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We developed a measurement method for the characteristics of microdisplays specifically aimed at vertically aligned nematic reflective cells. It allows determination of contrast ratio and cell gap, and gives good estimates for the pretilt angle and the elastic surface-coupling constant. The set-up consists of a laser source, high quality polarisers, a beamsplitter mirror, a quarter-wave plate and a sensitive photodiode. A model for the polarization changes in the light caused by each component allows the extraction of the initial phase retardation induced by the cell and gives a first estimate of the thickness. Simulation of the director configuration in liquid crystals is then used to enhance the accuracy by taking into account the properties of a real LC cell. Matching of the simulation and the measurements yields the required values together with a calibrated simulation model. 1.INTRODUCTIONLCoS devices are serious candidates for becoming the dominant lightvalve technology in many applications 1 . The vertically aligned nematic LC mode is ideally suited for LCoS applications, offering high performance in terms of contrast, response time and driving voltage requirements 2 . As these devices become more common, a measurement method to determine characteristics and ensure quality of production will be needed. For transmissive devices, especially employing twisted LC configurations, display metrology methods are thoroughly investigated and standardised 3 . Publications concerning measurements for reflective devices are not so common and most are aimed at LC modes employing twist 4 . METHODOLOGYThe display characteristics of interest typically include the contrast ratio, the response speed, cell-gap uniformity and some information about the liquid crystal orientation angles. The methodology described concentrates on contrast ratio and cell gap, giving information on orientation angles and anchoring energy as a side effect. Response speed measurements are very straightforward and thus not discussed. OPTICAL SET-UPDue to the size of a typical LCoS device, illumination with a laser spot is most convenient if one wants to scan the active area to detect non-uniformities. The laser is also the ideal source of light to measure the inherent contrast ratio of the cell. The set-up is an on-axis configuration. Figure 1 schematically shows the arrangement of the optical elements in our measuring device. The laser source is a HeNe-laser emitting virtually non-polarised light at 543 nm. After being deflected by a mirror, the beam is linearly polarised by a Glan-Thompson polariser. These polarisers offer high extinction ratios in the crossed configuration (a ratio of more than 10 5 is easily achieved), which will be needed in view of the expected high contrast of the cell itself. The light is then sent through a broadband dielectric beamsplitter and projected onto the display under test. The reflected beam again encounters the beamsplitter, where a part of the light is * dcuypers@elis.rug.ac.be; phone +32 9 264 3404; fax +32 9 26...

Atmospheric dependent, gas sensitive resistors seem to be good candidates for detecting critical air pollution levels. Recently, great progress has been made in the development of various sensor types, but less attention seems to be paid to the integration of sensor elements with different characteristics. The aim of this international project is to develop a smart hybrid gas multisensor module for environmental applications, i.e. by combining classical thick-and thin-film elements with polymer-film based sensors and also a signal processing ASIC within a single package, which should be useful for all sensor types. The module should enable multi-sensor operation as well, when connected to an intelligent signal-processing unit.