Figure 7.

Determining the number of distinguishable stimulus levels or “gray levels”. (A) Response vs. contrast curve from Figure 4B. The increment threshold or just noticeable difference is the minimum contrast increment that produces a discriminable response given the noise present. It is equal to the derivative of the response vs. contrast curve divided by the noise level as measured by the ideal observer. (B) An increment threshold is measured by the ideal observer for each level of contrast from the model data of Figure 4. The dip in threshold at 0.1–0.2 contrast is due to the nonlinearity in the contrast response near zero from (A). (C) The contrast sensitivity is the inverse of the increment threshold. Integrating the area under the contrast sensitivity curve gives the number of gray levels. The gray level compression is the contrast sensitivity divided by the number of gray levels; in this example, the maximum gray level compression (at 0.05 contrast) = ~3. The curves were fit by eye.