All reflected light obeys the relationship that the angle of incidence equals the angle of reflection. Just as images are reflected from the surface of a mirror, light reflected from a smooth water surface also produced a clear image. We call the reflection from a smooth, mirror-like surface specular (as shown in Figure 2a). When the surface of water is wind-blown and irregular, the rays of light are reflected in many directions. The law of reflection is still obeyed, but the incident rays (Fig. 2b) strike different regions which are inclined at different angles to each other. Consequently, the outgoing rays are reflected at many different angles and the image is disrupted. Reflection from such a rough surface is called diffuse reflection and appears matte.

• circularly polarized light propagating along z. (. )2. 1 cosθ. ∝ +. (. )2. 1 ... = expected circular polarization. 1. 0. ⎛ ⎞. ⎜ ⎟. ⎝ ⎠

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.

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!

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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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:

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.

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.

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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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Figure. 2 Light reflection from a) smooth surface (specular reflection ) and b) rough surface (diffuse reflection). In both cases the angle of incidence equals the angle of reflection at the point that the light ray strikes the surface.

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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.

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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.

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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.

Diffused light Ceiling

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.

Light is also reflected when it is incident on a surface or interface between two different materials such as the surface between air and water, or glass and water. Each time a ray of light strikes a boundary between two materials - air/glass or glass/water - some of the light is reflected. The laws of reflection are obeyed at all interfaces. The amount of reflected light at the interface depends on the differences in refraction between the two adjoining materials.

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.

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).

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Diffuse lightingPhotography

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).

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What is it about objects that let us see them? Why do we see the road, or a pen, or a best friend? If an object does not emit its own light (which accounts for most objects in the world), it must reflect light in order to be seen. The walls in the room that you are in do not emit their own light; they reflect the light from the ceiling "lights" overhead. Polished metal surfaces reflect light much like the silver layer on the back side of glass mirrors. A beam of light incident on the metal surface is reflected.

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.

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.

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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.

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Figure. 1 Light reflected from a metal surface with angle of incidence i equal to the angle of reflection i'. The dashed line (normal) is perpendicular to the surface.

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.

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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.

Reflection involves two rays - an incoming or incident ray and an outgoing or reflected ray. In Figure 1 we use a single line to illustrate a light ray reflected from the surface. The law of reflection requires that two rays are at identical angles but on opposite sides of the normal which is an imaginary line (dashed in Fig. 1) at right angles to the mirror located at the point where the rays meet. We show in Fig. 1 that the angles of incidence i and reflection i' are equal by joining the two angles with an equal sign.

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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).