AN ALEXANDRITE EFFECT AND OPTICAL-PROPERTIES OF HOLMIUM METAPHOSPHATEGLASS

Holmium phosphate glass of composition near to that corresponding to m etaphosphate R(PO3)(3) shows an effect of the alexandrite type: its pe rceived colour changes when it is viewed under different light sources . The optical absorption and fluorescence spectra conform with those e xpected if the holmium ions are present as Ho3+. Although this glass f luoresces, the strength of the measured argon ion laser excited fluore scence spectrum is small so that the color changes cannot be due to li ght induced fluorescence. Comparisons made between the emission spectr a of light sources, whose radiation causes the glass to appear to have a different colour, and the glass transmission spectra obtained using these same sources shows that the light transmission through optical windows in the glass absorption spectrum is the dominating effect in d etermining colour. The colour changes in this glass resemble those fou nd in the laser host crystal alexandrite and have a similar spectral o rigin.