Curved and High-Resolution Display Metrology
June 4, 2015, Society for Information Display Symposium, San Jose, CA—Karlheinz Blankenbach from Pforzheim University in Germany presented the results of studies comparing flat and curved TVs. The key parameter they measured was viewing-angle degradation and reflections.
The ability to manufacture concave curved displays may provide advantages compared to flat and convex displays of the same size. The advantages of the curve displays include larger field of view with the same display size, reduction of viewing angle degradation, and may show less specular reflections.
A curved LCD display is in effect a combination of a vertical flat mirror and a horizontal concave mirror. Optics theory says that a light source at the radius would be magnified and appear to spread across the entire surface. Currently, a 55 inch curve television has a radius of curvature of about 4 m, whereas the recommended viewing distance is 2 to 3 m. Therefore, a room light at the 4 m distance could obscure the television image.
The first step was to measure the actual curvature of a test device. Two flat screen 55-inch sets and one 4k UHD curved set comprised the test sets. Measurements included distortions from ideal curvatures and other optical defects. The distortions and focus set are challenges for evaluations of curved displays. The actual curvature was closer to 4.5 m versus a stated 4.2m.
After measuring the curvature and identifying defects, we measured viewing angle performance of the three sets. Test setup was observer distance of 2.2 m ( the focal length of the curved mirror surface) and observations at 0, ±15, and ±30 degrees. Display parameters like luminance and color were measured with an Instrument Systems Lunicam 1300.
The curved display followed the geometric optics laws and disturbing magnification effects happen for objects 3-5 m from the screen. Reflected objects are magnified horizontally and somewhat reduced vertically. In addition, non-uniform curvature is easily visible as waviness of the horizontal lie of reflection and geometric distortions. The defects are quantifiable as convex or concave relative to the overall curvature. Dents are visible down to 0.3 mm.
The curved screen is highly symmetrical, so measurements from either side matched the correlating angles. In general, color shifts depend on the grey level and are smaller for the curved screen than for the flat ones. For a viewer sitting on-axis of the curved screen, the viewing angle color shifts are in the range of 1-2 ?E, which is virtually unnoticeable on a 5-inch TV. ?E= 20 is the minimum threshold for fore-to background discrimination. ?E approached 70 for the flat panel, about 4 times the value for the curved display.
