dye precursor molecules — normally rhodamine base — held in a transparent matrix are reactive with acids , bases , ions or radicals — and in the case of rhodamine are reactive with acids — to produce dye molecules — i . e ., rhodamine — having markedly different spectroscopic properties . light - sensitive molecules - normally a compound of ortho - nitro - aldehyde , in particular o - nitro - benzaldehyde or , preferably , 1 - nitro - 2 - naphthaldehyde — in the same matrix undergo photochemical reaction when selectively exposed to light so as to form at least one of the acids , bases , ions or radicals with which the dye precursor molecules are reactive — preferably nitroso acid . chemical reaction of rhodamine base dye precursor molecules with photochemically produced nitroso acid within domains that are radiatively - selected two - dimensionally or within voxels that are radiatively - selected three - dimensionally , by first - frequency “ write ” radiation — including as may be realized by two - photon absorption — produces stable rhodamine dye in the radiatively - selected domains / voxels . subsequent illumination with a single , second - frequency “ read ” radiation induces strong fluorescence in the dye of the written domains / voxels while leaving all chemicals / photochemicals unchanged . the induced fluorescence may be imaged to a detector , such as a charge coupled device , to reliably realize a high signal - to - noise , non - degrading , optical memory of the write once , read many type .