A glass sorting machine optically sorts glass cullet according to different color characteristics by irradiating the cullet with red, green and blue light as the cullet are passed through a sensing region between a light source and light sensors responsive to the different light colors. The attenuation levels of the different light colors are measured by the sensors as the cullet passes through the sensing region. The machine analyzes the attenuation levels to determine the color characteristics of the cullet. A collimator is typically used to enhance shadow definition. In one embodiment, a pixel grid map is made of the sensing region and each cullet that passes through the sensing region. Mapped on to each grid is a red, green, and blue digital signal. Attenuated signals are compared against baseline values to determine the attenuation for given cullet. The pixel grid map may be used with a data erosion technique to compensate for refraction. To reduce errors associated with optical impurities, the machine normalizes the baseline values when cullet are not in the sensing region by taking the derivative of the sensed signal and normalizing the baseline signal when no cullet are present.

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