MIL-DTL-62420B(AT)
b. Measurement. The preferred wavelength range for this measurement is 380
nanometer (nm) to 760 nm: any range that includes the range 400 nm to 700 nm is acceptable.
Photopic transmission shall be derived from multiplying measured spectral transmission data,
taken every 5 nm at 0 +2 degrees incidence along the optical axis of the periscope, by the
photopic luminous efficiency values that make up the standard human eye curve established by the
Commission Internationale del'Eclair (CIE) (see ASTM E308).
c. Calculation.
The PT is derived from:
760
760
∑
∑
X=
T(w) S(w)V(w)dw
Y=
S(w) V(w) dw
380
380
PT = x/y
Where,
w = Wavelength
T(w) = Filter transmission characteristics
S(w) = CIE source A characteristic (1931)
V(w) = Photopic visibility function (1931)
dw = 5 nm intervals
4.7.1.5 Optical density measurement using a spectrophotometer. The periscope shall be
measured at 0 +2 degrees incidence and shall be tested in accordance with the following:
a. Apparatus. One of the following systems shall be used:
(1)
A constant current tungsten lamp
A monochromator
A detector
(2)
A double beam spectrophotometer
For either system, the light incident on the periscope shall have a collimated beam diameter of
10 +3 mm (1/e2) and the system shall be calibrated over the optical density range of 3 through 4
using neutral density filters traceable to the National Institute of Standards and Technology
(NIST). Prior to testing, system alignment shall be verified by measuring the periscope's
transmission or optical density at an unattenuated wavelength (approximate range 500-550 nm).
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