Effects of static and dynamic image noise and background luminance on letter contrast threshold

We performed a pilot psychovisual experiment to determine the contrast threshold and slope of the psychometric function for a target embedded in two levels of static and dynamic external image noise. Sloan letters were presented in a local background surrounded by a global background, both varied over four luminance levels: 58.62, 155.97, 253.50, and 347.47 candela per square meter. Uncorrelated Gaussian noise with normalized standard deviation 0.019 and 0.087 was added to the stimuli. A noise-free stimulus was also tested. No systematic effect of global background luminance was found. The contrast threshold was approximately 1% in the noise-free stimulus and increased monotonically with rms noise contrast, following a power law relationship. Thresholds were higher in static noise. The model will be incorporated in a no-reference, task-based medical quality metric for x-ray sequences.