With a recording photometer of photopic sensitivity, measurements were made of many points in the sky during twilight for solar altitudes <i>H</i> = +5° to -15° for clear air and no clouds at two stations, one in Maryland, altitude 30 meters, and one on Sacramento Peak, New Mexico, altitude 2800 meters. The sky polarization on the meridian through the sun, and the illumination on a plane at various orientations exposed to the sky, were also recorded. For <i>H</i> from about -3° to -11° the entire sky changed in brightness at about the same rate of a factor of 10 for each 2° change in <i>H</i>. Except at the horizon the Sacramento Peak sky was about ⅔ to ½ as bright as the Maryland sky because of clearer air; at the horizon the two were about the same. At Sacramento Peak the ratio of the polarized components reached a minimum of about 0.06 at the zenith for <i>H</i> = -3°.
M. J. KOOMEN, C. LOCK, D. M. PACKER, R. SCOLNIK, R. TOUSEY, and E. O. HULBURT, "Measurements of the Brightness of the Twilight Sky," J. Opt. Soc. Am. 42, 353-353 (1952)