A plastic Fresnel lens positioned just before the diffusion screen in a projection system can provide significantly superior corner illumination, enhancing overall display luminance uniformity. Fresnel lenses are also used within the light engine to collimate light through the LCD panel and focus light through the projection lens. Disadvantages include the added cost of the lenses, ghosting, printout of the Fresnel rings and moiré patterns. Birefringence is important to control in polarization-sensitive applications.Without reduction of ghosting artifacts, Fresnel lenses will have limited application in text-based RP systems. Ghosting is described in some detail including how it is caused, quantified and reduced. Efforts to reduce cost and birefringence will also be discussed.

Aim Globally, most landscape burning occurs in the tropical savanna biome, where fire is a characteristic of the annual dry season. In northern Australia there is uncertainty about how the frequency and timing of dry season fires have changed in the transition from Aboriginal to European fire management. LocationIn the tropical eucalypt savannas that surround the city of Darwin in the northwest of the Northern Territory of Australia. MethodsOur study had three parts: (1) we developed a predictive statistical model of mean mass ( µ g) of particulates 10 µ m or less per cubic metre of air (PM 10 ) using visibility and other meteorological data in Darwin during the dry seasons of 2000 and 2004;(2) we tested the model and its application to the broader air shed by (a) matching the prediction of this model to PM 10 measurements made in Darwin in 2005, (b) matching the predictions to independent measurements at two locations 20 km to the north and south of Darwin and (c) matching peaks in PM 10 to known major fire events in the region (2000-01 dry seasons); and (3) we used the model to explore changes in air quality over the last 50 years, a period that spans the transition from Aboriginal to European land management. ResultsWe demonstrated that visibility data can be used reliably as a proxy for biomass burning across the largely uncleared tropical savannas inland of Darwin. Validations using independent measurements demonstrated that our predictive model was robust, and geographically and temporally representative of the regional airshed. We used the model to hindcast and found that seasonal air quality has changed since 1955, with a trend to increasing PM 10 concentrations in the early dry season. Main conclusionsThe results suggest that the transition from Aboriginal to European land management has been associated with an increase in fire activity in the early months of the dry season.

The degree to which a forecast changes from one issue time to the next is an interesting aspect of a forecast system. Weather forecasters report that they are reluctant to change a forecast if they judge there is a risk of it being changed back again. They believe such instability detracts from the message being delivered and are reluctant to use automated guidance which they perceive as having lack of stability. A Flip‐Flop Index was developed to quantify this characteristic of revisions of fixed‐event forecasts. The index retains physically meaningful units, has a simple definition and does not penalize a sequence of forecasts that show a trend, which is important when assessing forecasts where a trend can be interpreted as a forecast becoming more confident with a shorter lead time. The Flip‐Flop Index was used to compare the stability of sequences of automated guidance with the official Australian Bureau of Meteorology forecasts, which are prepared manually. The results show that the forecasts for chance of rain from the automated guidance are often more stable than the official, manual forecasts. However, the official forecasts for maximum temperature are more stable than those based on automated guidance. The Flip‐Flop Index is independent of observations and does not measure skill, but it can play a complementary role in characterizing and evaluating a forecasting system.

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Aim Globally, most landscape burning occurs in the tropical savanna biome, where fire is a characteristic of the annual dry season. In northern Australia there is uncertainty about how the frequency and timing of dry season fires have changed in the transition from Aboriginal to European fire management. LocationIn the tropical eucalypt savannas that surround the city of Darwin in the northwest of the Northern Territory of Australia. MethodsOur study had three parts: (1) we developed a predictive statistical model of mean mass ( µ g) of particulates 10 µ m or less per cubic metre of air (PM 10 ) using visibility and other meteorological data in Darwin during the dry seasons of 2000 and 2004;(2) we tested the model and its application to the broader air shed by (a) matching the prediction of this model to PM 10 measurements made in Darwin in 2005, (b) matching the predictions to independent measurements at two locations 20 km to the north and south of Darwin and (c) matching peaks in PM 10 to known major fire events in the region (2000-01 dry seasons); and (3) we used the model to explore changes in air quality over the last 50 years, a period that spans the transition from Aboriginal to European land management. ResultsWe demonstrated that visibility data can be used reliably as a proxy for biomass burning across the largely uncleared tropical savannas inland of Darwin. Validations using independent measurements demonstrated that our predictive model was robust, and geographically and temporally representative of the regional airshed. We used the model to hindcast and found that seasonal air quality has changed since 1955, with a trend to increasing PM 10 concentrations in the early dry season. Main conclusionsThe results suggest that the transition from Aboriginal to European land management has been associated with an increase in fire activity in the early months of the dry season.