on the emulsion itself, since such markings pass unharmed through the developing process. 14, Exposures The luminous fireball emits a small fraction of its total energy in the form of visible light. The radiant emittance (surface brightness) of the fireball varies by a very large factor, from the maximum brightness which it obtains early in the explosion until the minimum which occurs approxi- mately at the time the shock front becomes nonluminous. The variation in surface brightness is shown, for a nominal 20-kiloton device, in Figure 2.92 of The Effects of Nuclear Weapons. * The portion of fireball growth which provides the best information, as far as yield-scaling is concerned, occurs just before the minimum in the light curve. It is consequently necessary to choose the camera expo- sure to give images of good density during this portion of the fireball growth. As a result the earlier stages are heavily overexposed, and are of less precision Owing to spreading of the image and halation. For megaton explosions, it has been found that the surfice brightness over the period of interest, is of the order 10,000 watts per square meter in the spectral band 0.4 - 0.7 microns. This corresponds to a black-body having a temperature of 2200 to 2500° Kelvin. The equivalent reading of the Weston photographic expo- sure-meter would be 200, 000. One additional factor to be considered, before exposures and aperture settings can be computed, is the loss of light due to absorption and scattering inthe twenty-mile path of air between the fireball and the camera. Ob-' viously one cannotpredict just what transmission may obtain at shot time, and it is necessary to allow for a considerable variation. Experience has shown that at best, only 50 percent of the light can penetrate a twenty-mile path, and that unacceptable images result if we have less than 3-percent transmission. To compensate for the variations that may be expected in atmospheric transmission, we will choose different lens apertures for the several cameras; as a result we may have only one film with excellent images, but the probability is that two or three will be usable. It is expected that no filters will be used on the cameras. 9s. Glasstone (ed.), The Effects of Nuclear Weapons. Printing Office (1957). -21- U.S. Governme ° |