Illumination for Vision Systems Lighting is the one area of installing a vision system that usually requires considerable experimentation. It is virtually impossible to predict the exact lighting arrangement that will give the best results for a new application. 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. While lighting may seem to be mundane and not very exciting, it is hard to overestimate how important this topic is for developing robust and reliable vision systems. Many vision companies have failed because they did not address this area in an effective fashion.

Diffuse Front Illumination Diffusers are used in front of light to reduce directionality and shadows and increase uniformity. Contrast is often reduced and some features may become clearer Diffusers significantly reduce light intensity.

Lighting and Rapid Motion Video Cameras operate in a similar fashion to photographic cameras in that both integrate light over some period of time. For fast moving production lines a fast camera shutter speed is needed to reduce motion blur. Fast shutter speeds in turn requires higher levels of illumination. When object motion is very fast, a strobe-type illumination source is often used.

Directional Front Lighting Useful for detection irregular surface features that cast shadows such as burrs or depressions. This technique is used for objects where the presence or absence of surface features requires detection. This technique is unlikely to work for reflective surfaces.

Grazing Illumination Grazing illumination is very useful for detecting surface imperfections and is closely related to dark-field illumination which is described later.

Back Illumination 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 appears black against a white background. Care should be taken not to over-saturate the sensor in the camera or the object will appear reduced in size. Experiment by altering the distance of the light behind the object and masking off areas of the backlight surrounding the object

Dark Field Illumination Features appear as bright objects against a dark background. This approach is capable of providing very high contrast. Very small (below system resolution) features can be detected but not resolved! The main disadvantage of this approach is that dust and minor contamination can cause false rejects.

Use of Polarizers Polarizing filters allow unwanted reflections to be dramatically reduced in intensity. When using a single polarizer, it can be rotated to the optimum angle for reducing reflections. The filter should be locked in place once it has been adjusted properly. With two polarizing filters, nearly complete cancellation of reflections is possible which is especially useful when inspecting specular surfaces. One polarizer should cover the entire light while the second polarizer should be mounted at 90 degrees to the direction of polarization of the light source.

Setup using 2 Polarizing Filters One polarizer is rotated to maximize attenuation of specular reflection. Diffusely reflected light is depolarized so it is attenuated by only half instead of nearly total cancellation. This approach can be used to enhance specular reflections and provide some suppression of diffuse reflections by aligning the filters.

Structured Lighting A Line of light is projected onto a surface to obtain 3D information. Object is moved and a series of images are acquired.

Fiber-Optic Illumination A wide range of fiber optic lighting is available. It can be used as the light source for virtually any lighting technique. Fiber optic sources tend to have higher cost than other light sources, so are normally only used when required by a given application. 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. A fiber-optic source causes much less heating compared to the presence of a lamp in close proximity to the inspected objects.

Diffuse Back Illumination Useful for opaque parts that have relatively low reflectivity. Can also be used to inspect transparent or translucent parts made out of materials such as plastic and glass.

Light-Tent Illumination “All-around” illumination casts no shadows. Objects often have a very unusual appearance with this type of illumination. A wok painted flat white paint on the inside and a circular fluorescent light makes a good light tent!

Magnification Problems In some applications, it is difficult to control the distance of the part to the camera. In other applications, parts have significant depth. In both cases, features that are nearer the camera will appear larger than more distant features. Hole sidewalls in thick parts, especially when viewed off axis, will be visible so that accurately measuring their size is problematic. Telecentric optics provides an efficient solution in many instances for these types of problems.

Telecentric Back Illumination Collimated source is obtained by placing a lens at its focal distance from a point light source. Setup and adjustment is somewhat critical.

Telecentric in Object Space A lens is placed between the object and the camera. The telecentric stop is usually the aperture of the camera lens

Telecentric in Image Space A Lens is placed between a stopped-down light source and the object. The size of the aperture determines how collimated the light source will be.

Acceptance Angle The size of the aperture is a very useful means for adjusting the sensitivity in many vision applications Refractive defect detection is addressed very well with this approach

Telecentric Application Example Internal gear teeth are difficult to gauge with ordinary optics. Gear Part Normal View Telecentric Image

Latteral Inhibition From a variety of measurements, it appears that the HVS performs spatial filtering. Some studies have suggested that the nature of this filtering is the difference of two Gaussian filters.

HVS Follies!

Black Body Radiation and Temperature Radiation from a black body radiator increases significantly with temperature. Most of the radiation is in the IR portion of the spectrum. Incandescent lights have similar characteristics to black body radiators. One result of this is that small changes in filament voltage result in significant changes in light output due to temperature changes which in turn change the spectrum of the source.

LED Sources LED sources are not very bright by conventional standards but are suitable for a number of applications. Expected lifetime is on the order of 100K hours. A variety of spectral properties are available including white. Can be used in strobe applications to obtain much higher peak light levels.

Fluorescent Illumination Sources Fluorescent sources are useful for applications that require large area illumination or very long illuminators. Light intensity is relatively low compared to incandescent sources which limits use in vision applications. This is a very efficient type of illumination source and has very little IR output. Fluorescent sources have somewhat unusual temporal properties which can impact vision applications significantly.

Fluorescent Light Properties Fluorescent lights feature long lifetimes, on the order of 10K hours. While not readily apparent, this type of source exhibits substantial instability when initially turned on and requires a number of minutes before light output is reasonably constant. Standard fluorescent sources have substantial ripple in their light output but a special high-frequency (~25 kHz) ballast eliminates this problem.

Fluorescent Light Flavors This type of light comes in many shapes and sizes. A variety of spectral outputs are available including ultraviolet lamps which have no phosphors.

Human Visual System The human visual system (HVS) has been studied for many years and provides some interesting capabilities which may be useful in machine vision. Humans are far more able to analyze real-world scenes than machines under most circumstances. As a better understanding of the HVS is developed, it may be possible to transfer this understanding to improve vision system technology.

HVS Spatial Frequency Response Color response is primarily in the lower spatial frequencies. Monochrome response exhibits a distinct peak and has poor low-frequency response.

Human Spectral Response The human visual system (HVS) has a peak response around 550 nm for daytime (photopic) viewing. The nightime (scotopic) response is shifted to shorter wavelengths.

Mach Bands Gray levels are perceived differently near discontinuities. This implies the presence of high-pass filtering in the HVS.

Simultaneous Contrast Even though the two gray rectangles have the same gray-scale value, they appear to be different. The gray level of the background affects how the foreground is perceived.