The intent of this regulation is to provide the technologist performing the examination reasonable assurance that there will be consistency and reproducibility between the images produced using the same type cassette. However, a facility may use cassettes specifically designed to be of different speeds to deal with various clinical problems. In addition, cassettes of different sizes, even from the same manufacturer, may yield different screen speeds due to differences in design and the use of different types of screens. In those cases where a facility has clearly and permanently identified groups of cassettes of different speeds, has established mammographic technique charts to compensate for the different speed cassettes, and has made these charts available to all their radiologic technologists, the facility can group these cassettes for purposes of the screen speed uniformity test. As long as the difference between the maximum and minimum optical density of all cassettes within a group does not exceed 0.30, the requirement has been met. For any group of cassettes used to image the standard breast, the facility must assure that the radiation dose does not exceed the requirement limit of 3.0 milligray (mGy).