Conoscopy Assesses Displays

Jörg Schwartz

Liquid crystal displays are gaining market share. As prices fall, users want brightness and viewing angles comparable to that of conventional displays. A display's visual performance usually is measured with goniometric methods based on mechanical angular scanning of emission and/or reflectivity. The technique is sensitive, has high resolution, but is slow.
Autronic-Melchers GmbH has developed ConoScope, an all-optical system that quickly measures viewing performance.
The key component of the system is a high-performance Fourier transform lens. It covers an aperture angle of up to ±80°, so that it can simultaneously collect and analyze a 160° cone of light from a point on the display. All contained light directions transform to points in the rear focal plane, producing the "conoscopic figure," which is a feature of the spatial emission characteristics.
ConoScope records luminance and chromaticity vs. viewing direction. The receiver module is available with ±80° or ±60° aperture optics. Its measuring spot diameter is adjustable between 0.2 and 2 mm. A high-sensitivity photometric/colorimetric charge-coupled device (CCD) array analyzes the conoscopic figure.
Besides measuring the stationary angular distribution of light intensity and chromaticity, the system measures temporal variations and emission spectra. Because this requires faster acquisition than the 20 ms of the CCD, system designers use a fiber bundle as a "side door" to the focal plane of the Fourier lens. The fibers leading out are split and connected to a fast photometer and a spectrometer.