Image intensifier: Phosphor screen
The phosphor screen emittes photons if accelerated electrons hit the material. The most common use of phosphor screens are cathode ray tube displays which are used in the early TV's and oszilloscopes. Phosphors for these cathode ray tubes were standardized and designated by the letter "P" followed by a number. The phosphor screen of image intensifiers converts the electron avalance from the micro channel plate back into photons.
The phosphor screen converts accelerated electrons into photons.
Typical conversion factors of the used phosphor screens are between 20 and 200 photons per electron, depending on the phosphor type and the kinetic energy of the electrons, i.e. the acceleration voltage. In order to increase the number of photons emitted in the direction towards the CCD sensor, the backside of the phosphor is coated with an aluminum layer that reflects photons towards the proper direction, as shown in the enlarged detail above.
Several different phosphors typs are available which differ in the emitted spectrum and in decay time, i.e. in fluorescence lifetime, as shown in the according pictures below.
The quantum efficiency of different phosphor screens.
The decay times (fluorescence lifetime) of different phosphor screens.
An optimum phosphor screen will be choosen for the specific requirements of your application. There are three important considerations in choosing a phosphor screen. First the efficiency, second the phosphor decay time and last the spatial resolution.
The two mostly used phosphor screens for image intensifiers are P43 and P46. The P43 phosphor screen has a higher efficiency and higher spatial resolution due to smaller grain size. However, it has a long decay time. For fast applications e.g. double frame mode with a interframing time of 500ns the faster decaying P46 phosphor screen is necessary to avoid gost images from the previous exposure. The trade-off of the P46 phosphor screen are lower efficiency and lower spatial resolution.
Especially, at the double frame mode it need to be ensured that the fluorescence of the phosphor from the last image has sufficiently died down before the CCD sensor read out the second image. This is to avoid any loss of light and, even more important, to avoid crosstalk to the next image.
|Conversion Efficiency||200||95||photons per electron|
|decay time 100% => 10%||1500||0.2||µs|
|decay time 100% => 1%||3200||20||µs|