Optimal backlighting determination apparatus and method

#The invention relates to a method of calculating an optimal first and second backlight driving level, for a color display having a backlight which can be controlled to produce a first amount of light with a first spectrum in accordance with the first backlight driving level and a second amount of light with a second spectrum in accordance with the second backlight driving level, and the color display having a first and second light transmission valve plus color filter combination, arranged to create from the backlight spectra a respective first and second color primary light output, the chromaticity of at least one of the color primaries depending on the first and second backlight driving level, and corresponding apparatus unit, which can be incorporated in displays and cameras, and software.

#A number of displays create their pictures by having an in-display light creation unit which is placed behind a modulation unit, e.g. for each (sub)pixel a combination of a filter to create a local color, and a valve to create an amount of color. E.g. a transmissive LCD has the property that the amount of light exiting (ignoring for the moment the spectral behavior) is dependent via typically an S-shaped transfer function on the applied voltage. Other alternative principles valve by redirecting light, e.g. reflecting an amount towards a screen.

#It is also known to make multiprimary displays of the above type, in which the optimal 3-color gamut (RGB) is replaced by a gamut spanned by several color primaries, e.g. red, yellow, cyan and blue, or for increased luminance RGBW, where W is a white color, e.g. D65. In this case, the 4 or more valves need the appropriate driving values to reproduce an input standardized RGB, or XYZ color, which is because of the underdeterminedness a somewhat difficult task, although in the past a number of techniques were developed that are applicable to one or several of the available multiprimary displays.

#In the drawings

#FIG. 1 schematically illustrates a display with a dependent variable primary problem;

#FIG. 2 schematically illustrates the change in displayable gamut (GAM—4N to GAM—4S) as a function of variation of a backlight dependent white primary;

#FIG. 1 shows for explanation purposes a very simple display 100 (e.g. LCD) in which a primary chromaticity (i.e. hue and saturation; not of course only the trivial luminance dependence) dependence occurs, namely a rather strong variability of the white.

#Blue backlight 102, and the green and red backlights 104, 106 each produce corresponding backlight spectra SB, SG, SR, in graph 150. These backlights can be, e.g., Light Emitting Diode (LED) arrays, homogenized by homogenizer 108.

#Pixel color values are realized by valving (i.e. transmitting a fraction) the backlight with respective valve+filter combinations. E.g., blue filter 110 (or similar green 112, red 114, white 116) may consist of an LCD material (the color transfer characteristics are at present for simplicity assumed to be a pure non-linear luminance transmission function of the valve drive level VB) and a color selective filter, the spectrum FR of which is shown in graph 152. The final light output spectrum PB in graph 154 follows from the multiplication of SB and FB—the height of FB being able to take into account how much the valve transmits—, since in this simplistic example it is assumed that the different backlight spectra fit entirely in their respective color filter spectrum, and these filter spectra are not overlapping.