Glare and Polarized Light in Machine Vision Applications

Anyone familiar with Machine Vision knows that lighting is everything—or just about. Proper choice of lighting is essential when integrating successful vision applications, and provides more robust systems. Good image quality, stronger inspection detail/contrast, and depth of focus are all related to the type of lighting used.

In many cases, proper lighting can even eliminate undesirable glare that blocks image features, causes pixel “blooming”, and washes out image contrast. Examples of these forms of glare are shown below.



Increased Contrast –OCV Application, Same bottle on left and right under different lighting techniques

 

Backlight Clear PET Bottle Inspection - Glare from Ambient Lighting (Left) reduced by using Red Bandpass Filter with Red Backlight (Right)

Black Substance Measurement Application - Filtering of Glare via polarization filters (visible) and blocking IR/UV light that cannot be polarized by the polarized filter

In each case, elimination of glare is a tremendous step toward success. There are applications, however, where glare is desirable and can be used to provide increased feature contrast.  Before going through the effort to eliminate glare, make sure that it cannot be made useful first!

So what is glare, how is it produced, and how can it be used or reduced to provide the best feature contrast possible? These are the questions I aim to answer in this article.

What is glare?

Glare is defined as a “strong reflection of light.” To determine a correct method for eliminating glare, the surface of reflection must be considered. For example, material that diffuses light may still produce unwanted effects of glare.  The diffuse material is more strongly lit in one area than another, with respect to the Camera. The geometry of the lighting application, properly adjusted, will often correct the “washed out” effect of the strong light reflections. “Indirect Lighting” is a typical solution in which the incident angles of light (on the surface of the inspection material) differ at all points from the angle of the Camera to that material. 

Lighting non-diffuse surfaces (or shiny material) often requires more than simple adjustment of the light-to-product-to-camera setup. Reflected light on non-metallic surfaces is somewhat polarized. Polarized light can be reduced by the use of polarized filters in many cases. 

Properties of light: polarization

Light is an electromagnetic wave that propagates through a medium with electric and magnetic orthogonal “wave” components. It is a “transverse” wave (as opposed to longitudinal), meaning the electric and magnetic wave components oscillate perpendicular to the direction of travel.

Electromagnetic Wave (Light).¹

Unpolarized light is defined as a random assortment of the electric fields orthogonal to the direction of travel, whereas polarized light has a specific orientation of these fields.

Light can become polarized after being transmitted or reflected off of the surface of a non-diffuse material. The exception to this polarization, involves light reflecting off of metallic surfaces. Metallic surfaces will not polarize light.  However, Light reflected off of non-metallic surfaces can become polarized at low incident angles.  In fact, at an angle known as the Brewster Angle, the reflected light is completely polarized. Other angles will produce only partially polarized light.

Incident light reflection on a surface resulting in polarization.

Stress shown in a polycarbonate lenses².

Another use of polarized light is analysis of the distribution of stress and strain in plastics and glass materials, as shown in the image of a pair of glasses to the right.

Utilizing the polarization of light for elimination of glare

One common technique to elimination of glare, is to use polarizing material on both the light source and the camera lens. By polarizing the light source, all the light becomes polarized in a specific direction, for instance, {|} vertical. This direction depends on the orientation of the polarizing material. When the light hits a diffuse surface, most of the light becomes randomly polarized and reflects in random directions. Areas where strong reflections exist, however, will maintain much of the incident light state (or angle) of polarization. By placing another polarizing filter on the Camera lens, rotated 90deg relative to the light source angle of polarization (in this case, {--} horizontal), the strong reflections of polarized light are absorbed by the camera lens filter.  The diffused light (non-glare area) is transmitted and glare is eliminated.

Using a polarizing filter for both the light source and the camera, and crossing them, is not always necessary. The correct technique is largely dependent on the cause of the glare. Earlier, I mentioned the Brewster Angle. In this case, unpolarized light incident on a non-metallic surface (at the Brewster Angle) will become completely polarized in a plane parallel to the surface of the material. Elimination of the unwanted effects of “polarization by reflection”, often requires a single polarizing filter on the Camera Lens, rotated perpendicular to the polarization angle of the reflected light.  

Difference between Linear and Circularly Polarized Light

Circularly polarized light is possible through the wave properties of light. Interestingly, circular polarized filters can cause a “cool” or “warming” effect, in the light it transmits, when paired with a linear polarizer.  This is due to the birefringent material of the circular polarizer. For the vast majority of applications, regarding the elimination of glare on a non-metallic surface, linear polarizers are sufficient and most commonly used.

Circularly Polarized Light³.

 A Special Case of Polarization – 360deg Inspection Application

I am interested to know if anyone has ever come across a polarization filter, neither linear nor circular, but orientated as spokes and crossed with concentric circles. I had an application that needed one of these types of polarizing filters, due to the mechanical limitations of the application. A cylindrical object was inspected by multiple cameras and a single light source was used. Since polarization and glare are related to the geometry of the lighting/camera setup (in this case), each camera required a particular orientation of polarized light to eliminate glare within its field-of-view. A semi-spokes-like filter was made to polarize the light source, and reduce intense reflections. A commercialized filter with continuous properties would have been preferred.

References:

1.       Electromagnetic Spectrum. David J. Schneider. Department of Geological Engineering and Sciences Michigan Technological University. <http://www.geo.mtu.edu/rs/back/spectrum/>

2.       Polarization (waves). Spigget. 27 February 2010. Wikipedia. <http://en.wikipedia.org/wiki/Polarization_(waves)>

3.       Circular Polarization. Dave3457. 29 March 2010. Wikipedia.  <http://en.wikipedia.org/wiki/File:Circular.Polarization.Circularly.Polarized.Light_Right.Handed.Animation.305x190.255Colors.gif>