  The operation of visible-sensitive gaseous- and, to some extent,
vacuum-photomultipliers is critically affected by secondary electrons
originating from ion impact on the photocathode. A simple method for indirect
measurement of the effective ion-induced secondary-electron emission (IISEE)
coefficient from the photocathode into a gas medium, $\gamma_+^{eff}$ was
developed. The experimental results with visible-sensitive K-Cs-Sb, Na-K-Sb and
Cs-Sb photocathodes, yielded $\gamma_+^{eff}$ - values between 0.02 and 0.03 in
Ar/CH$_{4}$ (95/5) at 700 mbar; these are in good agreement with theoretical
calculations. The corresponding vacuum IISEE coefficients, $\gamma_+$, were
estimated, based on a theoretical model, to be 0.47, 0.49 and 0.47 for K-Cs-Sb,
Na-K-Sb and Cs-Sb photocathodes, respectively. The ratio of gas
$\gamma_+^{eff}$ and vacuum $\gamma_+$ IISEE coefficients, calculated to be
$\sim$0.06, is the fraction of secondary electrons surmounted the
backscattering in the gas media.
