Photographic measurement of the effects of viewing angle on the luminance and contrast of liquid crystal displays
An actively cooled charge couple device detector in combination with a 4 mm focal length lens (camera) was used to evaluate the luminance and perceived contrast properties of a liquid crystal display (LCD). The circular field of view (FOV) of the camera occupied an angular range (9) of ±42.5° from normal in all directions. Uniform field images corresponding to 17 equally spaced grayscale values in the 8 bit digital driving level (DDL) range of the display system were acquired. The 12 bit grayscale digital images produced by the camera were converted to luminance (cd/m 2) units via the measured DDL vs. luminance response of the camera. The Barten model of the grayscale response of the human visual system was used to compute the perceived contrast of the display within the angular FOV of the camera and throughout the 8-bit DDL range of the display. 1D profiles were extracted from the 2D measurements and compared to measurements acquired from a similar display using a Fourier-optics-based luminance meter and published methods. The results of the two methods generally agreed to within 5%. Greater discrepancy was realized for the lowest portion of the DDL range. The photographic methods used were straightforward and resulted in accurate display assessment measurements over a FOV that is relevant for the clinical use of LCDs.
