1 low voltage spotlight and the VSE 440 LED floodlight. For larger objects the IM-FSL1 narrow strip light can be used. 

 This technique will highlight dust on a surface, small dents, screw threads and scratches. 

 Back lighting

 Where only the silhouette of an object is required, back lighting can be used. This is simple to set up and is a very robust technique. A diffuse light source is placed behind the object. A camera views the resulting profile from the front. As this technique requires access to both sides of the object it may not be suitable in all cases. 

 Diffuse back lighting

 Back lighting provides high contrast and is easy to set up. It is used in applications where all features to be inspected can be seen in the silhouette of the object. The object is placed between camera and the backlight. On some production lines it may not be possible to achieve this. The object appears black against a white background. Care should be taken not to over-saturate the sensor in the camera or the object will appear smaller than it really is. Reduce the aperture or reduce the shutter speed as necessary to avoid over-saturation. Experiment by altering the distance of the light behind the object and masking off areas of the backlight surrounding the object. 

 Suitable lights for small objects are the VSE M30/30 back light. For larger objects the VSE M60/90 narrow backlight or the VSE M120/120 wide backlight can be used.  

Back lighting is used for measuring objects, through hole inspection, counting edge features and calculating areas.

 Complex lighting arrangements

 For some applications, simple lighting techniques are unsuitable. When looking at highly reflective surfaces, inside holes, at colored objects or measuring heights more complex solutions are required.

 Dark field lighting

 Diffuse back lighting cannot normally be used for inspecting transparent or translucent objects. Dark field illumination highlights defects in such materials. A black opaque mask in the shape of the object is cut out and placed on or above the backlight. This mask should appear slightly larger than the object to be inspected in the image. The object appears black, discontinuities in the object appear bright.

 Dark field lighting highlights edges, cracks, defects and small objects touching the surface of glass and clear/translucent plastic objects. 

 Highly diffuse lighting

 For very reflective surfaces highly diffuse lighting is required. This prevents parts of the surface from acting like mirrors and giving bright glints in the image. Important details could be hidden within these glints. These can either be diffuse material lit from behind, or opaque material lit from below/side. The diffusers act like a very large and even light source.

Experiment with size and position of the diffusers and direction of the light. The hemisphere can either be diffuse material lit from outside, or opaque material lit from below. This technique gives very even illumination, but may be difficult to implement on many production lines. Note that the hole in the top may be reflected in the object being inspected.

 This technique is used for inspecting curved, reflective surfaces and for detecting the presence of features on polished components.

 Polarized lighting

 Polarizing filters allow unwanted reflections to be dramatically reduced in intensity. The left hand diagram shows a single polarizer mounted in front of the camera lens. The polarizer can be rotated to the optimum angle for reducing the reflections. The rotating part of the filter should be locked in place once it has been set up. The right hand diagram shows cross polarization of the light source and the light reflected from the object. The light polarizer should cover the entire light. The second polarizer should be mounted at 90 degrees to the direction of polarization of the light source.

 The single polarizer reduces reflections when viewing surfaces from an angle. The twin polarizers reduce reflections from complex surfaces, such as when trying to view objects that are packed in polyethylene bags.

 Color filtered lighting

 Most inspection applications use gray level imaging. Different colors in an image often appear to have the same brightness. Where certain colors need to be differentiated, color filters can be used. The left hand diagram shows an object illuminated by white light and a color filter placed between the object and camera. By selecting a suitable filter, some colors will appear dark and others bright. They can then be inspected by their relative brightness. These lights will in themselves accentuate certain colors. A color filter can be used to reduce the effect of ambient lighting by filtering out colors other than the color generated by the LEDs. Infrared and ultraviolet light sources that use frequencies outside the visible spectrum can also be very useful in particular instances.

 Color filtering adds color information to an image. It can be used to check that the correct color item is present. It can also be used to improve contrast between various parts of multicolored objects.

 Coaxial lighting

 This technique illuminates objects with light that appears to come from the lens itself. A semi-silvered mirror is placed at an angle of 45 degrees in front of camera. Diffuse light is shone onto the mirror from the side. Half the light is reflected towards the object. The other half passes through the mirror and is lost. Half the light returning from the object passes through the mirror and generates an image in camera. The other half is reflected by the mirror and is lost. The mirror is mounted within a light proof blackened enclosure to prevent stray light effecting the image.

 Coaxial lighting allows holes to be illuminated with very even lighting, even if the holes are deep. Any other form of lighting would generate shadows at the bottom of the holes. Suitable lights for small objects are the VSE-FV40R DOAL. For larger objects the VSE-FV50R DOAL or the VSE-FV70R DOAL can be used.  

Example of use:

Lighting VSE FV70R
Work piece Bearing
Visual field 45 mm
WD 50 mm
Purpose Inspection of surface for flaws
Result Minuscule flaws are clearly detected

 Structured lighting

 Structured lighting allows depth information to be extracted from a front view of an object. A line (or series of lines) is projected onto the object at an angle to camera. This line is normally created using a small laser fitted with a line generating lens. The ambient lighting is controlled so that the line appears very bright relative to the rest of the image. As can be seen on the image schematic in the diagram below, the line in the image is deflected in proportion to the height of the object. By measuring the position of the line at various points, the profile of the object can be calculated. This technique works in either of the two configurations shown in the diagram. A simpler version of this method is to use a narrow beam spotlight, and to determine the height of features on the object by measuring the length of shadows they cast.

Some examples of structured lighting are:

 Measuring the heights of push buttons or keys

Checking the insertion depth of bushes and sleeves at the ends of tubes

Checking the shape of curved surfaces

Detecting the gaps between steel bars

Measuring height of ceramic plate rims above plate centers

Counting the number of cards in a stack

Checking for the presence of a blind hole

Counting the number of turns in a spring

Locating the edge of a large steel fabrication

Checking the number of plates within a car battery

Checking presence of plastic components where there is no contrast between them

 Fiber optic lighting

  A wide range of fiber optic lighting is available. It can be used as the light source for virtually any lighting technique. Fiber optic lights tend to be more expensive than other light sources, so they are normally only used where other lights are unsuitable. Fiber optic lighting is particularly useful when viewing small objects or where very intense lighting is required over a small area. It is also used in applications where space is limited around the object to be inspected and there is insufficient room to accommodate conventional lighting.

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Last Updated December 2001

©1998 Vision Systems Engineering Ltd. All rights reserved.