Blinded by the Light, 2020

Visual (not thermal) glare is not restricted to daylight. London’s Crossrail has lodged a formal objection against the proposed MSG Sphere in Stratford, east London, warning that giant LED advertising screens on the 90m-tall entertainment venue would compromise safety by impairing train drivers’ ability to pick out signals vital for running high speed trains on a complex part of the network. It is feared the effect, called masking, would occur because the luminance of the glaring LED screens behind signals at the side of the tracks would be several orders of magnitude greater than the luminance of the signals.

Whereas there is still no internationally accepted methodology to demonstrate glare from reflective facades, methods exist which can be used to estimate the sensation of glare to which passing motorists and pedestrians would be subject, such as the Hassall method as described in his book Reflectivity (5). When designing for network Rail 20 years ago, my practice was made fully aware of the dangers of glare for train drivers, and we were obliged to demonstrate that solar reflections would not diminish a driver’s ability to see the colour of signals.

Accurately calculating glare is still problematic, although David Hassall’s book Reflectivity was published in 1991 with glare templates. The first glare-related legislation, one suspects because of that publication and lobbying, was adopted by Sydney City Council in 1992: ‘Veiling luminance’ suggested a maximum reflected solar glare of 500 candelas (cd) /m2 on vehicle drivers. Later, the same council limited the exterior surface reflectivity of a building to 20%, specifying that all materials including window glass will have a reflectivity below 20%. Singapore and Rotterdam have adopted similar legislation. The City of London brought in a Planning Advice Note on the subject in 2017, with reference to a BRE Information Paper IP 3/87 ‘Solar dazzle reflected from sloping glazed facades’ (IHS BRE Press, Bracknell, 1987) on how to carry out the calculations.

Obviously glare should be considered when contemplating the massing of a building or a larger development. This requires evaluating the geometries and surfaces of neighbouring buildings with specular facades, potential secondary and tertiary reflections, the Earth’s movement in relation to the sun, local topography, viewing points, and types of public and transport spaces.

This brings immediate awareness of the basic laws of optical physics: angle of incidence equals angle of reflection, and the trajectory of nature’s own laser – the sun. Such calculations, including sciagraphy used to determine the perspective projection of shadows suggest immediately that a concave surface will focus light, and a convex surface will scatter it, though still potentially create moments of glare. During the design process of any structure it would seem sensible to avoid surface geometries – particularly parabolic and multiple-angled – that could focus sunlight to cause glare and overheating.

Intelligent building orientation and facade design can clearly mitigate glare, and limiting a building envelope’s reflectivity to prevent glare might be considered a banal approach when it is clearly possible for an architect or technical advisors to demonstrate the level of risk using sun-path analysis of a reflective façade. In interior lighting, good design practice either diffuses the light to reduce the luminance or shields the source from view. There is no reason the same practice should not be applied to reflected sunlight.