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125 METHODOLOGY


The disability glare metric used as the basis for this modelling work was published in a research paper by D. Hassall*. 1. DATA COLLECTION FROM FAÇADE MOCK-UP AT BORDESLEY:


• Full size mock-up of a facade section (approx 20m x 10m);


• Luminance measurements;


• High dynamic range photography for digital analysis;


• Data used for calibration of the software model. 2. COMPILE A 3D CITY MODEL OF THE ZONE OF INFLUENCE AROUND BIRMINGHAM NEW STREET STATION FROM A VARIETY OF SOURCES: • Digital terrain mapping of Birmingham city centre;


• 3D railway track alignment model; • Architect’s Rhino model of the proposed station building.


3. SUN REFLECTION SIMULATION: This exercise involved Radiance 3D sunlight modelling throughout the daylight hours of each solstice and equinox to understand the location and movement of the sunlight reflected from the facade. The warping of the facade and constantly moving sun position could potentially pose an infinite number of sunlight reflection possibilities which would require computer modelling. The objective of this exercise was to determine when sunlight reflections of significant scale and duration land on a particular zone of track where the train driver may be trying to visually locate a signal in the distance. These significant reflections are then analysed for disability glare. 4. SUNLIGHT REFLECTION INTO SIGNALS: As a result of the sun reflection simulation, Arup discovered that sunlight could be directed into a railway signal Fresnel lens. The resultant out- comes were that the signal lens could on occasions appear to be illuminated when it is not artificially illuminated and when the sunlight is of sufficient high intensity that it obliterates the colour of the signal.


Arup subsequently completed an analysis of a variety of railway signals, simulating the optical performance of the signals using measured weath- er data.


5. DISABILITY GLARE ASSESSMENT: Having established the location of significant sun- light reflections the approaches to five platforms from the east and west directions were tested for disability glare (veiling luminance) using an algorithm based on the Holladay-Stiles** formula for veiling luminance: Lv


= 10E/θ2 . The veiling luminance (Lv) depends upon the angu-


lar distance from the source. ‘E’ is the illuminance in the plane of the eye due to the glare light source and ‘θ’ is the angular distance in degrees. The Hassall paper proposes that the veiling lumi- nance threshold is 500cd/m2


. A veiling luminance


measured or predicted above this value will result in disability glare.


The ability of the train drivers to view the approaching signals is related to the speed of the train. As a train approaches the station it has a maximum speed of 10mph. Using Radiance, Arup produced an animation of the train driver’s field of view travelling at 10mph towards the station. The animation included conditioning of each frame with a human visual response function which simulated internal scattering in the human eye resulting in the loss of visible contrast near bright sources and offers a visual prediction of the glare effects.


Each frame of the animation was numerically analysed for veiling illuminance and a profile of the driver’s journey created to establish the mag- nitude and duration of any disability glare effects.


Top to bottom Glare studies revealed the potential for disability glare for train drivers; then the lack of glare after the façade redesign; Arup engaged in reflection studies to ascertain the way the sun would reflect off the façade surface at different points in the day; a mock-up was built by Arup to test glare conditions.


* D. Hassall, Reflectivity: Dealing With Rogue Solar Reflections, University Of New South Wales, 1991. ** Holladay LL. 1926, The fundamentals of Glare and Visibility, J. Opt. Soc. Am. 12(4):271-319


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