Phosphor persistence in oscilloscopic displays: its luminance and visibility

Phosphor persistence and its potentially confounding effects in visual experiments were discussed recently in these pages (Groner, Groner, Miiller, Bischof & Di Lollo, 1993). In that report, we showed that phosphor persistence can remain visible for extended periods (in the order of hundreds of msec) on oscilloscopic screens coated with P31 but not with P15 phosphor. In a dissenting research note, Westheimer (1993) dismisses our evidence as stemming from “very indirect exper- iments” and concludes that the persistence of P31 phos- phor, as measured by a photometer, decays within about 2 msec. This 2-msec estimate could be easily taken as applying equally to photometers and to human observ- ers. To obviate this misinterpretation, the generality of Westheimer’s conclusion needs to be qualified. A common reason for measuring the temporal course of phosphor decay is to provide engineering specifica- tions for display oscilloscopes (e.g. Bell, 1970). For this purpose, the measuring instrument of choice is a photo- meter, not the human eye. As pointedly noted by Westheimer (1993), human eyes would supply only indirect estimates. We agree: instead of the direct lumi- nance readings provided by a photometer, visual re- sponses yield only