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The efficiency of S20 multialkali photocathodes is a function of thickness, wavelength, polarization and angle of incidence of light into the cathode. Therefore no single set of conditions can maximize performance over the entire spectral range. Consequently, two prototype photocathodes have been made with a gradation in thickness so that during monochromatic analysis the optical beam can be addressed to different thickness regions at preferred angles of incidence. This potentially enables a spectrum to be recorded under optimal conditions at a single interaction event. Relative to normal commercial S20 photomultipliers the quantum efficiency (QE) has been significantly raised across the entire spectral range. The current data were primarily obtained between 450 and 800 nm and at 450 nm it resulted in values of at least 65% QE, which is the highest value ever cited at this wavelength. Signals at longer wavelengths, for example at 750 and 800 nm, were recorded with up to 20 and 10% QE, respectively. Once again these are new record values that match performance from multiple interactions in waveguide cathodes. The data from this new design of photocathode underline the potential for improvements in efficiency for non-normal incidence in graded thickness photocathodes and indicate that current S20 technology could be significantly enhanced. Alternative enhancement methods are mentioned, particularly for spectrally dispersed signals. The enhancements are compared with data for a standard high quality S20 photocathode.

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Biological luminescence stimulated by optical excitation results in signals which are characteristic of the host tissue. The spectrum of the emitted light, the intensity, and the excited state lifetimes are modified as the result of disease or by activation through addition of cell selective phosphors. There is an opportunity to identify diseased tissue both by the spectral signals from activators or, in some cases, by the differences of the natural luminescence responses. For practical reasons, defined by the sensitivity range of standard luminescence detectors, much of the current work has focussed on the short wavelength emissions driven by laser activation. However, the techniques are poised to undergo a dramatic expansion in scope with the advent of higher sensitivity photocathodes with high efficiency responses at long wavelengths. It is now possible to utilise a greater range of emission features with improved discrimination. Further, movement to longer wavelength excitation, and emission, open the way to probe deeper beneath the surface of tissue. The current overview will focus on recent examples from detection of cancer to tooth caries and indicate how the non-destructive luminescence probes can distinguish between tissue changes at an early stage of development.

PACS 07.60.-j , 78.55-m, 78.60.-b, 79.60.-i Measurement of luminescence from 700 to 1,000 nm has been improved both by retrofit designs on existing photomultiplier tubes and design of new cathode structures. Based on the S20 type multialkali photocathodes the enhanced systems offer improvements over conventional PM tubes from a factor of ~2 from 200 to 700 nm, increasing to ~25 times on progressing to longer wavelengths from 900 to beyond 1,100 nm. These gains offer major changes in sensitivity for detection of weak long wavelength emission bands, with improved signal to noise and reductions in problems of second order light. Indeed, previously ignored red emission bands may dominate the photon energy spectra in some cases.