d. Illumination Time.
As a field expedient, yield may be estimated from the measurement of the
illumination time of a nuclear burst, especially during hours of darkness
or poor visibility; however, this method should be used only if it is
impossible to obtain cloud parameters as discussed earlier, since this
method only gives a yield estimate on the order of a factor-of-10.
Techniques for measuring illumination time will vary depending on the
situation, but under no circumstances should the observer attempt to look
directly at the fireball since this can result in permanent damage to the
eyes. The illumination time may be estimated by the observer who has
taken shelter in a foxhole by noting the light reflected into the
foxhole. The observer can look at the floor of the foxhole and still
sense the duration of the flash or reflected light. Counting in seconds
will probably be the most effective way of determining the illumination
time since the "dazzle" (flash blindness) effect will preclude the
reading of watches.
The chart below (Figure 2-4) shows rough estimates of yield, using
Yield Estimation (Illumination Time)
Nuclear Yield Calculator.
For rapid calculation of a nuclear burst yield, the M4A1 Nuclear Yield
Calculator may be used. The M4A1 Calculator is a component of the M28A1
Nuclear Calculator Set.