How on earth do you measure light flicker?

GL SPECTIS 1.0 Touch + Flicker Meter Metrics is an upgraded version of the very successful handheld Spectis 1.0 Touch Spectral Light Meter. This device can now measure the increasingly important parameter of light flicker in addition to the extensive range of standard photometric and colorimetric values already available. The Spectis 1.0 Touch Flicker Spectrometer is now equipped with additional electronics and a fast photodiode to measure flicker frequency, flicker index, and flicker ratio. Designed and developed in consultation with industry leaders and standards committees, this device provides all the measurement quantities required to accurately measure and understand flicker.

http://www.saelig.com/MFR00137/gl-spectis-flicker.htm