This paper presents the application of Signal Detection Theory (SDT) concepts to the certification of optional systems that provide operational or system safety benefits. The method and analysis yield quantitative requirements for the system performance that account for the risks and benefits of the potential system. This is in contrast to the standard certification approach which only focuses on Failure Conditions, and does not examine potential system benefits. A case study of an aircraft spotter on an electronic moving map illustrates that substantial safety improvements may be achieved even with the relatively low levels of software reliability associated with Commercial Off-the-shelf Software (COTS). The method makes few domain assumptions, and is based on the underpinnings of SDT and Bayesian probability theory, with well-established validity and reliability. Accordingly, the technique should have broad application to the certification of all optional aircraft systems.