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BACKLIGHTING APPARATUS AND METHOD
CROSS-REFERENCE TO RELATED PATENT
 This application is a An application claiming the benefit under 35 USC 119(e) U.S. Application 60/688,895, filed Jun. 8, 2005, incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
 The present invention relates to displays, and more particularly to backlighting of display panels using lightemitting devices.
 The first industry standards for colour video displays were established by the Federal Communications Commission in 1953 through the National Television Standards Committee (NTSC). These standards included specific CIE 1931 xy chromaticities, (i.e., colours) for the red, green, and blue phosphors, or generically the "primaries" used in the cathode ray tubes (CRTs) to ensure that colour reproduction of broadcast images was consistent regardless of the CRT manufacturer.
 Phosphor technology for CRT displays has advanced in the half-century since the NTSC specifications were published. In North America, colour television primaries are now specified by the Society of Motion Picture Engineers (SMPTE 2004), while in Europe, television colour primaries are specified by the European Broadcast Union (EBU 1993) and high-definition television (HDTV) colour primaries are specified by the Radiocommunication Sector of the International Telecommunications Union (ITU 1990). The Reference display primary chromaticities as defined by each of these standards are provided in Table 1.
 These standards equally apply to the colour primaries of liquid crystal displays (LCDs), plasma screen displays, field emission displays (FEDs), micro-mirror digital light projectors (DLPs), and other colour television and computer monitor display technologies. For example, the colour primaries of LCDs refer to the white colour of the fluorescent lamp backlight as respectively filtered by red, green, and blue pixel microfilters, the polarizing films, the liquid crystal material, and the various layers of transparent support and diffusion materials. Therefore these primary chromaticities refer to the colour of the red, green, and blue pixels as observed by a viewer, and are thus independent of the display technology.
 Having particular regard to colour creation, it is known that colour science is predicated on Grassman's three laws of colour additivity. The first law states that any colour C can be matched by a linear combination of three other colours R, G, and B, for example, SMPTE or EBU/ITU
display primaries, provided that none of the three colours can be matched by a combination of the other two and can be defined as follows:
where a, b, and c are constants of proportionality.
 Grassman's second law of colour additivity states that any two colours C1 and C2 can be matched by a linear combination of any three other colours R, G, and B that individually match the two colours C1; and C2. Wherein this law can be defined as follows:
C=dC1+eC2=(a1+a2)R+(b1+b2)G+(c1+C2)B (2) where 3^, &2i ^1' ^2' ^1'
c„ d, and e are constants of
 Grassman's third law states that colour matching persists at all luminance values within the range of photopic vision which can be defined as follows:
where d, e, and k are constants of proportionality.
 The above identified standards, namely NTSC, SMPTE and EBU/ITU, can place restrictive requirements on manufacturing tolerances for the primary chromaticities. For example, SMPTE 2004 specifies chromaticity tolerances of ±0.005 units for both x and y in the CIE 1931 chromaticity diagram, while EBU 1975 specifies that chromaticity variances should be less than ±0.003 units in the CIE 1960 uv Uniform Colour Space. These tolerances can provide a means for meeting needs for skin tone reproduction, for example.
 While cold-cathode fluorescent lamps are commonly used to provide backlighting for LCD panels, some television manufacturers have recently introduced products that use a combination of red, green, and blue light-emitting diodes (LEDs) to generate white light for backlighting purposes. The primary advantage the colour LEDs offer is that they are narrowband emitters with spectral bandwidths of between approximately 15 and 35 nanometers (nm). As most of the broadband emission generated by fluorescent lamps must be blocked by colour filters in order to achieve the requisite primary chromaticities, the narrowband emissions generated by LEDs may not require filtering, and therefore LEDs may offer the opportunity of higher backlight efficiency and brighter displays.
 In general, the colour LED chromaticities do not coincide with those specified by the SMPTE 10 and EBU/ ITU 12 standards, as illustrated in FIG. 1. This, however, is not important as long as the colour gamut 14 defined by the red, green, and blue LED chromaticities exceeds that of these standards as illustrated in FIG. 1. It is important to note however that LED chromaticities vary widely, particularly for green and blue LEDs. Current manufacturing technologies require LED manufacturers to test each LED for dominant wavelength, which is a measure of its colour and subsequently "bin" the LED accordingly with like LEDs. Typical binning criteria for blue and green LEDs are 10 nm intervals for their dominant wavelength, which can result in chromaticity differences greatly in excess of SMPTE and EBU/ITU requirements.
 Additional problems that can arise with the use of LEDs for backlighting occur due to temperature dependencies of LEDs. For example, the dominant wavelength of blue and green LEDs has a typical temperature coefficient of approximately 0.04 nm/° C, while for red LEDs it is approximately 0.05 nm/° C, where the temperature is that of the LED junction. Assuming that the LED backlighting is designed to be dimmed over a range of 10:1, an expected junction temperature variation in the range of 30° C. may be possible. This temperature range would result in a shift in the dominant wavelength for blue LEDs of approximately 1.2 nm and which results in a corresponding change in chromaticity which exceeds the SMPTE and EBU/ITU requirements.
