One method for correcting the effects of non-uniform illumination in a digital image is to measure background brightness at a number of points. These values can then be used to generate a background using a polynomial function. Usually either a second or third order polynomial will provide a good fit to the nonuniformity that results from optical vignetting or the fall-off in illumination due to condenser lens settings or copy stand lights. This interactive Java tutorial allows the user to select points from which to fit a background to be subtracted, illustrating the result of subtracting a polynomial function to correct nonuniform illumination.

	Interactive Java Tutorial
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The tutorial initializes with a randomly selected specimen imaged in the microscope appearing in the Specimen Image window. The Choose A Specimen pull-down menu provides a selection of specimen images, in addition to the initial randomly chosen one. The Images radio buttons choose a set of either Color or Grayscale specimen images. Click on the marked points on the image and drag them to the locations to be used for fitting the polynomial. The size of the sampled area at each location is controlled by the Control Point Size slider. The amount of brightness offset added to the result after the polynomial has been subtracted is adjusted using the Brightness Offset slider. The Display Image radio buttons show the Background image or the Subtraction image. Click on the Background Image radio buttons to display either the results of the polynomial fit to the user-selected locations, or a pre-recorded actual background image for comparison.

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