Machine Vision Guide - Lighting Fundamentals

Lighting Fundamentals

Lighting Techniques

The basic approach to lighting for a particular application is easily determined. It is a function of the type of object and the features to be measured. The more detailed lighting design builds on this basic technique (table 3).

Lighting Solid Angle: Point or Diffuse

Lighting solid angle is the area of a unit sphere, centered on the object, that the illumination occupies (figure 15). Just as angles are measured in radians, with 2p radians in a full circle, solid angles are measured in steradians, with 4p steradians in a full sphere. Illumination from a small solid angle is called point-like; illumination from a large solid angle is called diffuse.

Illustration Type	Solid Angle	Direction	Advantages	Disadvantages
Directional Front Illumination Incandescent lamp or fiber bundle illuminates object from the top	Point	Front	Easy to implement; good for casting shadows; fiber-optic delivery available in many configurations	May create unwanted shadows; illumination is uneven
Coaxial Lighting Illumination from the precise direction of the imaging lens, either through the lens or with a beamsplitter in front of the lens	Point	Front	Eliminates shadows; uniform across field of view	Complicated to implement; intense reflection from specular surfaces
Diffuse Front Illumination Fluorescent lamp, fiber illuminator with diffuser, or incandescent lamp with diffuser, illuminates object from the front	Diffuse	Front	Soft, relatively nondirectional; reduces glare on specular surfaces; relatively easy to implement	Illuminator relatively large; edges of parts may be fuzzy; low contrast on monocolor parts
Light Tent Diffuse illuminator surrounds object	Diffuse	Front	Eliminates glare; eliminates shadows	Must surround object illuminator is large; can be costly
Dark-Field Illumination Point-like source at near right angle to object surface	Point	Side	Illuminates defects; provides a high-contrast image in some applications	Does not illuminate flat, smooth surfaces
Diffuse Backlighting Source with diffuser behind object	Diffuse	Back	Easy to implement; creates silhouette of part; very-high-contrast image; low cost	Edges of parts may be fuzzy;must have space available behind object for illuminator
Collimated Backlighting Point source with collimating lens behind object	Point	Back	Produces sharp edges for gauging	Must have space available behind object for illuminator
Polarized Front Illumination Point-like or diffuse front illumination; polarizer on illuminator; analyzer in front of imaging lens	Point or Diffuse	Front	Reduces glare	Reduces light to lens
Polarized Backlighting Diffuse backlight; polarizer on illuminator; analyzer in front of imaging lens	Diffuse	Back	Highlights birefringent defects; relatively easy to implement	Only useful for birefringent defects; edges of parts may be fuzzy; must have space available behind object for illuminator


Figure 15. Solid angle

Point-Like Lighting

Point-like lighting is generally easy to implement because the illuminators are small and can be located at a distance from the object. Incandescent lamps, optical fiber bundles, ring lights, and LEDs are examples of point-like illuminators. Some, like fiber optic bundles, are directional, so light can be directed onto the object from a distance.

Point-like illumination provides high intensity and light efficiency. It is good for creating sharp image edges, casting shadows, and accenting surface features. Their small size makes the illuminators easier to mount and integrate than diffuse sources.

The same shadows and surface features that are useful in some applications can be distractions in others. With specular objects, point-like illumination creates very bright reflections that may saturate video cameras. Away from these reflections, specular objects appear dark.

Diffuse Lighting

By definition, diffuse lighting must cover a large solid angle around the object. Fluorescent lamps (both straight tubes and ring lights) are inherently diffuse. Diffusers in front of point-like sources make them more diffuse.

Diffuse illumination of specular surfaces allows imaging without bright reflections. Surface texture is minimized, and there is less sensitivity to surface angles on parts.

Diffuse illumination can be difficult to implement because the illuminator must surround much of the object. For example, when reading characters stamped on textured foil, sources with solid angles approaching 2p steradians are required. These �light tents� are difficult to construct effectively because the lens, camera, and handling equipment must be mounted around the illuminator. Diffuse illumination can also cause blurred edges in images. In general, a diffuse illuminator is more complex than a point-like illuminator.

Lighting Direction
Bright field In bright-field illumination, the light comes in approximately perpendicular to the object surface (figure 16). The whole object appears bright, with features displayed as a continuum of gray levels. Normal room lighting is bright-field illumination. This sort of illumination is used for most general-vision applications. An important special case of bright-field illumination is coaxial illumination. Here, the object is illuminated from precisely the direction of the imaging lens. This requires a beamsplitter, either within or in front of the imaging lens. Coaxial illumination is used to inspect features on flat, specular surfaces, to image within deep features, and to eliminate shadows. Dark field If the object is illuminated from a point parallel to its surface, texture and other high angle features appear bright while most of the object appears dark. This low angle illumination is called dark-field illumination. Dark-field illumination is useful for imaging surface contamination, scratches, and other small raised features.

Figure 16. Lighting angles

Backlight
Backlight illumination means the illuminator is behind the object. It can be either point-like or diffuse. Point-like lighting, projected through a collimator whose axis is parallel to the lens axis, is similar to coaxial lighting. There are two distinct uses of backlighting: viewing translucent objects in transmission and silhouetting opaque objects. Sheet glass is an example of a translucent product inspected using backlight. Point-like lighting that is not coaxial with the lens highlights surface defects (scratches, gouges) as well as internal defects (bubbles, inclusions). Backlighting is more commonly used to silhouette opaque parts. Silhouettes are easy images to process because they are inherently two dimensional and binary. Flexible parts feeders frequently use back lighted images to determine the orientation of mechanical parts to be picked up by a robot for assembly.
Lighting Color
Most machine vision applications use unfiltered light. However, in some cases, monochromatic illumination provides better feature contrast. A narrow spectrum also reduces the effect of any chromatic aberration in the imaging lens and therefore provides improved resolution. Filtering does, however, reduce the amount of illumination and may be unsuitable for applications where there is a shortage of light.
Polarization
Polarized illumination is used to reduce glare from specular surfaces. A polarizer is placed in front of the illuminator, and another polarizer (called the analyzer), whose polarization axis is perpendicular to that of the first, is placed in front of the imaging lens. Light that is specularly reflected from the object retains its polarization direction, and is therefore blocked by the analyzer. Light scattered from the object is randomly polarized and is passed by the analyzer.
Light Sources
Several types of light sources and illuminators are available for machine vision applications. Table 4, below, summarizes the properties:

Light Source Type	Advantages	Disadvantages
LED Array of light-emitting diodes	Can form many configurations within the arrays; single color source can be useful in some applicationS. Can strobe LEDs at high power and speed	Some features hard to see with single color source; large array required to light large area
Fiber-Optic Illuminators Incandescent lamp in housing; light carried by optical fiber bundle to application	Fiber bundles available in many configurations; heat and electrical power remote from application;easy access for lamp replacement	Incandescent lamp has low efficiency, especially for blue light
Fluorescent High-frequency tube or ring lamp	Diffuse source; wide or narrow spectral range available; lamps are efficient and long lived	Limited range of configurations; intensity control not available on some lamps
Strobe Xenon arc strobe lamp, with either direct or fiber bundle light delivery	Freezes rapidly moving parts; high peak illumination intensity	Requires precise timing of light source and image capture electronics. May require eye protection for persons working near the application