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|  MIL-L-27160 (USAF)
d. Determine if at least two lamps are used.
e. Determine if any portion of the display depends solely upon one lamp for its
illumination. Not more than 70 percent of the brightness on any portion of the display
shall be illuminated by one lamp.
4.5.4 Life and reliability. The lighting system shall be subjected to a life test with 5.0
0.05V 400 Hz applied to the lighting terminals. The duration of the test shall be 1,000
hours or for a period equal to the life test time required by the applicable instrument
specification, whichever is greater. Wherever possible, this test shall be performed
concurrently with the reliability or life test, or both, of the instrument specification. The
power supplied to the lighting circuit shall be interrupted every 57 minutes for a period of
three minutes. Measurement of color and brightness shall be made prior to and after this
test. When this test is run as part of the reliability test, the brightness and color shall be
monitored each 500 hours of testing and failure of the instrument to meet either the
brightness or color requirements shall be counted as a reliability failure of that instrument.
Pattern failures (the same type of failure on several instruments) shall require redesign and
retesting.
4.5.5 Light (calibration and measurement of brightness and color of white lighted
indicators). Light tests shall be made with 4.50 0.050V applied to the lighting terminals.
All measurements shall be in complete darkness and with the axis of the photometer
perpendicular to the coverglass.
4.5.5.1 Equipment. Equipment required for the light test shall be as follows:
a. Photo Research Corporation model UB 1/2 Spectra Brightness Spot Meter, or
approved equivalent.
b. Microspectar lens.
c. Filter holder between meter and lens.
d. External foot-lambert meter (internal impedance must match photometer).
e. Regulated power supply, 1 percent accuracy (regulating voltage to the indicator).
f. All voltmeters used shall be accurate to with in 1/2 percent of the specified voltage
(AC voltmeters should provide readings that are independent of wave form of the applied
voltage).
g. Lamp and opal glass combination calibrated by the National Bureau of Standards
(NBS) for 2,854K (or applicable specified color temperature). This is the color
temperature of the light emitted by the opal glass.
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