Cathode luminous sensitivity

Several different definitions are used to speeify the effieieney of a PMT eathode. Often the sensitivity of a PMT is speeified in units of eathode luminous sensitivity". This is the cathode eurrent per lumen incident light from a tungsten lamp operated at a temperature of 2,856 K. Beeause the intensity maximum of the lamp is at about 1,000 nm, the luminous sensitivity may not represent the efficiency at a given wavelength. Photocathodes of different spectral sensitivity are therefore not directly comparable. Moreover, the cathode luminous sensitivity does not include the effieieney of the eleetron transfer from the cathode into the dynode system and the possible loss of photon pulses due to incomplete resolution of the pulse height distribution (see Fig. 6.27, page 241). 

Fig. 6.27 Measurement of the Cathode Luminous Sensitivity (Ze/f) and the Anode Luminous Sensitivity (right)...

The Anode Luminous Sensitivity" is measured in a similar setup as the Cathode Luminous Sensitivity. However, the PMT is operated in the normal way, i.e. by applying the specified voltage distribution to the dynode chain (Fig. 6.27, right). Of course, a calibrated filter has to be inserted in the light path to attenuate the light of the lamp to a level that does not overload the PMT. As mentioned before, the anode luminous sensitivity is not very useful in characterising a PMT for TCSPC. 

The eathode luminous sensitivity" is the eathode current per watt incident light power from a tungsten lamp operated at 2,856 °C. The cathode luminous sensitivity is the integral of the product of the eathode radiant sensitivity and the lamp speetrum. Beeause the lamp has its emission peak in the NIR the cathode luminous sensitivity lets the sensitivity of NIR-sensitive cathodes appear higher than it aetually is. 

In the test sheets of PMTs, the manufacturers occasionally specify the measured anode luminous sensitivity" instead of the cathode sensitivity. The anode sensitivity is the cathode sensitivity (including the electron transfer efficiency) multiplied by the gain of the tube. Because almost any gain can be obtained by increasing the supply voltage, the anode luminous sensitivity cannot be used to compare the photon counting performance of PMTs. 

In NEA Si cold cathode devices, a planar diode is biased to inject electrons from an n-type substrate to a p-type NEA surface layer, where current is drawn off by an electric field. (Other cold cathodes of different design have been demonstrated in III-V structures [5.130-132].) This efficient emission could be useful in replacing hot (thermionic) cathodes in IR sensitive devices where a low luminence source with small electron energy spread is desirable, but requirements of zero contamination along with electron emission-density restrictions and long-term instability problems have so far prevented practical use. 

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