According to the recommendations of the American College of Radiology (ACR), the SPEED of an x-ray film is typically measured at the step closest to 1.20 Optical Density. Also by this recommendation the 1.20 density is typically located at step #11 of the test film. This convention helps to ensure the Toe to Shoulder results described above. Please note that this is NOT a requirement - - rather it is a recommendation. Even so, it is not uncommon for a customer to 'tweak' his or her sensitometer to better match its exposure to their lab conditions.
In the initial setup of a processor, where quality control AIM values are being established for the first time, the user must average all 21 steps of five films exposed and processed over five days (one each day). From the average obtained, he then finds the step that yields a density closest to 1.20. If the test film exhibits a full Toe to Shoulder profile there is no need to adjust the sensitometric setting. If Toe is heavily fogged or should the Shoulder not exhibit full saturation then the sensitometric exposure should be adjusted.
Exposure from the sensitometer increases from exposure setting #1 to setting #7. The units are preset to setting #3 at the factory. This is generally considered the 'normal' setting.