Classic photomultiplier tubes (PMT) work in the ultraviolet, visible, and near-infrared range of the electromagnetic spectrum. These are electron tubes that pick up weak light signals (up to individual photons) and amplify them to such an extent that free electrons produce more free electrons of lower energy, whereby the weak input signals are converted into measurable currents. They typically consist of a photocathode and a downstream secondary electron multiplier (SEM) in an evacuated glass bulb. PMTs can have dead times of tens of ns. A further development of the PMT, the hybrid photodetector, combines this with avalanche photodiodes.