Which function is primarily based on percentage modulation?

The function primarily based on percentage modulation is the Temporal Modulation Transfer Function (TMTF). This function assesses the visual system's ability to perceive changes in luminance over time, specifically how well it can detect modulation in brightness at various frequencies.

The TMTF evaluates how the timing and intensity of light variations can be perceived, which is expressed in terms of percentage modulation. Essentially, it tells us how effectively the visual system can process dynamic visual stimuli at different rates of temporal change. If the modulation is above a certain threshold, the visual system can efficiently detect it, which the TMTF characterizes.

In contrast, the other functions mentioned focus on different aspects of visual perception. Spatial Modulation Transfer Function primarily relates to spatial frequency and how contrast is perceived across different spatial scales rather than temporal changes. The Contrast Sensitivity Function is related to the ability to perceive contrast at various spatial frequencies but does not explicitly emphasize modulation over time like the TMTF. Visual Acuity Function pertains to the resolution capability of the visual system, measuring the smallest detail that can be resolved but also does not focus on modulation in temporal terms.