The brightness of a pixel is proportional to the amount of light
directed by the surface patch which projects on to the pixel. Light
reflected from an object is characterized as being either diffusely reflected or specularly reflected. The intensity of
diffusely reflected light in all directions
is proportional to the cosine of the angle
of the incident light and the reflectance or albedo of the
surface (Lambert's law). On the other hand, specularly reflected light is
reflected only along a direction where it is equal to the angle of
incident light. In real life surfaces are a mixture of Lambertian
(i.e. diffusely reflecting or satisfying Lambert's law) and specular
(from the Latin speculum meaning mirror). An example, sort of
canonical in the image processing literature is shown in Figure
1. Each pixel has an intensity 
.
Figure 1: Showing a digitized image from a camera
A basic operation that one can perform with images is thresholding.
This operation consists in determining the maximum and minimum of the
intensity values in the image denoted 
and replacing the
pixel intensity 
by
where
is a parameter used for scaling the threshold
These demos are visualized in
http://robotics.eecs.berkeley.edu/sastry/ee20/index.html
for different values of the threshold parameter
.