Single parameter irradiance measurements commonly used for the determination of photo-resist exposure times are incapable of giving unique values of either the broad band irradiance from a mercury-arc lamp, or the photoresist exposure times. Resolution of these measurement incapabilities through a series of irradiance measurements of narrow, approximately monochromatic radiation bands is discussed. A basis for establishing photoresist sensitivity indices that can be used to calculate. unambiguous exposure times by applying Van Kreveld's additivity law to these narrow band irradiance measurements is suggested. The thickness variation of photoresist films is included in the method of calculating exposure time. Conditions that may simplify the calculations of exposure times or reduce the number of irradiance measurements when a mercury lamp is used as the exposure source are discussed. Suggestions are made for the use of these indices as photoresist specifications, and for the real-time automatic monitoring and control of exposure times in wafer fabrication where optical density variation in the photomasks and thickness variations in the photoresist films are automatically incorporated into the control by a microprocessor.