Critical Flicker Frequency (CFF)

The critical flicker frequency (CFF) method is similar to that of heterochromatic flicker photometry(hfp). The major difference is that the reference field used in heterochromatic flicker photometry is set to some arbitrary level. In the CFF method the reference field is zero radiance (i.e. a dark field). Consequently, whereas in hfp the test stimulus rapidly alternates with a bright reference field, in CFF the test rapidly alternates with a dark field.

While the impetus for this section of the book is to discuss using CFF to measure spectral sensitivity, it would be of interest to consider at least one of CFF's broader implications. There are many situations in our everyday environment where we are presented with flickering lights. We may not appreciate this fact because under normal conditions the visual system can not discriminate or appreciate the fact that these sources are in fact going on and off. Two typical examples are fluorescent lamps and movies. Fluorescent lamps turn on and off at a sufficiently high rate so that they appear to be steady. Movies are presented in discrete frames. The light in the projector is physically blocked several times for each frame. So what is presented to the visual system is a flickering series of images. Fortunately, the flicker is so rapid that it appears steady to the visual system.

There is a considerable scientific literature aimed at understanding the visual systems ability to discriminate flickering lights. One small area of which is concerned with the ability to measure human spectral sensitivity using CFF.

Marks & Bornstein (1973) found that with flicker frequencies ranging from 20 to 35 Hz spectral sensitivity functions with the CFF method were nearly the same as Judd's modification of the CIE photopic luminous efficiency function. However, they found that at 40 to 45 Hz photopic sensitivity measured by the CFF method decreased at wavelengths longer than 560 nm (i.e. in those parts of the spectrum that usually appear yellowish to red.)

Those who would like a more extended discourse on CFF are encouraged to see Human Color Vision. Additional references will be found here as well.

Table of Contents [When not using frames]