Linear Reference Comparators. When using SPIPs or DDAs, a measuring scale or an object of knowndimension shall be used to serve as a linear reference comparator. The comparator shall be attached to the SPIP holder orDDA prior to exposure. As an alternative, when using SPIPs a transparent scale with opaque gradations may be placedon the SPIP prior to processing. In any case the reference comparator shall not interfere with interpretation of the image.