Phototubes

A phototube (also called photoelectric cell or vacuum photodiode), invented by Julius Elster and Hans Geitel in 1893, is a photoemissive detector based on a small glass tube containing electrodes where the external photoelectric effect (or photoemissive effect) is utilized. Such tubes are often evacuated or sometimes filled with a gas under low pressure.

An ordinary phototube contains only two electrodes: the photosensitive cathode (photocathode) and an anode. During operation, some voltage (e.g. 15 V or 50 V) is applied to the electrodes (the positive pole to the anode), so that photoelectrons are rapidly swept from the cathode to the anode, and a photocurrent can be measured. For an evacuated phototube (vacuum phototube), the photocurrent depends on the incident optical power according to the equation

where ($\eta$) is the quantum efficiency, ($e$) is the electron charge, and ($h\nu$) the photon energy. (The quantity ($S$) is called responsivity.) The usable dynamic range can be quite large, for example with photocurrents between a few picoamperes (pA) and a few microamperes (μA), although the maximum allowed photocurrent is usually far below that of a photodiode. There is hardly any temperature influence on the responsivity.

The electronics used in conjunction with phototubes can be similar to those for photodiodes (see the article on photodiode amplifiers), except that the required voltage tends to be higher for phototubes.

A simple circuit with a load resistor for converting the photocurrent into a voltage may be used. An advantageous aspect is the typically lower electrical capacitance of phototubes, at least when compared with photodiodes with large active areas. This allows one to use a higher load resistor for given detection bandwidth, so that a higher voltage is obtained.

A transimpedance amplifier, receiving the photocurrent as input and providing a voltage output, is also suitable for phototubes. Of course, low-bias operational amplifiers should be used to avoid an apparent dark current.

For operation in the picoampere region, care must be taken to avoid leakage currents e.g. on a circuit board or at the contacts of the phototube; one should not touch such things with bare fingers.

A special form of the phototube is the photomultiplier tube. This contains additional electrodes, with which a high amplification of the photocurrent can be achieved based on secondary electron emission. Such devices are normally called photomultipliers, i.e., a phototube is normally considered to be a simple tube with two electrodes only.

Nowadays, phototubes are largely replaced by solid-state devices like photodiodes, where the internal photoelectric effect is used. For some applications, however, phototubes can still have substantial advantages:

• They can have a lower dark current, leading to a lower noise equivalent power.
• They can be made with a large photosensitive area while still having a high detection bandwidth.
• The high dynamic range and high stability of phototubes can be advantageous for precise measurements, e.g. in spectrometers.
• Higher detection bandwidth can be achieved, also in combination with high sensitivity.
• Particularly for UV applications, the high stability (compared with semiconductor devices, for example) can be a substantial advantage.

On the other hand, phototubes have a lower quantum efficiency, are more sensitive to mechanical vibrations and shocks, typically need a high operation voltage and can handle only a very limited amount of photocurrent. Particularly for high-sensitivity applications, it must be avoided that bright ambient light hits the photocathode while the device is turned on. In environments where helium can be contained in the air, it may happen that helium gas diffuses through the glass tube and then affects the performance of the phototube.