Discriminability measures for predicting readability

Several discriminability measures were correlated with reading sped over a range of screen backgrounds. Reading speed was measured using a search task in which observers tried to find one of three works in a short paragraph of black text. There were four background patterns combined with three colors at two intensities. The text contrast had a small positive correlation with speed. Background RMS contrast showed a stronger, negative correlation. Text energy in the spatial frequency bands corresponding to lines and letters also showed strong relationships. A general procedure for constructing a masking index from an image discrimination model is described and used to generate two examples indices: a global masking index, based on a single filter model combining text contrast and background RMS contrast, and a spatial-frequency-selective masking index. These indices did not lead to better correlations than those of the RMS measures alone, but they should lead to better correlations when there are larger variations in text contrast and masking patterns.