 A further problem with the use of LEDs for backlighting occurs due to spectral broadening with increasing LED junction temperature. The full width half maximum (FWHM) spectral bandwidth of red LED spectral distributions can be predicted by:
where the dominant wavelength X is in nm and the LED junction temperature T is in Kelvin. The spectral broadening of blue and green LEDs can be ill-defined, but typically can exhibit similar behaviour. The result of this spectral broadening is a decrease in excitation purity, or saturation of the LED colour and can result in a further change in LED chromaticity.
 Changes in LED chromaticities however, may not be a problem if: a) the resultant colour gamut fully encompasses the colour gamuts defined by the SMPTE or EBU/ ITU primaries; and b) the display system includes a colour sensor to monitor the backlight chromaticity and continually adjusts the LED drive currents to maintain constant chromaticity for the displayed colours. These objectives may be achieved through careful colour binning of the LEDs by dominant wavelength, although this can be costly as only a small portion of the manufactured LEDs can be used for this purpose.
 An advantage of LED backlighting is that it can offer the opportunity to achieve a larger colour gamut than is possible with CRT display phosphors and LCD panels that are backlit with cold-cathode fluorescent lamps. However, with this comes the need for stringent colour binning requirements for the LEDs, as the range of LED chromaticities must always encompass the specified colour gamut for the display device.
 A further advantage of LED backlighting with colour feedback is that studio-quality CRT displays typically must be manually calibrated at frequent intervals to maintain colourimetric reproduction accuracy. LCD panels with LED backlighting and colour feedback can offer the possibility of self-calibrating displays. However, the need to allow for manufacturing tolerances in LED chromaticities and their temperature dependencies tends to restrict the colour gamut that can be achieved.
 There is therefore an evident need for a method whereby the choice of LEDs is not limited to a small range of dominant wavelengths in order to achieve a desired colour gamut, and there is also an evident need for an apparatus and method for backlighting using light-emitting elements, wherein said colour gamut may be maintained despite LED chromaticity shifts due to changes in LED junction temperature, LED aging, and other factors.
SUMMARY OF THE INVENTION
 An object of the present invention is to provide a backlighting apparatus and method. In one aspect of the present invention there is provided a method for generating light having a desired primary chromaticity having a dominant wavelength, said method comprising the steps of: providing one or more first light-emitting elements for generating first light having a first dominant wavelength, said first dominant wavelength being greater than the dominant wavelength of the desired primary chromaticity; providing one or more second light-emitting elements for generating second light having a second dominant wavelength, said second dominant wavelength being less than the dominant wavelength of the desired primary chromaticity; and driving said one or more first light-emitting elements and said one or more second light-emitting elements, wherein combining the first light and second light creates light having the desired primary chromaticity.
 In another aspect of the present invention there is provided an apparatus for generating light having a desired primary chromaticity having a dominant wavelength, said apparatus comprising: one or more first light-emitting elements for generating first light having a first dominant wavelength, said first dominant wavelength being greater than the dominant wavelength of the desired primary chromaticity; one or more second light-emitting elements for generating second light having a second dominant wavelength, said second dominant wavelength being less than the dominant wavelength of the desired primary chromaticity; a feedback system for monitoring a combination of the first light and the second light, said feedback system for generating feedback signals based thereon; and a control system operatively connected to the feedback system for receiving the feedback signals and for controlling activation of said one or more first light-emitting elements and one or more second light-emitting elements, wherein said control system activates the one or more first light emitting elements and the one or more second light-emitting elements in order that the combination of the first light and the second light creates light having the desired primary chromaticity; wherein the apparatus is adapted for connection to a source of power for activation of the one or more first light-emitting element and one or more second light-emitting elements.
 In another aspect of the present invention there is provided a backlighting apparatus comprising: one or more first light-emitting elements for generating first light having a first dominant wavelength, said first dominant wavelength being greater than a desired first primary dominant wavelength and one or more second light-emitting elements for generating second light having a second dominant wavelength, said second dominant wavelength being less than the desired first primary dominant wavelength; one or more third light-emitting elements for generating third light having a third dominant wavelength, said third dominant wavelength being greater than a desired second primary dominant wavelength and one or more fourth light-emitting elements for generating fourth light having a fourth dominant wavelength, said fourth dominant wavelength being less than the desired second primary dominant wavelength; one or more fifth light-emitting elements for generating fifth light having a fifth dominant wavelength, said fifth dominant wavelength being greater than a desired third primary dominant wavelength and one or more sixth light-emitting elements for