Lighting for Color Machine Vision

Inappropriate, non-uniform, and/or unstable lighting are probably the most common causes of unsatisfactory performance of machine vision systems. Good lighting with a smooth broad spectral distribution is particularly important for color vision. The more stable the lighting is in the presence of line voltage fluctuations, temperature changes, and the aging of bulbs, the better it can handle the inevitable variations in component colors, temperature based drift of cameras and frame grabbers, etc.

For some applications, where there is good color contrast, lighting may not be a problem. On the other hand, for many machine vision applications lighting is the most challenging part of system design. Integrators inexperienced in machine vision lighting may wish to consider using the services of an experienced professional and/or taking one of the courses offered in the subject.

Except in applications where a strobe is required, lighting is typically high frequency fluorescent or DC tungsten halide incandescent. White LED (Light Emitting Diode) lighting may be suitable for some specialized applications but spectral distributions remain far from ideal for color work. For most assembly inspection applications a regulated source with internal light feedback is recommended.

Light from incandescent sources is typically transmitted to the work area using fiber optics. Delivery methods can vary from simple line or ring lights to more elaborate diffuse ring illuminators and diffuse-on-axis lighting. The best choice to obtain maximum uniformity and color contrast depends on the light delivery system, the geometry of the assembly, its components and the inspection fixture. When designing the inspection station, the best results are usually obtained when all of these factors are taken into account.