52.2 / J. Bergquist 52.2: Display with Arbitrary Primary Spectra Johan Bergquist Display Research, Device R&D Common Technologies, Nokia, Tokyo, Japan Abstract Individual differences in colour sensation of a single display (ob- server metamerism) can be reduced by synthesizing and adapting the primary spectra to the physiological spectral response of the observer. The technique can also be used to synthesize the eigen- spectra in multispectral imaging, and thereby provide an alterna- tive to multi-primary displays. 1. Introduction The colour appearance in real life is generally determined by an illumination source with a continuous light spectrum, and an ob- ject with a continuous reflectance spectrum. The light hitting the observer’s retinas has a spectrum which is a product of the illumi- nation and reflectance spectra and is therefore also continuous. In electronic displays, metameric matching is used to reproduce the same colour sensation as in real life. That is, a combination of three or more primary light sources whose combined spectra give the same colour sensation as the original product of the source and reflectance spectra. The human visual system (HVS) combines the primary stimuli, e.g. RGB, either spatially (subpixels) or tempo- rally (field-sequential colour, FSC) to obtain the desired colour sensation. Two stimuli that give the same colour sensation for any observer under any circumstances need to have the same spectra; they are said to be isomers. Two colours that appear identical but originate from different spectra are called metamers. When the appearance of an object or displayed image is different for differ- ent observers in the same viewing conditions, it is referred to as observer metamerism. Approximating a continuous spectrum with only three primary sources inevitably results in observer metamerism, particularly if the primary sources are saturated [1, 2]. This is because even a small shift in the observer’s sensitivity spectrum by, for example, aging or changing field-of-view (FOV), has a large impact on the product of the colour matching functions and the narrow primary spectra. On the other hand, the HVS is correcting the stimuli from experience and different people therefore tend to see the so-called memory colours in the same way, even when their physiological spectral sensitivities are different. But if the object colour is un- known or there could be several candidates for a particular colour, memory colour in our visual system does not support the observa- tion. In this case, a spectral match closer to real life would give an appearance that is more equal between observers, i.e. it reduces observer metamerism. 2. Observer metamerism in displays Displays based on organic light-emitting diodes (OLED), RGB LEDs, or LASERs, all offer a wide gamut that enable a colour reproduction hitherto impossible. From a device engineering point of view, narrow-spectrum RGB LEDs also give higher luminous efficiency [3]. On the other hand, there is a trade-off between wide gamut and observer metamerism which, so far, has been eased [4, 5] by multi-primary colour (MPC), i.e. displays with more than three primaries. MPC also provides a solution to the inevitable trade-off between colour gamut and luminance in displays with broad-band light sources and absorbing colour filters. MPC has been implemented in several configurations, for exam- ple, six-primary displays realised by two projectors with two dif- ferent sets of colour filters [6] or direct-view LCDs with six dif- ferent colour filters [7]. However, a finite number of primaries still result in metameric mismatches, and increasing the number of primaries further would reduce the temporal and/or spatial resolu- tion of the display. Reducing observer metamerism is also possi- ble by employing broader primary spectra [1, 8] but this inevita- bly means sacrificing of the gamut. FSC displays with adaptive gamut (programmable chromaticities) have been proposed for projectors [9] and direct-view displays [10] using temporal and spatial superposition of the primaries. Such displays can boost luminance by up to 100% per primary for unsaturated content, decrease colour break-up, increase moving image quality, and improve ambient contrast [10]. Compared to MPCs, they solve the luminance-saturation trade-off without any increased display complexity or reduction in temporal and/or spa- tial resolution. However, they are based on narrow-spectrum pri- mary sources and are thus prone to observer metamerism, espe- cially in the case of low-chroma images for which the HVS is very sensitive to variations in memory colours such as white and skin colours. This problem also manifests itself in on-screen metamerism [4] of RGBW displays where W is generated directly from the backlight; White by RGB and white by W look different. One objective of this paper is to propose a display with adaptive gamut and reduced observer metamerism. Desaturation is achieved through primary spectra synthesis (broadening) rather than superposition of fixed narrow spectra. While not touched upon here, it is also possible to adapt such a display to observers with colour vision impairments or age-related physiological changes, without transformation of the image data. 3. Multi-spectral imaging To accurately capture and reproduce the appearance of any object for any observer under any circumstances, it is necessary to record the reflectance spectrum for each pixel and multiply with the spectrum of the desired light source (multi-spectral imaging). While such an approach involves redundancy and produces an exceedingly large amount of information, a linear combination of eigenspectra can fully describe the original spectrum. For exam- ple, it has been found [11] that only three and five eigenspectra are sufficient to fully reproduce the original reflectance spectra of human tissue and oil paintings, respectively. For any image, a maximum of seven primaries is necessary to reproduce the origi- nal spectrum [12]. Therefore, a display with eigenspectrum prima- ries requires only between three and seven channels and the un- compressed video bandwidth to the display would therefore be approximately the same as in conventional three-primary or multi- primary displays. However, established compression techniques