A new non-intermittent sensitometer has been constructed which consists of two sector wheels rotating about the same axis at different angular speeds. During the exposure, the larger disk makes only one revolution. In this interval, the small disk makes many revolutions but the large wheel covers the photographic material except for one revolution of the small wheel.
With the ordinary sector wheel sensitometer, the exposure range is rarely more than 256 to 1 because of the difficulty of cutting the wheel. It is shown that by using two sector wheels, the exposure range may easily be increased to 16,384 to 1. An extension of this principle to three disks would not involve any serious mechanical difficulties and would increase the exposure range to more than one million to one.
ARTHUR C. HARDY, "A NEW NON-INTERMITTENT SENSITOMETER," J. Opt. Soc. Am. 10, 149-154 (1925)