MIL-DTL-62422D(AT)
procedure II for five 24-hour cycles. After environmental testing, the filters shall show no
evidence of physical damage and shall again meet the optical requirements as specified in 3.5.
4.5.3 Optical density measurement. To determine conformance to 3.5.1, filters shall be
subjected to the measurements as specified in 4.5.3.1, 4.5.3.2, and 4.5.3.3.
4.5.3.1 Optical density measurement using a laser (first article).
4.5.3.1.1 Apparatus. National Institute of Standards and Technology (NIST) filters shall
be used to calibrate the measurement system for optical densities 3 through 4. The detector
configuration shall be such that each pulse, in its entirety, shall be monitored prior to passing
through the filter (for example, see figure 2). Optical density shall be measured for all angles as
specified and at least 0.50 inches away from any edge. The sample detector shall have a
signal-to-noise ratio of 10 or more and shall be located at least 200 mm from the laser. If the
system is calibrated for optical densities 3 through 4, the optical density of the filter can be
inferred by comparing the sample detector reading for the calibrated filter to the reading
achieved for the filter.
4.5.3.1.2 Optical density (first article). Detector responsivity calibrations shall be as
specified in 4.5.1.2. Each filter shall then be tested at a wavelength of 694 nm using a linearly
polarized laser with a collimated beam diameter of 4.8±1.7 mm (1/e2). The energy distribution
over the area of the laser pulse shall be a gaussian shape. Measurements of the noise level (flash
tube pulse but no laser light to the detectors) without the filter in the optical train shall be a
reading determined as an average of at least 100 pulses. The value of 100 pulses shall be used
assuming that the minimum value achieved with the filter in the optical train is 1.5 orders of
magnitude above the noise level. Similarly, readings 1 through 3 shall be values determined as
an average of at least 100 pulses (see 4.5.3.5 for exemption to 100 pulse requirement). All
readings specified in 4.5.3.1.4 and 4.5.3.1.5 shall be laser energy transmitted through different
areas of the filter as shown on figure 1, except that only one center point shall be used. The laser
energy incident on the filter shall be "P" polarized with a minimum 100:1 ratio of "P" to "S".
The measurements shall be made such that the beam incident energy spreads in the fashion
depicted by the rectangles on figure 1.
4.5.3.1.3 Reading 1. The laser energy incident on the filter shall be measured. This
measurement can be performed directly or by using a calibrated beamsplitter as on figure 2 and
then determining the energy incident on the filter. If the reading is performed directly, the laser
energy incident on the filter shall not drift by more than 5 percent. If a calibrated beamsplitter is
used, reading 1 shall be performed simultaneously with reading 2 or reading 3 as required.
4.5.3.1.4 Reading 2. The filter shall be placed in the beam at 2±1.5 degrees incidence
and the beam energy transmitted-fitted through the filter shall be measured at one center point
only and each edge as shown on figure 1.
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