Variation of calibration factor over time for high dynamic range photography in a single daylit interior scene

This paper describes a study on the variation of calibration factor over time for High Dynamic Range (HDR) photography in a single daylit interior scene. Field measurements were conducted in a classroom under stable sky conditions. The luminance of each color of the X-Rite ColorChecker chart measured with a calibrated luminance meter was compared with the luminance value of the corresponding pixels extracted from the HDR image fused with low dynamic range (LDR) photographs of multiple exposures taken using a consumer grade digital single-lens reflex camera fitted with an ultra wide angle lens. The ratio between the luminances obtained from these two methods was computed and defined as the calibration factor. It was found that for each color there was a weak and not significant correlation between the calibration factor and the daylight levels ranging from about 75 to 30,000 lx. Without any significant relationship with the daylight levels, the calibration factors of each color obtained in the measurements were averaged, so that for any daylit scene with a dominant color, the real luminance could be determined by multiplying the average calibration factor of that color to the HDR luminance. The calibration factor of a 'daylit scene' in which all the 24 colors were evenly distributed was also computed, and it was 0.53 with a standard deviation of 0.03. Having more understanding about the feasibility of applying HDR photography technique as a luminance data acquisition system, researchers can have more confidence in its application for the investigation of the relationship between various luminance-based parameters and interior daylighting performance.Department of Building Services Engineerin