Reflective!Xerox arm dpiX is presenting a paper at this week's Society for Information DisplayConference in San Diego which investigates full colour reflective displays. Steve Bush examines its findings Ink on paper has been good enough to convey messages for at least two thousand years. As reflective technology, it works well in varied lighting conditions, from brilliant sunshine to candlelight.
Children reading under the bed clothes with a torch because mum's turned out the light, is one of the few situations that anyone chooses to supplement ambient light for reading paper. So, as parents don't hold as much sway over the average laptop user, why do portable computers have power hungry emissive displays when a reflective one would work well in most circumstances? The answer is that full colour reflective displays are far more difficult to make than emissive types.
Despite, and because of, the difficulties, one of the most active areas in display research at the moment is colour reflective LCDs. Evidence of this is to be found in the number of papers presented on the subject at this year's Society for Information Display Conference in the US. Reflective displays offer more than just power reduction and sunlight readability. The eye and brain are used to looking at objects that reflect light. We automatically compensate for variations in both the colour and intensity of ambient light and the effect of this on reflective objects. Reflective things look more natural, although possibly less eye catching.
Xerox, under the banner of its Palo Alto-based subsidiary dpiX, is heavily involved in reflective colour LCD research. Greg Crawford of dpiX is presenting a paper at this week's SID conference in San Diego, which sets out some of the difficulties of making reflective LCDs and some possible solutions.
In the paper, Crawford identifies the key to successful reflective displays is providing high spectral reflectance. That is, good colour selectivity with adequate reflected luminance. Current emissive LCDs are transmissive types with a lamp behind. These transmissive displays do not actually transmit much light at all.
Crawford claims that a typical full colour twisted-nematic display only transmits between three and five per cent. Putting a mirror behind this would result in a reflective display with, at most, 1.5 percent reflectivity.
Losses come from the polarisers, which eat 60 per cent of incident light, and the absorptive colour filters that lose more light as their colour selectiveness increases. To make matters worse, the mosaic approach used in colour LCDs only gives a third of the total area of the display to each primary colour.
Crawford's group has used a sophisticated modelling package to estimate the performance required of a reflective display.
The starting premise was that it should offer at least VGA resolution and at least the colour performance of a CRT or active-matrix LCD. The predicted minimum reflectivity for a display to be readable in-office and outdoors is 40 per cent.
The paper describes two technologies that promise this kind of performance, both are based on polymer stabilised liquid crystal (PSLC).
This is a technology that is already exploited to make bistable monochrome displays by US-based Kent Display Systems. These displays use liquid crystals in which polymers are dispersed. They have both a transparent and a highly reflective state and the displays do not requiring polarisers to function.
The difference between the two techniques in the paper is that one uses scattering to reflect light and the other uses Bragg reflection (as the Kent displays do). Bragg reflection is a phenomenon that happens when light hits layers that are spaced at a multiple of its wavelength. Constructive interference is responsible for the reflection. In both the scattering and the Bragg case, the reflectivity varies with optical wavelength (colour).
Both of the proposed displays use three layers, one for each primary colour. Scattering type displays reflect a maximum of 50 per cent of incident light. Using another model, the team has shown that an overall reflectance of 34 per cent could be achieved using three stacked elements with 40 per cent individual reflectivity.
To avoid the 66 per cent light loss inherent in side by side RGB mosaic structures, a full colour reflective LCD must have three separate colour layers. The liquid crystal in each layer must reflect only one colour and pass all others.
The Bragg type of display, called holographic-PDLC (H-PDLC) display, has been shown to be capable of reflecting 100 per cent of light at the Bragg wavelength and transmitting all others. In the off-state, these have planes of droplets at predetermined positions in the liquid that set up a modulation of its refractive index. This results in Bragg reflection. In the on-state, the periodicity vanishes, and with it, the reflection, resulting in a clear liquid.
The modulation intensity, and therefore the reflectivity, can be varied by changing the applied voltage. The Bragg wavelength (the colour) is set during manufacture.
Modelling has shown that a three layer Bragg reflector display could offer better colour performance than a standard phosphor but still with only 34 per cent reflectivity. The reason for the lack of improvement in reflectivity is given as the restricted thickness of the layers used in the model. These only allow for a limited number of droplet planes.
Further modelling work has resulted in several optimised H-PDLC configurations with a minimum reflectivity of 54 per cent.
In addition, the paper claims that a monochrome display with a reflectivity of 73 per cent has been designed. This is said to be equivalent to good quality paper.