NIH Image

Performs spatial calibration so that results from length and area measurements are presented in calibrated units, such as millimeters. Before setting the scale, use the line selection tool to make a straight line selection that corresponds to known distance. Then, bring up the Set Scale dialog box, select a unit of measurement from the Units pop-up menu, and enter the known distance in Known Distance.

Setting Pixel Aspect Ratio to a value other than 1.0 enables support for different horizontal and vertical spatial scales, for example 100 pixels/cm horizontally and 95 pixels/cm vertically. Before setting the aspect ratio, calibrate to a known horizontal distance. Then enter the pixel aspect ratio into Pixel Aspect Ratio. To find the pixel aspect ratio, measure the width and height (in pixels) of a digitized object with a known 1:1 aspect ratio. The pixel aspect ratio is computed by dividing the width by the height. Note that only the horizontal scale is shown. Divide the horizontal scale by the aspect ratio to compute the vertical scale. Note that the Measure command sets the ellipse major and minor axis lengths to zero when the pixel aspect ratio is not one.

Once the scale is set, you can switch to other units of measurements, for example from inches to centimeters. If you know what the scale for an image is (e.g., 300 dpi), then that value can be directly entered into Scale. Select Pixels from the pop-up menu to disable spatial calibration. To use a unit of measurement that is not predefined, select Arbitrary Units and manually type in the unit. As a shortcut, double-click on the line selection tool to bring up the Set Scale dialog box. Note that Image displays a black diamond in the title bar of spatially calibrated images.