In this film evaluation, I speak about how much film speed was lost during two-minute time exposures. In reality, film speed remains the same throughout a time exposure. However, during a long exposure, the effects of reciprocity law failure makes some films less efficient at retaining the latent image in very dim light. The dimmer the target, the greater the amount of reciprocity law loss, and thus the greater the perceived loss of film speed. But remember, this is a perceived effect and in reality the film speed remains the same at all times.
Camera stand and flood lamp for testing film reciprocity law failure.
Sample results of low and high reciprocity law failure. Each color was partitioned from the other by black foamboard partitions. The white target was ruled at one-inch intervals to allow measurement of the film's sensitivity in each color. The goal in using the two red LEDs was to see which films continued to record beyond the H-Alpha wavelength. Unfortunately, the 650 nanometer LED turned out to be much dimmer than the others and barely showed up at all on many films.