Optical dating was developed primarily for sediments (though application to pottery has also been made) and has the particular advantage over thermoluminescence (TL) that at deposition the latent signal is orders of magnitude smaller; this is particularly important for sediments deposited in water. For routine dating of quartz the signal is stimulated by green light, from an argon ion laser, from a filtered xenon lamp, from a filtered halogen lamp or from light-emitting diodes (though at much lower intensity); the dominant emission wavelength is in the ultraviolet. With quartz, the limitation in age range is due to saturation rather than poor stability. For many types of feldspar there is a remarkable resonance in the stimulation spectrum in the near infra-red (IR). This leads to the widespread use of IR-emitting diodes for stimulation; the whole range of visible wavelengths is then available for detection and this gives the possibility of some degree of mineral selectivity by means of colour filters. The IR signal appears to have comparable stability to that of the green-stimulated signal and TL - no worse, no better; it has the advantage over both that it is more easily bleachable by daylight. In general, the signal from feldspars is much brighter than from quartz and this has allowed the development of 'single aliquot dating', with advantages in precision and convenience as well as in application to poorly bleached sediments.
