Phosphors-image intensifiers

In the optoelectronic X-ray image intensifier (Fig. 86), [5.427], the X-ray phosphor screen (input screen) is in direct optical contact with a photocathode that converts the luminance distribution of the X-ray screen into an electron-density distribution. The liberated electrons are accelerated in an electric field between the photocathode and an anode (20-30 kV) and are focused by electron lenses onto a second phosphor screen (output screen), where conversion of the electron image to a visible image takes place. 

For medical (X-ray) and military (infrared) uses, X-ray and IR fluorescence and image intensifiers were developed. An input phosphor converts X-ray (or IR) photons to electrons, which are electronically intensified and... 

In a TVD, X rays are converted by a fluorescent phosphor (gadolinium oxysulfide or ZnS) into visible light, which is amplified by an image intensifier (essentially a second phosphor) and detected by the photocathode of a high... 

Fig. 18. (a) Ultrafast electron diffraction apparatus consisting of an electron gun chamber, a diffraction chamber, and a detector chamber. Two fs laser pulses are used, one to initiate the chemical change and the second to generate the electron pulse, (b) Detector system incident electrons either directly bombard a small CCD or strike a phosphor-coated fused fiber-optic window. Light emitted from the phosphor is amplified by an image intensifies and brought to a scientific-grade CCD. Both CCDs are thermo-electrically cooled [reproduced with permission from (96), p. 1601. 

Figure C3.1.5. Schematic diagram of an intensifier-gated optical multichannel analyser (OMA) detector. The detector consists of a microchannel plate (MCP) image intensifier followed by a 1024-channel Reticon photodiode array. Light dispersed across the semitransparent photocathode ejects photoelectrons. These are accelerated toward the entrance of the microchannels by the gate pulse. The photoelectrons collide with the channel walls to produce secondary electrons, which are accelerated in turn by the MCP bias voltage to produce further collisions and electron multiplication. Electrons leaving the microchannels are further accelerated by the phosphor bias voltage.

Fig, 4.100a,b. Cascade image intensifier (a) schematic diagram with cathodes C/, fluorescence screens P/, and ring electrodes providing the acceleration voltage (b) detail of phosphor-cathode sandwich structure... 

A manufacturer of a two-stage optical image intensifier states that incident intensities of 10 W at A = 500 nm can still be seen on the phosphor screen of the output state. Estimate the minimum intensity amplification, if the quantum efficiency of the cathodes and the conversion efficiency of the phosphor screens are both 0.2 and the collection efficiency of light emitted by the phosphor screens is 0.1. The human eye needs at least 20photons/s to observe a signal. 

Figure 3.1 schematically represents time-resolved experimental setup used in our experiments. The excitation sources were pulsed lasers, such as excimer XeCl (308 nm), nitrogen (337 run), three harmonics of Nd-YAG (266, 355 and 532 nm), and tunable dye and OPO, which deliver pulses of 10 ns duration. The spectra observed at the geometry of 90° are analyzed by intensified CCD matrix. Image intensifiers comprise three main components a photocathode, microchannel plate (MCP) and phosphor screen. The standard operation of these devices starts when the incident photons become converted into electrons at the photocathode. The electrons then accelerated towards the MCP where they are multiplied to an amount... 

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