TEST OF AN INSTRUMENTAL IMPLEMENTATION OF AN EDGE-LOSS ERROR CORRECTION SYSTEM

摘要

Reflectance measurement data for plastics and other translucent materials (e.g. paper) can be in error due to the lateral diffusion of light within the sample. Illuminating light from the measuring instrument can laterally diffuse to sample areas outside of the area which is viewed by instrument photometric measuring system. Figure 1 illustrates an idealized representation of a 45/0 measuring system in which some of the illuminating light laterally diffuses to areas which are outside the view of the measuring detector. This results in a type of measurement error which the author has characterized as lateral diffusion error (LDE) to note the physical process which causes the error. Elsewhere in the literature it has been described as translucent blurring error, translucency error, edge loss error, and stray darkness. While the example in figure 1 shows a 45/0 measurement configuration, LDE can occur with all measurement configurations (e.g. 0/45, d/8, etc. ). It is caused by the instrument illuminating light diffusing lateral within the sample. Lateral diffusion is present in all non-metallic samples to some extent. A common solution for minimizing LDE is to make the area viewed by the detector larger than the area illuminated by the light source. Thus, in the case of the idealized instrument configuration of figure 1, doubling the area viewed by the detector would allow all of the light reflected directly from the illuminated area and the light that was laterally diffused and emerged from the unilluminated surface to be viewed by the detector. An alternative is to leave the area viewed by the detector untouched and to double the area illuminated by the light source. While this may not intuitively seem equivalent to increasing the area viewed while holding the area illuminated constant, optical reciprocity, as defined by Clark and Perry, indicates that this method should give equivalent error reduction. The exact amount of over-viewing or over-illumination area required to minimize LDE is very dependent on the translucency of the sample being measured. This explains, in part, why instruments used for measuring plastics are general not useful for measuring graphic arts products and vise-versa. Also, ideally all areas of the sample illuminated and viewed should have the same surface and color characteristics. This can present a problem in the color control of graphic arts printing where color control strips are included and take up otherwise marketable product space. As a result, the elements of these color strips are kept very small to minimize unmarketable content. This limits the size and amount of over-illumination or over viewing that can be used to minimize LDE.