Sensitivity

In an image intensifier, the photocathode sensitivity measures the efficiency of the photocathode at converting incoming light into electrons.

Photocathodes have a spectral response, which means that their sensitivity differs depending on the spectrum of the light.

As measuring the sensitivity requires measurement of the current being produced by the photocathode, it can only be measured in isolation prior to assembly of the image intensifier tube.

There are two different types of photocathode sensitivity, luminous transmission sensitivity and radiant sensitivity.

Luminous Transmission Sensitivity

Luminous transmission sensitivity is measured in microamperes per lumen (μA/lm). It defines how much current the photocathode produces when exposed to a blackbody radiation spectrum of a certain temperature and intensitiy, usually covering the spectrum visible to the human eye.

This measurement is useful to determine how well the image intensifier will pick up visible light.

Measurement

The MX-10160/AVS-6 specification defines the test setup under which luminous transmission sensitivity at a light temperature of 2865K is measured.

• Photocathode supplied with no more than 800 Vdc
• 16mm diameter circle of photocathode illuminated with 0.001 lm of 2865K tungsten lamp radiation

The sensitvity is calculated from the output current, corrected for leakage and dark current, divided by the input light intensity (in lumen).

Radiant Sensitivity

Photocathode radiant sensitivity is measured in milliamperes per watt (mA/W). It defines how much current the photocathode produces when exposed to certain amount of input radiation of a specific frequency, usually in the infrared band.

This measurement is useful to determine how well the image intensifier will pick up infrared light, which is especially advantageous when using infrared illuminators or markers.

Measurement

The MX-10160/AVS-6 specification uses the same test setup used to determine the luminous sensitivity to measure the radiant sensitivity at a wavelength of 830nm and 880nm respectively.

• 830nm / 880nm filters inserted between the photocathode and the lamp
• The 16mm circle illuminated with a filtered radiation level between 1 x 10^-6 and 1 x 10^-4 watts

The sensitivity is calculated from the output current, corrected for leakage and dark current, divided by the input radiation level (in watts).