Monochromatic light sources in testing image intensifier tubes

Military standards and literature sources recommend use of polychromatic light source of 2856K color temperature in testing image intensifier tubes. Tungsten halogen bulbs are standard sources of radiation used in commercially available test stations. However, there are two main disadvantages of tungsten bulbs: short life time and temporal changes of both light power and light spectrum. Application of monochromatic semiconductor sources in testing of image intensifiers as possible alternative of halogen bulbs is analyzed in this paper. It is shown that, if calibrated properly, monochromatic semiconductor sources can be used in stations for testing image intensifier tubes. Military standards require use of polychromatic light sources of 2856K color temperature in testing image intensifier tubes(1.a.i.(1)-1.a.i.(6)). Tungsten halogen bulbs have been standard sources of radiation used in commercially available test stations for last few decades in testing image intensifier tubes (1.a.i.(7),1.a.i.(8)). However, tungsten halogen bulbs have several significant limitations. First, they are not stable and emitted flux varies significantly in time. Second, they are characterized by short life time. Third, due to significant temporal inertia it is necessary to use mechanical shutters to enable tests of temporal characteristics of image intensifier tubes (II tubes). Sensitivity of image intensifier tubes depends significantly on wavelength. Therefore, illumination of the photocathode with 2856K color temperature light is not equivalent to illumination with monochromatic light of the same radiation power. However, let us analyze if properly calibrated monochromatic sources can replace 2856K color temperature halogen bulbs in test equipment to be used for testing image intensifier tubes and if accurate measurement of photometric parameters of II tubes is still possible. Quantitative relationships between input illuminance and output luminance is measured during the photometric tests of II tubes. The MIL standards state very precisely the necessary illuminance levels of 2856K color radiation to be used during measurement of photometric parameters of II tubes. The conversion coefficient between the illuminance level of 2856K light and the equivalent illuminance level of monochromatic light depends on spectral sensitivity curve of the tested tube that can vary with different tubes. Therefore we should use different conversion coefficients in order to calibrate properly the monochromatic source used during tests of tubes of different spectral sensitivity.