The noisy-bit method for digital displays: converting a 256 luminance resolution into a continuous resolution

Visual psychophysics often manipulates the contrast of the image on a digital display screen. Therefore, the limitation of the number of different luminance intensities displayable for most computers (typically, 256) is frequently an issue. To avoid this problem, experimenters generally need to purchase special hardware (graphic cards) and/or develop specific computer programs. Here, we describe an easy-to-implement method, consisting of adding noise to the displayed stimulus, that we call the noisy-bit method. This random dithering method, generalized to 256 luminance intensities, is equivalent to displaying continuous luminance intensities plus a certain amount of noise. Psychophysical testing using a standard spatiotemporal resolution (60 Hz and 1,024 x 768 pixels) demonstrated that the noise introduced by the noisy-bit method has no significant impact on contrast threshold and is not visible. We conclude that the noisy-bit method, combined with the standard 256 luminance levels, is perceptually equivalent to an analog display with a continuous luminance intensity resolution when the spatiotemporal resolution is high enough that the noise becomes negligible (which is easily attainable with the typical spatiotemporal resolutions of present-day computers).