In the past decade, several persistent phosphors were intensively studied, such as CaAl(2)O(4):Eu,Nd (blue) and SrAl(2)O(4):Eu,Dy (green). Yellow-to-red emitting persistent phosphors have also been identified. All these phosphors are normally evaluated by measuring their afterglow as a function of time in photometric units. However, the eye sensitivity curve shifts from photopic vision to scotopic vision upon decreasing light intensity, with a corresponding decrease in red-sensitivity. Performance of red-emitting persistent phosphors thus cannot be described with usual photometry. An alternative description is presented, based on recent vision research in the intermediate (mesopic) region and using a unified system of photometry. It turns out that this effect seriously compromises the development of red-emitting persistent phosphors. This is illustrated on a number of commercially available persistent phosphors.