A replaceable unit for an image forming apparatus according to one example embodiment includes a housing and at least one transmissive member positioned on an exterior of the housing. The at least one transmissive member is arranged to receive optical energy from the image forming apparatus and has a transmissivity for modifying an amount of the optical energy that leaves the at least one transmissive member relative to an amount of the optical energy received by the at least one transmissive member. The at least one transmissive member indicates information relating to a characteristic of the replaceable unit.