What is the best lighting for visual inspection?

Testing and inspection of parts and components during design, manufacturing and testing is an essential part of the engineering process. During testing and inspection, small defects which are often invisible to the naked eye, can be found and fixed before they cause problems later down the line.

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To successfully inspect small components, adequate inspection lighting is required. The best lighting for visual inspection is most often LED, as it provides a bright light while remaining energy-efficient. LED visual inspection lighting for microscopes also has better longevity compared to traditional halogen bulbs meaning fewer lamp replacements over its lifetime. LED lighting in microscopy helps to reduce eye-strain while providing even lighting across the entire inspection area, improving overall reliability.

At Optimax, we stock a range of the best lighting for visual inspection including Photonic LED lights for endoscopes and microscopes, as well as ring lights, spotlights, swan necks and segmented lighting to suit all metrology requirements.

e offer product demonstrations of all our microscope lighting at our onsite laboratory in Market Harborough. If you&#;d like to find out more about our lighting for visual inspection or want to book an appointment, call us on to speak to a member of the team.

Polarization

Unlike microscopy applications, light polarization in machine vision has been employed primarily to block specular glare reflections from surfaces that would otherwise preclude a successful feature identification and inspection. Normally, two pieces of linear polarizing film, applied in pairs with one placed between the light and object (polarizer) and the other placed between the object and camera (analyzer &#; Fig. 58a). It is common for the polarizer to be affixed to the source light and the analyzer to be mounted in a filter ring and affixed to the camera lens via screw threads or a slip fit mechanism if no threads are present, allowing the analyzer to be freely rotated.

However, it&#;s first important to comprehend the nature of unpolarized light passing through space and its behavior with respect to this polarizer/analyzer pair. As indicated earlier, light is a propagating transverse electromagnetic wave, meaning the electric field fluctuations, modeled and depicted as a sine wave, &#;oscillate&#; in random planes perpendicular to the light propagation direction &#; exhibiting unpolarized behavior (Fig. 58b). Further, the wave magnitude is related to the amount (or intensity) of light.

Fig. 58

Relative optical path positions of the polarizer and analyzer in, a &#; Front-lighting arrangement, b &#; Light oscillation planes through a linear polarizer and resulting single wave oscillation.

In the following graphics, for clarity of demonstration, we illustrate only 2 perpendicular oscillating light waves to demonstrate how they respond to polarization.

Typical iodine-acetate based linear polarization film is composed of roughly parallel lines of long-chain polymers (Fig. 59). This structure allows us to define a Polarization Axis (Transmission for an analyzer) and an Absorption Axis, oriented at right angles to each other. In looking at the film on a molecular level with respect to the perpendicular light wave fronts (Fig. 60), we see that it is these parallel strands of polymer chains that block (absorb) all but one plane of oscillation.

Fig. 59

Idealized polarizer demonstrating the transmission / polarization axis (blue), absorption axis (red) and partial transmission axis, oriented at 45 degrees to the polarizer (black).

Fig. 60

Molecular view of two perpendicular light waves interacting with the long-chain iodine complex molecules. Note that the wave oscillation in the horizontal plane (red dashed line) is effectively absorbed by the polymer chains, whereas the perpendicular wave will pass through the chains.

However, it is important to note that the film&#;s long-chain polymers are oriented perpendicular, rather than parallel to the transmission and absorption axes &#; unlike the picket-fence analogy commonly depicted in the literature, which can be misleading if interpreted literally. This analogy is not incorrect as long as we only analogize the &#;pickets&#; in a fence with a light polarization or transmission axis and not a physical grate of chains, oriented parallel to the wave amplitudes. What is most important to understand is that the long-chain molecules absorb the electric field oscillation component whose amplitude is parallel to the polymer chains but passes the perpendicular component more readily.

To understand how unpolarized and polarized light are affected when they pass through a succession of polarizing films, we look to Malus&#; Law. Briefly stated &#; the intensity of plane polarized light that passes through an analyzer varies as the square of the cosine of the angle between the polarizer polarization axis and analyzer transmission axis. We can then infer that the plane polarized light is fully or partially transmitted or blocked completely (Figs. 61a-b).

The mathematical relationship is described by the following equation:

I = I0cos2 Θ, where:

I0 = Original pre-analyzer light intensity

I = Post-analyzer light intensity

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Θ = Angular difference between the polarizer polarization & analyzer transmission axes

For example, a simple calculation applying basic trigonometry: if the polarizer and analyzer transmission axes are parallel (Θ = 0 degrees), cosine of 0 = 1, meaning the plane polarized light passes 100% through, whereas if Θ = 90 degrees, cosine of 90 = 0, that plane polarized light is 0% transmitted. Finally, if Θ = 45 degrees, we would be correct in our supposition that ½ of the plane polarized light is transmitted.  We see the relationship between light oscillation planes and respective transmission and absorption axes in Fig 61c.

