Dark field illumination can be used to highlight edges, making it perfect for edge detection, inspecting engravings or damage and stains on metal surfaces, and reading laser markings.

Another typically challenging part to illuminate effectively is the biomedical culture or sample titer tray. These parts are often presented on a tray with a matrix of regularly spaced wells of varying size and depth. In this particular example, the wells are relatively small (5 mm wide) and shallow (3 mm deep), and spaced at approximately 8 mm in X and Y. Additionally, each well has a laser-etched 2-D matrix code that must be read and verified by a vision system as part of an FDA-requirement for sample correlation and tracking history.

It can be challenging to select the right illumination for your application. With the wide range of lighting options on the market, it is crucial to choose the right illumination early in the design phase to save cost and integration efforts. But how do you select the optimal lighting type for your vision application?

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The right lighting in a computer vision setup significantly impacts image processing results. Here, we explain the key factors that contribute to optimal lighting.

In this respect, it is similar to beam illumination in a dark field arrangement, but the key difference is that dark field illumination does not focus on any particular direction.

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The FX0808 light is part of the Ai “Expandable Series” of lights.  This enables our team to build a light of this type from 1” x 1” (25 x 25 mm) to 24” x 24” (610 x 610 mm) in 1” (25 mm) increments, all available in 2 weeks.

How to diffuse lightfor photography

It’s also important to pair the lens focal length correctly in order to prevent vignetting, or “port-holing” – which is particularly true of the Diffuse Dome Light.

Advanced illumination offers a diffuse source to address the application shortcomings of other diffuse, bright field lights: the FX0808 Flat Diffuse Light and FD0808 Back-lit Flat Diffuse Light.  These Flat Diffuse Lights are a highly diffuse source with a viewing port in the center, allowing it to be used as front, or projection light (Figure 2).

If we instead apply the same lighting schemes to a longer working distance view (Figures 6A-6E & 7A), one that may only require 1-2 image frames per tray, we see that only the Flat Diffuse Light and the DL097 Diffuse Dome Light are effective (Figures 6D & 6E, respectively).  Of course, a much higher resolution camera would be necessary to resolve the 2-D laser etched codes.

Figure 7A illustrates the entire tray lighted using the FX0808 Flat Diffuse Light at a 12” (305 mm) working distance.  In conjunction with a high-resolution CCD or CMOS camera, the FX0808 provides for a very robust and efficient inspection solution for larger fields of view.

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Dark field illumination can be used to highlight edges, for example surface irregularities such as scratches or engravings.

Unlike normal ring lights, this type of light does not illuminate the object directly; this way the light reflected into the camera is mainly from the edges of surface features.

2) to maintain a high solid angle for even light distribution over curved or topographic surfaces, the low angle component from the dome interior must also be incident on the surface.

Figure 4 illustrates the limited size application of the 2”x2” Square Coaxial Light, where large size and set working distance are critical.

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The right illumination is critical in a vision system. Properly combining lighting with the camera and lens ensures the best possible results in your application.

Surfaces prone to uneven reflectivity are often challenging to inspect – particularly with bright-field point sources commonly used in machine vision systems.  To achieve more uniform contrast over such surfaces, one solution is to apply diffuse lighting techniques.

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While the Diffuse Dome Light is very effective on specular, curved, and topographic surfaces, and the Coaxial Light is effective for specular, flat, and angled surfaces of varying heights, both techniques have very specific application criteria that can present a challenge under some lighting circumstances.

We also see that this short working distance contributes to an effective lighting geometry.  However, this is not an efficient sampling strategy, as the 75 x 125 mm tray would require multiple X & Y traverses to inspect the entire tray.

We see that the Diffuse Dome Light and Flat Diffuse Light work equally well for relatively small areas of the PCB (Figures 3A & 3B).  In comparison, the large Diffuse Dome is less effective than the Flat Diffuse Light on larger fields-of-view, primarily because the dome has to be oversized by at least 50% of the intended field-of-view size (Figures 3C & 3D).

The image depicted in Figure 6E demonstrates that the DL097 Diffuse Dome Light effectively illuminates the tray at longer working distances.  But for larger sampling areas, we still require larger domes, which can be bulky, expensive, and may require longer lead times.

For illustration purposes, we can see the 2-D matrix code in close-up views (See Figures 5A–5D).  A 2” (50 mm) working distance is necessary for the code to be read and verified with a standard resolution CCD camera, while also being typical of the working distances required for effective diffuse and coaxial source lighting.

Let’s view images from samples that illustrate the advantages of applying the Flat Diffuse Light versus other diffuse lights, including a Diffuse Dome Light and a Square Coaxial Light.

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The right lighting is key for any image processing system. It helps determine whether an inspection task is handled efficiently or not.

PCBs can be challenging to illuminate correctly, particularly those with large or odd-shaped components, like capacitors or heat sinks. Figures 3A-3D illustrate the differences among the three diffuse lighting options: dome, flat, and coaxial.

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Specifically, as illustrated in Fig 1A, a Diffuse Dome Light illuminates from nearly an entire hemisphere; hence it is described as having a relatively larger “solid angle”  than the directional Spot Light depicted in Figure 1B.

Our Lighting Advisor provides individual product recommendations by guiding you step by step to a selection of suitable lighting and components. Select the lighting for your vision system as early as possible during your design process.

To maintain sufficient incident light intensity, uniformity, and angular distribution, Diffuse Lights are best placed close to the intended inspection surface.  This is particularly crucial for the Diffuse Dome Light for two reasons:

Application example: edge detection, inspection of engravings, inspecting metal surfaces for damage or stains, and reading laser markings.

As discussed in our previous Lighting Education Blog Post, diffuse lighting in machine vision applications may be classified as full bright field illumination as opposed to partial, or directional bright field lighting.  The underlying concept behind full bright field illumination, defined as illumination with a larger “solid angle”, is that light is sourced from a large area and is incident on the object surface from multiple angles (Figure 1A).

It’s not difficult to imagine that a high solid angle light can become an increasingly lower solid angle and less intense light (inverse square rule) as the working distance is increased, effectively turning it into a more directional point source!