Image intensifier: Photocathode
The photocathode converts the incoming photons to photo electrons by collisional ionization. The photocathode is, therefore, directly behind the entrance window of any image intensifier. Multiple photocathodes cover the spectral range from UV, VIS to IR. The photocathode sensitivity is one important property to choose the most suiting photocathode for your application or the predominant imumination.
The graph shows the quantum efficiency of selected photocathodes.
Photocathode incoming photons to photo electrons.
The specific material composition of the photocathode enabels to cover different spectral ranges. The incoming light first meets the photocathode of the ICCD cameras image intensifier. The photocathode converts the incoming photons to photo electrons by collisional ionization. To obtain a maximum signal to noise ratio in the images the spectral sensitivity of the photocathode should be well fitted to the applications light spectrum. A maximum number of photoelectrons per incoming photon is generated in this way, i.e. a maximum integral quantum efficiency is achieved. The photocathode of the image intensifier will therefore be selected according to the specific requirements of your application. A selection of popular photocathode materials is shown in the diagram above. Furthermore the sensitivity of these individual photocathodes can be extended to the UV by using a MgF2.
Spectral sensitivity is selected by photocathode and entrance window
Before the photons hit the photocathode, they have to pass the entrance window. Quarz is used as standard material for the window. However, for UV applications a MgF2 window may be applied. The ultraviolet transmission factors of both materials are shown in the appropriate diagram below.
The entrance window and the photocathode as sectional drawing.
The graph below shows the transmission of the MgF2 and quarz window in comparison.
Transmission of input windows. The MgF2 window extends UV and VUV.