Luminance and Chromaticity Uniformity

When panels are constructed, complex layering processes occur. These processes create inconsistencies in their output of luminance (brightness) and color uniformity (chromaticity). Luminance uniformity describes brightness being completely uniform in all parts of a display, ensuring there are no dark spots or illegibility of information on the screen. Color uniformity (chromaticity) is measured by color replication accuracy, with areas of color variation being able to affect contrast, color rendering and legibility as well (see picture at right). Most uniformity problems exist outside the center of the display, with extreme uniformity errors existing around the outside edges of most panels (see picture at left). This is partly due to the placement of the backlight in LCD panels, making it incredibly difficult to create a completely uniform LCD display.

In addition, these problems can stem from imperfections in the manufacturing process, like uneven thickness of a liquid crystal cell, problems with optical backlight sheets, color filters or from problems like non-uniform backlight radiance. In turn the non-uniformity of luminance and chromaticity directly affect the monitor’s gamma levels and the gray scale for color, often creating errors, as seen in the above pictures. With differences in color and contrast, visual problems are created, which in turn affect legibility, color rendering and color replication. These problems can be so bad that identical pictures side by side can visually be seen as different colors, creating color replication problems. Deviating luminance can also have different shapes and sizes, which further create problems for accurate color replication, in addition to creating more problems when trying to correct non-uniformity.

Achieving Uniformity

With various ways for luminance to be uneven and with screens having different non-uniformity states, Delta E values (calculated result of the difference in color between two points) can be extremely inaccurate, to the point where the naked eye can see color errors from the display (Delta E ≥2). Areas of the panel can have different brightness levels and chromaticity which is reflected by luminance variance values (Delta Lv) and Delta E values respectively. Brightness is measured by a luminance variance value taken from the center of the screen, displaying a deviation percentage at each other measured area of the panel. These measurements show how accurate a particular panel is when measuring for uniformity. With each monitor producing non-uniformity errors in a unique way, it is near impossible to standardize a solution during the manufacturing stage. To achieve uniformity, ViewSonic has developed a uniformity correction function, which balances any luminance and color uniformity imbalance.