Tips for choosing the optimal lighting solution for a machine vision application.
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Shaina Warner | Microscan
Proper lighting is critical to the success of a machine vision application, and should be the first consideration when setting up a system. A well planned lighting solution will result in better system performance and save time, effort, and money in the long run. This article presents the following tips:
1. Use Bright Light to Detect Missing Material
2. Use Appropriate Wavelength for Accurate Component Placement
3. Use Non-Diffused Light to Detect Cracks in Glass
4. Use Diffused Light to Inspect Transparent Packaging
5. Use Color to Create Contrast
6. Use Strobed Light for Rapidly Moving Parts
7. Use Infrared Light to Eliminate Reflections
8. Use Infrared Light to Diminish Color Variation
Before & After Correct Lighting
Machine vision systems rely on quality images for quality output. High quality images enable the system to accurately interpret the information extracted from an object under inspection, resulting in reliable, repeatable system performance. The quality of the image acquired in any vision application is highly dependent on the lighting configuration: the color, angle, and amount of lighting used to illuminate an object can mean the difference between a good image, resulting in good performance, and a poor image, yielding poor results.
Machine vision lighting should maximize feature contrast while minimizing contrast of the rest, thereby allowing the camera to clearly "see" the part or mark. High contrast features simplify integration and improve reliability; images with poor contrast and uneven illumination require more effort from the system and increase processing time. The optimal lighting configuration is dependent on the size of the part to be inspected, its surface features and part geometry, and the system needs. With a broad range of wavelength (color), field of view (size), and geometry (shape) options available, machine vision lighting can be tailored to specific application requirements.
Identifying missing material from plastic molding applications is important to ensure good sealing surfaces. When material is missing, you have a 'short shot' condition (i.e. insufficient quantity of material injected into the mold). To identify short shots, a NERLITER DOALR (Diffuse On-Axis Light) illuminator is used to highlight missing material on the sealing surface.
The bright field lighting technique relies on surface texture and flat topography. Light rays hitting a flat specular surface reflect light strongly back to the camera, creating a bright area. Roughly textured or missing surfaces scatter the light away from the camera, creating dark areas.
Identifying proper component orientation is a common machine vision application in PCB assembly. In this example, chips were incorrectly 'flipped' in an automated assembly step. Instead of being placed onto the substrate with copper side down for proper electrical connection, the chip is flipped over, silver side down, causing component and assembly failure.
To identify problem assemblies, a NERLITER DOALR illuminator with a blue wavelength is used to highlight chip orientation. This lighting technique relies on wavelength and coaxial lighting geometry. The blue wavelength (460 nm) sharply differentiates the silver and copper surfaces: copper absorbs the blue light and appears dark, silver reflects the blue light and appears bright. The unique NERLITER coaxial lighting geometry eliminates the root cause of false rejects: unwanted
glare, reflections and dark spots.
Detecting cracks during sidewall inspection of glass containers prior to packaged-goods shipment is an important way to minimize waste, decrease returns and increase consumer confidence. To detect cracks in transparent material, a NERLITER Area Array is used to highlight the imperfection.
In this application, dark field lighting is used to create a bright, easily detectable feature of interest within a dark background. Light rays are directed at the transparent bottle from a dark field region (i.e. from outside the camera's field of view). Most of the rays pass through the transparent object undetected by the camera. If a material irregularity such as a crack exists, some light rays strike that irregularity. Cracks, in particular, create an internal void, where light rays refract and reflect, scattering in many directions including back to the camera. These are the rays that turn an otherwise difficult to detect crack into a bright
feature on the dark background.
Detecting missing, damaged and/or incorrect contents in the cells of blister packages is an important inspection step for ensuring the quality of finished pharmaceutical goods. Illuminating the packages to avoid reflections from cell coverings can often be a tough challenge. To present an accurate image of the cells and their contents to the machine vision camera, a NERLITER CDI® (Cloudy Day Illuminator) employing white LEDs is used. The CDI eliminates reflections from the clear cell coverings, making them virtually disappear, while at the same time providing a high contrast, neutral color view of the cell contents.
The continuous diffuse lighting technique de-emphasizes surface textures and changes in elevation. It provides a very large solid angle of illumination to guide light rays to the object from a multitude of angles, thereby eliminating the reflections and shadows typical of unidirectional or single-plane light sources.
A useful method for creating a high contrast image in a machine vision application is to illuminate the object with light of a particular wavelength (color). A light's wavelength can make features with color appear either bright or dark to the monochrome camera. Using the color wheel as a reference, choose light of an opposing color to make features dark; choose light of the same color to make features light.
A light should be strobed when a part is moving quickly and the image captured appears blurred. Use the formula on the right to calculate Strobe Pulse Width (maximum acceptable blur image = 1 pixel).
In the formula, field of view (FOV) and pixels are from the axis of travel. For example, assuming a typical 1 inch FOV and a 640 pixel frame, and a part speed of 500 inches per second, a strobe pulse width of 3.1 μs is needed.
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Machine vision systems rely on transitions of gray levels in a digital image. In many vision applications, ambient light (i.e. overhead room lighting) sources contribute unwanted bright reflections that make it difficult or impossible for the vision system to detect the features of interest. An infrared (IR) light source can be used to eliminate this problem.
Infrared light can be used to diminish the grayscale difference between colored objects. Dark objects absorb infrared light waves, thereby creating uniformity in objects of otherwise varying shades. This lighting solution facilitates detection of inconsistencies where color or shade variation is expected and should not affect inspection.
Microscan's innovative NERLITE® line of products is the longest established brand of machine vision lighting, enabling machine vision and auto ID systems to perform reliably in thousands of applications worldwide.
A global technology leader with extensive solutions for ID tracking, traceability and inspection, Microscan's line of 1D and 2D barcode readers, machine vision systems, and machine vision lighting is backed by one of the world's most robust patent portfolios and years of proven performance in the field. Supported by a highly skilled global integrator network, Microscan products are known and trusted by customers worldwide for their high precision and reliability in industrial automation and control applications, with solutions ranging from basic barcode reading to complex machine vision inspection, gauging, and measurement.
More information on Microscan is available at www.microscan.com.
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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
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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