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Leica HyDTM

Hybrid detection combines the best of PMTs and APDs

It is the photodetector’s purpose to translate light into electrical signals. Hence, it is a critical part of the recording process. Not only should it harvest fluorescence light as efficiently as possible, but it should also have a large dynamic range, rapid response and low noise to produce crisp and quantifiable images. For a long time, photomultiplier tubes (PMTs) and their derivatives such as multianode arrays have been the standard in confocal photodetectors. Their large dynamic range and reasonable noise level contribute to this success.

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 APD with respect to secondary electrons which are further amplified by an avalanche effect in a semiconductor (Figure 1B, see also HyD tutorial on Leica Science Lab). This arrangement makes HyDs very sensitive, while producing little dark noise, and almost no afterpulsing. These properties make them suitable for a wide range of samples. For a thorough comparison of the three types of detectors, please see the corresponding article on Leica Science Lab by Rolf Borlinghaus. Since HyDs have a very short dead time and short response pulses in the range around 100 ps, they can apply photon counting to brighter samples than other photon counting detectors. Hence, quantification of stoichiometries in live samples, for example, is greatly facilitated. HyDs come in different varieties and are available as HyD SP for multispectral imaging and HyD-RLD for non-descanned detection in multiphoton imaging and for fluorescence lifetime imaging (FLIM).