We have fabricated 18mm format vacuum photodiodes incorporating GaAsP/A1GaAsP photocathodes (Eg 8eV) with P-20 phosphor screens. The photocathode response peaks at approximately 550nm. The quantum efficiency at 550nm is in excess of 55 (electrons per incident photon). The photocathode dark current for these tubes is less than i014 Amps/cm2 at room temperature. We have compared this cathode with the GaAs/AlGaAs photocathode.