Fig 1. The left-hand column shows data for observer V from De Lange [14] and the right-hand column, data for observer DHK from Kelly [15] both measured using sinusoidally flickering stimuli.

The data are shown twice: The upper panels show the logarithm of the reciprocal of the just-detectable contrast (the contrast sensitivity) as a function of frequency (Hz) plotted on a logarithmic scale. The lower panels show the same data replotted as the logarithm of the reciprocal of the just-detectable amplitude (the amplitude sensitivity) also as a function of frequency (logarithmic scale). Different symbols and colors denote different mean retinal illuminances (log₁₀ photopic trolands) as indicated in the key. The black curves are arbitrary smooth functions fitted to each dataset to facilitate comparison. Data are from Figure 5 in De Lange [14], who used a 2° diameter, centrally fixated, white flickering test field in a steady 60° diameter surround of same luminance and chromaticity, and from data tabulated in Table 1 in Kelly [15], who used a centrally-fixated, 50° diameter white target vignetted gradually from 50 to 68°.