Images from conventional video systems are being digitized in computers for the analysis of small particles. The method was developed to automate particle counting and area measurements of dark trash partides in cotton. However, it can be applied to many similar area measurement problems. Because video output is linearly proportional to the amount of light reflected, the best spectral band for optimum particle discrimination should be centered at the wavelength of maximum difference in reflectance between particles and their surroundings. However, due to the distribution of the illumination energy and the distribution of the detector sensitivity, peak performance bands are shifted. Reflectance from cleaned cotton samples and trash particles were measured to define spectral contrast. Pixel intensity histograms from the video system are reported for simulated area reference samples (painted dots on panels) and for actual cotton samples to demonstrate the particle discrimination mechanism. Particles from different parts of the cotton plant were used to demonstrate the importance of considering a particle's contrast in its area measurement.