However, PMTs do have limitations for quantification and low light imaging due to limited sensitivity and slow pulse response. To this end, avalanche photo-diodes (APDs) have been employed for low light imaging. Due to their small dynamic range and long dark states, APDs have remained special purpose detectors, however.
Now, alternatively, hybrid photodetectors as implemented in Leica HyDTM combine elements from both, PMTs and APDs. Instead of a long cascade of dynodes as in PMTs (Figure 1 A) with potential for photon loss and noise propagation, they use a simple geometry with an electron bombardment step, producing large (10³) gain in a single step.
Figure 1: Working principle of different photodetectors: photomultipliers (A) and hybrid detectors (B). Both detectors use the photoelectric effect at the photocathode to convert light into electricity. The downstream amplification greatly differs, however. PMTs use a cascade of dynodes to create gain, while HyDs employ a two-step process involving an electron bombardment step and avalanche gain. HyDs therefore produce images with good contrast, good signal-to-noise and have excellent photon counting properties for quantification.
Figure 2: Leica HyDTM are available as multispectral HyD SP (A) or HyD-RLD in the non-descanned position (B)
The second step resembles an