Innovations in 3-D colour LUTS for display calibration

Certain display technologies - for example, DLP, PDP, and AMOLED - produce colour mixtures that are intrinsically additive. Calibration of such displays is fairly simple. However, LCD displays are intrinsically nonlinear in two challenging ways. First, the intrinsic electro-optical conversion function (EOCF) of the LC cells has an S-shaped characteristic very different from the power-function response standardized for video displays. Second, the optoelectronic properties of the LCD cells themselves cause nonlinear colour crosstalk among the colour components. Linear crosstalk is easily correctable through 3×3 matrixing; however, crosstalk such as that exhibited by LCDs is nonlinear and therefore difficult to characterize and correct. 3-D colour lookup tables (LUTs) using trilinear or tetrahedral interpolation can be used to correct these nonlinearities. The LUT interpolation itself is fairly straightforward, and technical details are readily available in the literature. However, methods to derive the contents of the LUTs from device measurements are usually proprietary trade secrets. In this paper, we outline the mathematics used to build display calibration LUTs suitable for the demanding task of correcting LCD S-curves and colour crosstalk with studio performance. We will describe a state-of-the-art inversion technique that uses natural neighbour interpolation. The technique is suitable for calibrating LCD displays at professional, industrial, and consumer performance levels. The technique we describe offers significant improvement compared to other available solutions.