Lighting Research & Technology

The quality of light emitted by lighting systems based on high brightness LEDs is studied using colorimetric parameters (chromaticity coordinates, correlated colour temperature and colour rendering index) when operating with different forms of electrical power supply (continuous and pulsed current). Uncertainties, calculated by Monte Carlo simulations applied to relative spectral measurements and colorimetric parameters are presented.

This study investigated the impact of well geometry on vertical sky components in atria with square and rectangular forms under a CIE standard overcast sky. By comparing scale model measurements and analytical theory the vertical sky components, calculated using Radiance, were validated. More simulated data of vertical sky components for a very wide range of atrium geometries are given. From the results the attenuation of the vertical sky component on the wall of a square atrium is explained and some empirical functions are derived. In addition, the results from atrium models with rectangular floor plans show the relationships between the vertical sky component and the plan aspect ratio. Some guidelines for design are presented.

A new occupancy detection sensor network was developed, commissioned and installed in two private offices: data were collected to evaluate the utility of the sensor network for lighting control. Results show that there is considerable uncertainty associated with the determination of occupancy using measurements from a single detector in a space. A sensor network reduces uncertainty, because data from other detectors provides converging information that can be used to determine if a space is occupied. Sophisticated analysis techniques can be applied to the sensor network data stream to provide improved occupancy measurement and lighting control, compared to current systems.

This paper reviews the literature on occupancy-based lighting control as a prelude to the application of sensor networks to building management. Many buildings include systems to detect occupancy and control building services. Current systems use single measurement points to detect occupancy, and there can be significant uncertainty associated with the measurement of occupancy. Long time delay and high detector sensitivity settings compensate for this uncertainty, but these diminish the savings that could be achieved with more accurate occupancy measurement. More effective control may be provided by more extensive sensing, using a network of occupancy sensors, and more extensive analysis of sensor data. The literature reviewed in this paper establishes the need for an investigation of the performance of sensor networks when used for lighting control.

Review of previous studies suggests that mean preferred illuminances determined using an adjustment task tend to lie in the middle of the range of illuminances available, a stimulus range bias. This hypothesis was validated through the results of an experimental study. It is therefore suggested that the adjustment task is not an appropriate method for determining optimum illuminances.

This study was designed to explore the roles that long- and short-wavelength lights have on momentary mood and alertness at night. Twenty-two subjects participated in a mixed-design experiment, where we measured the impact of two levels of long- and short-wavelength lights on brain activity and on self-assessments of alertness, sleepiness and mood. Measurements were obtained 60 minutes prior to, during and after light exposure. Results showed that the red and the blue lights increased electroencephalographic beta power (12—30 Hz), reduced sleepiness, and increased positive affect relative to the previous dim-light period indicating that alertness and mood can be affected by light without necessarily stimulating the melatonin pathway. The impact of light was modest, however, compared to the increase in fatigue over the course of the night.

The experimentally known values of the 14 exponents associated with the exponent-rules for vacuum and gas-filled tungsten filament lamps are utilized to obtain the temperature dependence of performance parameters such as power demand, lumen output, life and efficacy. It is also shown that the data on emissivity of tungsten reported by Jones and Langmuir are consistent with theory in contrast to those obtained by Roser and Wensel. Further, the analysis predicts a higher power loss factor and a longer life for gas-filled tungsten filament lamps than for vacuum lamps.

This paper proposes a method for predicting fluorescent lamp lifetimes based on a minimum number of accelerated life tests. This prediction allows the life of the lamp in continuous operation to be estimated without having to know the nominal lifetime. This paper evaluates the possibility of making this prediction based on two accelerated tests, and finds that this is not generally to be recommended. It also evaluates the advisability of making the prediction based on three accelerated tests and fitting the results by a non-analytical iterative method. We conclude that under certain conditions, this method makes reasonable life predictions for fluorescent lamps associated with specific ballasts.

This year it is appropriate to celebrate an event of great significance for all illuminating engineers. This year is the 250th anniversary of the publication of major books by Pierre Bouguer and Johann H. Lambert. These books summarise the principles of photometry and of daylight science in particular. It is also an opportunity to mention the basic criteria of daylight design and evaluation still used today, that is the Daylight and Sky Factors together with the methods for the calculation of window solid angles developed by Lambert and several approximations derived later. All 20th Century daylight design tools, for example diagrams, nomograms or protractors are based on these. Although the simplified Daylight or Sky Factors are obsolete now, the solid angle for large area sources with a particular sky luminance specification is still needed for calculating the Daylight Coefficient or absolute illuminance in computer programs.