Positive and negative polarity contrast sensitivity measuring app

Abstract

Contrast sensitivity (CS) quantifies an observer's ability to detect the smallest (threshold) luminance difference between a target and its surrounding. In clinical settings, printed letter contrast charts are commonly used, and the contrast of the letter stimuli is specified by the Weber contrast definition. Those paper-printed charts use negative polarity contrast (NP, dark letters on bright background) and are not available with positive polarity contrast (PP, bright letters on dark background), as needed in a number of applications. We implemented a mobile CS measuring app supporting both NP and PP contrast stimuli that mimic the paper charts for NP. A novel modified Weber definition was developed to specify the contrast of PP letters. The validity of the app is established in comparison with the paper chart. We found that our app generates more accurate and a wider range of contrast stimuli than the paper chart (especially at the critical high CS, low contrast range), and found a clear difference between NP and PP CS measures (CS_NP>CS_PP) despite the symmetry afforded by the modified Weber contrast definition. Our app provides a convenient way to measure CS in both lighted and dark environments.

Figure 1

Measured pixel value- luminance correspondence with fitted polynomial curves for (a) low pixel range (0–15), (b) medium pixel range (15–240), and (c) high pixel range (240–255) for the three display types.

Figure 2

LogCS (log (1/C_threshold) of (a) the NP contrast letter stimuli measured from the Pelli-Robson paper chart, (b) our CS measuring app (on the IPS screen) with initial calibration, (b) PP contrast stimuli shown on our calibrated CS measuring app. The diagonal line represents perfectly calibrated chart. Error bars represent standard deviation of the measurement.

Figure 3

Improved contrast accuracy of the CS measuring app using bit-stealing technique for (a) NP contrast letter stimuli, and (b) PP contrast stimuli. The diagonal line represents perfect calibration. Note that with bit-stealing technique, the full letter contrast range used by the P--R chart is now supported and overall accuracy of the contrast simulation is improved for both polarity CS measures.

Figure 4

Measured CS for (a) subject 1 and (b) subject 2, with and without a simulated cataract (SC) clip-on, averaged over three measurements taken on three days. Binocular CS is higher than monocular CS. The P-R chart based CS measures are not different that of our app based measures in NP. Both subjects’ CS in PP condition is much lower (about 0.22 Log units) than the CS in NP condition.