Another important aspect of plane polarized light is that its intensity is ½ that of the original unpolarized light incident on the first polarizer. This is of importance to vision users if the application is already starved for light &#; any use of a single or especially a pair or more of polarizers may produce a considerable loss of image intensity. We will describe and illustrate these points in the following sections.

Fig. 61

a &#; Unpolarized light vibrating in the horizontal plane (depicted in blue) passing through the polarizer (P) and blocked (absorbed) by analyzer A1, b &#; Light passing through the same initial path as that in diagram a, but through analyzer A1 and A2 (rotated @ 45 degrees, blocking some of the plane polarized light. Note the light radiant power drops considerably with each P or A pass-through (if not blocked), c &#; Relative orientation of the transmission, absorption and partial transmission axes with respect to the long-chain polymers in the polarizer film.

As stated earlier, machine vision techniques have utilized light polarizer/analyzer pairs primarily to block reflective glare from parts &#; this glare reflection may be caused by the dedicated lighting used in the inspection and/or from ambient sources. These two cases may be treated differently:  Nonmetallic and transparent surfaces tend to partially polarize ambient incident unpolarized light, preferentially polarizing it in the horizontal plane (or more accurately in the plane parallel to the incident surface and perpendicular to the incident light plane), and hence only an analyzer, whose transmission axis is oriented at 90 degrees is needed to block it. This process is known as reflection polarization. An example of this phenomenon is reflected glare from a road or other smooth surface, such as a lake.

However, polarization by reflection isn&#;t always as complete as using film because photons with other oscillation directions can also be reflected, if not refracted by the part surface. This phenomenon of partial polarization explains why when rotating sunglasses (or turning your head while wearing them), the scene can get a bit brighter or darker, but not go to extinction &#; it all can&#;t be dialed out with an analyzer (vertically oriented transmission axis in this case). Metallic surfaces, on the other hand, typically reflect most, if not all the incident unpolarized light (no refraction into the material), so different strategies are often needed when it is not practical to polarize the ambient light before it is incident on a part&#;s surface.

However, dedicated light applied to the inspection area usually can be first polarized, then the offending light reflecting off the parts into the camera can be similarly dialed out using the analyzer. The very effective use of light polarization demonstrated by the image pairs in Figs. 62a-b does come with inherent compromises, however. Most notably, in this instance, the lens aperture had to be opened 2 ½ f-stops to create the same scene intensity, for example. Therefore, there is a lot less light to work with in those application situations requiring a considerable amount of light intensity, such as hi-speed inspections.

In Figs. 62c-e, we see that glare reflected from a curved surface, such as this personal care product bottle, can be controlled, but not eliminated with polarization (Fig.62d &#; center area). This is true because there are multiple reflection directions produced on the curved surface from a directional light source, and polarization filters cannot block all vibration directions simultaneously, thus always leaving some areas washed out.

In this case, a more effective approach to glare control, given the flexibility to do so, is to reconsider the lighting geometry. By simply moving the light from a coaxial position around the lens to a relatively high angle, but off-axis position, we can eliminate all specular reflection created by our light source (Fig. 62e).  Both of these application examples point toward the advantage of investigating an alternative to polarization by changing the part &#; light &#; camera 3-D spatial relationships.

Fig. 62

A change in &#;lighting &#; object &#; camera&#; geometry or type may be more effective than applying polarizers to stop glare,  a &#; Coaxial Ring Light w/o Polarizers,  b &#; Coaxial Ring Light w/ Polarizers (note:  2 ½ f-stop opening),  c &#; Coaxial Ring Light w/o Polarizers,  d &#; Coaxial Ring Light w/ Polarizers (note some residual glare), e &#; Off-axis (light optic axis parallel to the object long axis).

Another use of light polarization is illustrated in Fig. 63, namely for detecting stress-induced structural lattice damage in otherwise transparent, but birefringent materials, typically plastics. Recall that plane polarized light has wave oscillations in only one plane, unlike unpolarized light. When plane polarized light is transmitted through a stress-induced birefringent material, it resolves into two principal stress directions, each with a different refractive index, and thus the two component waves are said to be out of phase. They then destructively and constructively interfere, creating the alternating dark and light bands we see illustrated in Fig. 63b.

Fig. 63

Transparent plastic 6-pack can holder, a &#; With a red back light, b &#; Same, except for the addition of a polarizer pair, showing stress fields in the polymer.

Polarization References for further reading:

https://courses.lumenlearning.com/physics/chapter/27-8-polarization/

https://www.physicsclassroom.com/class/light/Lesson-1/Polarization

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