Smoothly varying illuminant

We have indicated that intensity dependent phenomena may be useful in at least two distinct ways. One is to obtain something approaching a "threshold detector" resist response. To obtain a threshold development response in typical positive resists is difficult, since the development rate is in general a smoothly varying function of the photochemical reaction progress. The application of a layer of polymer with the bleaching characteristics shown in Figure 5 provides a way to obtain such threshold response with conventional resists, provided an excimer laser is used in the illumination system. 

So far, we have mostly assumed that objects are directly illuminated by a light source. But what about shadows In real world scenes, one object may cast a shadow on another object. In this case, the area outside the shadow is directly illuminated, while the shadow area is diffusely illuminated by light that is reflected from other objects in the scene. Thus, for a natural scene containing shadows, we have at least two different illuminants, the light source and the illuminant of the shadow area. In Chapter 7, we have already seen a number of algorithms that can cope with an illuminant that varies smoothly over the image. A shadow, however, may have a sharp boundary. We discuss an algorithm for shadow removal in this chapter. 

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