MIL-PRF-38169A
the proper radii of curvature and thickness. The inherent power of the visor shall be calculated by
use of the following equations:
n′ - n
n - n′
t
F = F1 + F2 - F1 F2 ;
F2 =
;
F1 =
n′
r2
r1
Where:
F = Inherent power of the lens in diopters
Fl = Inherent power of the convex surface in diopters
F2 = Inherent power of the concave surface in diopters
n = Index of refraction of air
n′ = Index of refraction of the material
rl = Radius of first or convex surface
r2 = Radius of second or concave surface
t = Thickness in meters.
3.3.3 Luminous transmittance (see 6.3.2.1).
3.3.3.1 Class 1 visor and goggle lens. The luminous transmittance of the class 1 visor and goggle
lens shall not be less than 90% throughout the critical area. The noncritical area shall not vary in
transmittance by more than ± 2% of the critical area transmittance.
3.3.3.2 Class 2 visor and goggle lens. The luminous transmittance of the class 2 visor and goggle
lens shall be 15% ± 5% when measured at point C. The luminous transmittance of point C for the
left eye and point C for the right eye of the same visor and goggle lens shall not differ by more
than 20%. The total luminous transmittance at the top of the visor and goggle lens shall not
exceed the total luminous transmittance at the bottom of the visor and goggle lens (see 6.3.2).
3.3.3.3 Class 3 visor and goggle lens. The luminous transmittance of the class 3 visor and goggle
lens shall be 2.0% (± 0.5%) at any point in the critical areas. For the noncritical areas, the
luminous transmittance shall be between 1.5 and 3.0%. The difference between the luminous
transmittance measured at any two points in the critical or the noncritical areas shall not exceed
0.75%.
3.3.4 Optical distortion. There shall be no optical distortion visible in the critical areas of the
lens.
3.3.5 Haze. The haze value of the visor and goggle lens shall not exceed 5%.
3.3.6 Extended electromagnetic radiation transmittance of class 3 visor and goggle lens. The
average transmittance in the two ranges between 200 and 380 nanometers (nm) and 770 and
1,500 nm shall not exceed 2.5%. The average transmittance within the critical areas for the range
between 380 and 770 nm shall be 2.0% (± 0.5%). For the noncritical areas the average
transmittance for the range between 380 and 770 nm shall be 1.5 to 3.0%; however, the measured
values of transmittance within this range shall not vary more than 0.75% for any individual visor
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