The calibration of densitometers and microdensitometers is discussed in terms of the physical standards necessary to accomplish it. The need for calibration of these instruments is briefly considered, particularly for those forensic investigators looking at evidential photographic materials with a microdensitometer. The calibration process is reviewed for both instruments to prepare for a new procedure to follow. Since a great deal of microdensitometer work is carried out on materials about whose characteristics nothing is known, the problem of obtaining data in terms of diffuse (visual) density is almost impossible. However, a bootstrap calibration, based on the scan of a standard step tablet and scans of selected portions of the specimen is shown to be feasible. It requires a calibrated densitometer to quantify the results and has reasonable accuracy within the necessary assumptions. An example is given.