Comparison of conoscopic, telescopic, and goniometric methods for measuring angular emissions from medical liquid-crystal displays.

Although emissive displays exhibit a quasi-Lambertian emission, the anisotropy of the electro-optic effect that controls light transmission in liquid-crystal displays (LCDs) causes the pixel luminance to vary, sometimes strongly, with the viewing angle. These variations are not identical for all gray levels and can eventually cause gray-scale inversions. We compare methods currently used to measure angular luminance variations in the LCDs: the goniometric method, the telescopic method, and the conoscopic or Fourier-optics method. We show that, although they are the same at the high end of the gray scale, the results of the three methods differ significantly at lower gray levels. In some cases the measured luminance was as much as 38% higher for the conoscopic system, and 26% higher for the telescopic method, than the value obtained with the goniometric approach. This shift in the minimum luminance measurement translates directly into the contrast ratio, affecting the reporting of technical specifications of display systems.