We understand that the quality and suitability of lighting is one of the most important aspects of creating a standardized, accurate and timely vision inspection system. An understanding of lighting types and techniques, geometry, filters, sensor characteristics and colour, a thorough analysis of the inspection environment, including sample presentation and sample light interaction will provide the foundation for designing an effective lighting solution. Designing and following a rigorous lighting analysis sequence will provide a consistent and robust environment, thereby maximising time, effort and resources – items that are better spent on other important aspects of vision system design, testing and implementation. Perhaps no other aspect of consistent vision system design and implementation causes more delays, costs and overall stress than lighting. Historically, lighting has often been the last aspect to be specified, developed, and/or funded, if at all. And this approach is not entirely unfounded, as until recently there was no truly vision-specific lighting on the market, meaning that lighting solutions typically consisted of standard incandescent or fluorescent consumer products, with some supporting work around them. In this article, we will explore how light interacts with test specimens, how light color affects image quality, and how to find the best lighting solution for your project.

Specimen-Light Interaction

The way a specimen surface interacts with ambient and task-specific light is related to many factors, including its surface shape, geometry, and reflectivity, as well as its composition, topography, and color. Some combination of these factors will determine how much light, and how, is reflected to the camera, and then available for acquisition, processing, and measurement/analysis (see Figure 1). An important principle to remember when dealing with special surfaces is that light is reflected from these surfaces according to the angle of incidence - this is a useful property to apply to darkfield illumination applications (see Figure 3, right image for example).

Additionally, a curved, cube-shaped surface, such as the bottom of a soda can (Figure 2), will reflect a directional light source differently than a flat, diffuse surface, such as copy paper. Similarly, a topographic surface, such as a densely populated PCB, will reflect differently than a flat, but smooth or concave textured surface (Figure 3) depending on the type of light and geometry.

​Figure 1. Left: Light interaction on the sample surface.

Right: Point surface, angle of reflection = angle of incidence (Phi 1 = Phi 2).

Figure 2. Bottom of a soda can.

Left: illuminated with bright field ring light, but shows poor contrast, uneven lighting and sharp reflections

Right: captured with diffuse light, creating a uniform background that allows code reading.

​ Figure 3. 2D dot peen matrix code 

Left: illuminated by brightfield ring light

Right: captured with low-angle linear darkfield light.

A simple change in lighting pattern has created a more powerful and effective inspection.

 Warm Colors - Cool Colors - Color Analysis

Materials reflect or absorb a variety of different wavelengths of light, an effect that is valuable for both B&W and color imaging. As we all know from grade school, reflective colors brighten surfaces; conversely, absorbing opposite colors darken surfaces. By using a simple Warm vs. Cool color wheel (Figure 4), we can create distinct contrast between a part and its background (Figure 5), and even differentiate colored parts, with a limited, known color palette, with a B&W camera (Figure 6).

Figure 4 - Color wheel

​                                    Figure 5. a - Postage stamp photographed under Red light

                              b - Green light

   c - Blue light, which creates less contrast than green light

   d - White light, which creates less contrast than Blue or Green light.

   White light will contrast all colors, but it can be a contrast compromise

​ ​Figure 6. a - Candy pieces photographed under white light and color CCD camera

                                                              b - White light and B&W camera

                                            c - Red light, brightens both red and yellow, darkens blue

                                      d - Red and blue lights, which produce yellow, brighten yellow more than red

   e - Green light, brightens green and blue and darkens red

                                          f - Blue light, brightens blue colors and darkens other colors.