There are situations where a lighting device comprises an array of light sources; an example is the backlight of an LCD display, for use as a monitor, a TV, or the like. In the following explanation it will be assumed that the light sources are LEDs, but this is not essential.
A 2D backlight LED array for an LCD comprises a plurality of horizontal strips arranged above each other, each strip comprising a plurality of LEDs arranged next to each other. The LEDs may be ON continuously, but typically the strips are switched ON and OFF with the frame frequency, such that the strip aligned with the image lines currently being displayed is ON while the other strips are OFF. The LEDs may all produce the same light output, but better display results, especially a better contrast ratio, can be achieved if the light output of the LEDs is amended in conformity with properties of the corresponding image portion. For instance, for a darker portion of the image the corresponding LEDs can be dimmed, whereas for a brighter portion of the image the corresponding LEDs can be boosted. Such adaptation may be performed for an entire horizontal strip (1D dimming), but preferably the adaptation is performed on the level of individual LEDs (2D dimming).
A complication in this respect is crosstalk between adjacent light sources, which problem is heavier in the case of LEDs as compared to HCFL lamps. Crosstalk generally means that a segment of the display is illuminated by two (or more) light sources. This will generally be the case for display segments located midway between two adjacent light sources, but, especially with LEDs having larger opening angle, this may also be the case for display segments that should be illuminated by one associated light source only. With crosstalk, it may be that adaptation of the light output of one light source results in an undesirable change of the light available for illumination of a display segment associated with an adjacent light source. Such undesirable change should be compensated by appropriately adapting the light output of such adjacent light source.
Thus, when one light source is dimmed, crosstalk compensation may require the adjacent light sources to be boosted, as will be explained with reference to FIG. 1, which schematically shows a front view of a portion of a lighting device for an LCD screen. Individual LEDs are indicated by reference numeral 1. The LEDs 1 are arranged next to each other with some mutual horizontal distance in horizontal strips, which are indicated by reference numeral 2 and which are arranged above each other with some vertical distance. Hereinafter, the horizontal direction will be taken as X-direction, while the vertical direction will be taken as Y-direction. Individual strips 2 will be distinguished by addition of a Y-index j. Individual LEDs in the j-th strip 2(j) will be distinguished by addition of an X-index i and the Y-index j, as LED 1(i,j). Although not essential, it will be assumed that i ranges from 1 to a maximum iM and that j ranges from 1 to a maximum jM.
In FIG. 1, a circle 3(i,j) suggests a portion of the LCD screen illuminated by the LED 1(i,j). It is noted that in practice such portion will not have a sharp borderline. In can be seen that LED 1(i,j) produces an illumination contribution in screen segments corresponding to LEDs 1(i−1,j), 1(i+1,j), 1(i,j−1), 1(i,j+1). The illumination contribution in screen segments corresponding to LEDs 1(i−1,j−1), 1(i−1,j+1), 1(i+1,j−1), 1(i+1,j+1) will be ignored here.
Assume that LED 1(i,j) is to be dimmed. For compensating the crosstalk illustrated in FIG. 1, the LEDs 1(i−1,j), 1(i+1,j), 1(i,j−1), 1(i,j+1) should be boosted, while the remaining LEDs in strips 2(j−1), 2(j), 2(j+1) should continue to be driven at normal light output.