Perceived lightness difference with regard to spatial frequency and amplitude modulation

It has been found that the L* function defined in the CIELAB color space is not suitable to predict the human visual perception of modulated patterns at high spatial frequencies. For example, in multilevel halftoning (multitoning), when output levels are equally spaced in L*, it has been observed that the visibility of the resulting multitone patterns is not uniform across different parts of the tone scale. This leads to the hypothesis that the CIE L* function may not be a good metric to evaluate the perceived lightness differences at high-spatial frequencies as it was derived based on the perception of large area uniform patches. To investigate the relationship between suprathreshold lightness difference perception with regard to spatial frequency and amplitude modulation, we designed a psychophysical experiment, which was conducted using a lightness difference matching paradigm. The stimuli used in the experiment were horizontal square-wave gratings. The behavior of lightness difference perception under varying spatial frequencies and modulation amplitudes across the entire L* scale was studied. Consistent results were acquired that show a significant frequency-dependent effect where the effective lightness difference for high- frequency patterns is reduced for low L* values. The magnitude of this effect was found to be highly related to the spatial frequency of the modulation. Based on these results, we derived an effective lightness function that is dependent on spatial frequency. The effective lightness function can be applied to the selection of the output levels for multitoning.