Scalable Large Area Spectral Imaging for Thickness Maps
Publication Date: 2015-Nov-19
The IP.com Prior Art Database
This paper describes a novel method for measuring specular reflections suitable for white light interferometry and thickness measurements Unique to this method over traditional processes is cost effective scalability to large areas This is accomplished by combining off normal light with angular filtering and scaling factors
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Scalable Large Area Spectral Imaging for Thickness Maps Abstract
This paper describes a novel method for measuring specular reflections suitable for white light interferometry and thickness measurements. Unique to this method, over traditional processes, is cost effective scalability to large areas. This is accomplished by combining off normal light with angular filtering and scaling factors.
Inspection of a web in a roll-to-roll process is often critical for quality and process control. While many different inspection tools may be used, specular reflections, particularly when combined with spectroscopy, frequently contain useful information about many coating and web properties such as iridescence, thickness, color and chemical composition. Iridescence and thickness information are encoded in white light interference patterns from light reflected at each layer interface similar to a soap bubble or oil on water. Color and chemical composition information may appear when one or more layers absorb specific wavelengths of light. In visible wavelength, absorbances create color changes while infrared absorbances frequently relate to specific chemical structures. Even though all of these effects may also be seen in transmission, specular reflections often have much higher contrast and, unlike transmitted light, they can still be measured in the presence of backing layers that block or scatter light.
A major challenge when imaging specular reflections is scaling up the imaging to areas larger than a few inches. Using traditional imaging with collimated light normal to the surface, some of the optics need to be scaled to the size of the imaging area. A 52" web for example would require a 52" or larger lens. This is both difficult to integrate and cost prohibitive
This paper describes a novel method for measuring specular reflections suitable for white light interferometry and thickness measurements over a large area. This is accomplished by combining off normal light with angular filtering and scaling factors. As represented in Figure 1, light is reflected off of the sample pointed towards the camera. At the camera, an aperture is used (may be the camera lens pupil) to ensure that light from effectively only one angle reaches the camera from each point on the sample. The scaling factor is used to correct for the off normal angles.
Figure 1: Angular Light Transmission and Reflection
A sample is illuminated with a fiber line light. The light must be designed such that at each point along the line the emitted light includes a cross web angle that aims directly at the point where the spectral camera will be located (refer to Figure 1). A traditional forward pointing line light with 0.55 NA fibers will not produce sharp enough illumination angles with even intensity at the light ends to accommodate for a 60" working distance and web. Therefore, a custom...