Display Characterization for Contrast Sensitivity Testing.

SIGNIFICANCE: Contrast sensitivity measurements using a computer display have been reported to lack accuracy when displaying small contrasts. This report investigates if the characterization / calibration of display luminance may contribute substantively to these kinds of described inaccuracies.

PURPOSE: To investigate what errors in contrast sensitivity may result from characterizing a display by fitting a gamma curve through physical or psychophysical luminance measurement data.

METHODS: The luminance function of four different in-plane switching liquid crystal displays (IPS LCDs) have been measured for all 256 gray levels (the actual luminance function). This has been compared to a gamma-fitted luminance curve (the gamma luminance function). Calculated are the errors in displayed contrast that may arise when assuming the gamma luminance function instead of the actual luminance function.

RESULTS: The amount of error differs considerably between the displays. In general, for large contrasts (Michelson logCS <1.2) the error is acceptable (<<0.15 log unit). However, for smaller contrasts (Michelson logCS >1.5) the error may become unacceptably high (>0.15 log unit).

CONCLUSIONS: To improve the accuracy of testing contrast sensitivity with an LCD display, it is necessary to fully characterize the display, measuring the luminance of each gray level instead of fitting a smooth gamma function through limited luminance data.