Which phenomenon describes a flickering light that appears brighter than a steady light of the same average luminance?

The phenomenon that describes a flickering light appearing brighter than a steady light of the same average luminance is known as the Brucke-Bartley effect. This effect illustrates how the perception of brightness can be influenced by temporal changes in light rather than just its average luminance. When light flickers at a certain frequency, the human visual system can perceive it as being more intense. This is because the visual system is more sensitive to changes in illumination, which can create a heightened perceptual response compared to a constant light source.

As for the other concepts: the Granit-Harper law relates to visual acuity and threshold sensitivity rather than brightness perception; the Purkinje tree involves the perception of shadows from the retinal blood vessels, which doesn't pertain to flickering light; and the Troxler effect is about the fading of non-focusing visual stimuli. These distinctions underscore why the Brucke-Bartley effect is the correct answer in the context of how flickering influences brightness perception.