Psychophysical contrast calibration

Abstract

Electronic displays and computer systems offer numerous advantages for clinical vision testing. Laboratory and clinical measurements of various functions and in particular of (letter) contrast sensitivity require accurately calibrated display contrast. In the laboratory this is achieved using expensive light meters. We developed and evaluated a novel method that uses only psychophysical responses of a person with normal vision to calibrate the luminance contrast of displays for experimental and clinical applications. Our method combines psychophysical techniques (1) for detection (and thus elimination or reduction) of display saturating nonlinearities; (2) for luminance (gamma function) estimation and linearization without use of a photometer; and (3) to measure without a photometer the luminance ratios of the display’s three color channels that are used in a bit-stealing procedure to expand the luminance resolution of the display. Using a photometer we verified that the calibration achieved with this procedure is accurate for both LCD and CRT displays enabling testing of letter contrast sensitivity to 0.5%. Our visual calibration procedure enables clinical, internet and home implementation and calibration verification of electronic contrast testing.

Positive and negative polarity contrast sensitivity measuring app

“…To eliminate the need of physical measure of pixel value-luminance relations with a (rather expensive) light meter, the psychophysical method of contrast calibration [14] will be implemented, so that the contrast of stimuli can be correctly set for a given display.…”

“…This bit-stealing technique has been previously used to achieve the higher dynamic range needed to display the contrast levels needed for human threshold measurements [14]. …”

“…The self-illumination characteristics of the display and relative ease of cleaning the display surface makes calibration and maintaining the letter contrast simpler and possible even without a light meter [14]. Operator error can be reduced by automatic scoring and recording.…”

“…iPad 2 vs. iPad Air 2), as they have different intensity scales (ratio of the screen brightness relative to their maximum brightness) [18]. Therefore, even for Retina displays, individual display calibration [14] is essential to ensure the intended contrast of the stimuli to be properly represented.…”

The Effect of Central Vision Loss on Perception of Mutual Gaze

“…18 Contrast sensitivity was assessed at a 0.45 m testing distance using a custom computerized program, equivalent to the Pelli-Robson chart, for letters subtending 2.5°visual angle presented on a luminance-calibrated display. 19 Visual fields were assessed with kinetic perimetry using a computerized central visual fields program 20 at a 1 m test distance with a white square stimulus (0.7° visual angle) and a white fixation cross (0.7° visual angle) presented against a black background. Participants were encouraged to keep the fixation cross as clear as possible during the test ( i.e ., to use the preferred retinal locus for fixation).…”