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§161.013-5 Intensity requirements.
(a) If an electric light emits light over an arc of the horizon of 360 degrees, the light must:
(1) When level, have a peak intensity within 0.1 degrees of the horizontal plane;
(2) Have a peak Equivalent Fixed Intensity of at least 75 cd; and,
(3) Have a minimum Equivalent Fixed Intensity within a vertical divergence of ±3 degrees of at least 15 cd.
(b) If an electric light emits a directional beam of light, the light must:
(1) Have an Equivalent Fixed Intensity of no less than 25 cd within ±4 degrees vertical and ±4 degrees horizontal divergence centered about the peak intensity; and,
(2) Have a minimum peak Equivalent Fixed Intensity of 2,500 cd.
(c) The Equivalent Fixed Intensity (EFI) is the intensity of the light corrected for the length of the flash and is determined by the formula:
EFI = I × (tc − ti) / 0.2 + (tc − ti)
I is the measured intensity of the fixed beam,
tc is the contact closure time in seconds, (0.33 for this S-O-S signal), and
ti is the incandescence time of the lamp in seconds.
(d) An electric light which meets the requirements of either paragraph (a) or (b) of this section need not, if capable of operating in both manners, meet the requirements of the other paragraph.