Short answer
Because they reflect sunlight or bright lamps specularly — like a mirror — rather than diffusely, sending a concentrated beam towards people's eyes. Colour and light reflectance value describe how much light a surface returns; finish and geometry decide where it goes. The worst cases combine glossy or glazed surfaces with curved or angled façades, as at 20 Fenchurch Street in London in 2013, where a concave glass face focused sunlight onto streets hot enough to damage a parked car.
A surface's light reflectance value tells you what fraction of visible light it returns, averaged over all directions. It says nothing about the direction. A matt white render and a polished white stone can have similar LRVs; the render scatters light in all directions, so it looks evenly bright from anywhere, while the polished stone sends much of its reflection in one direction, producing a bright highlight for some viewers and a darker appearance for others. Glass, polished metal and gloss paint are strongly specular. Glare problems from buildings are therefore mostly about finish and geometry, with colour controlling only how much energy there is to redirect.
A flat mirror reflects the sun's image; a concave one concentrates it. At 20 Fenchurch Street, the concave south face acted as a curved mirror for a period of up to about two hours a day when the sun was in the right position during summer 2013, focusing a beam onto the streets to the south. Media reported spot readings above 90 °C, and parts of a parked car's bodywork were damaged; the developer paid for the repair and, with the City of London's approval, installed temporary screening while longer-term fixes were evaluated. The case shows that glare assessment belongs at design stage, using the sun's path and the façade's shape, not after occupation.
Solar glare is predictable. The sun's position for every hour of the year is known in advance, so reflections from a proposed façade can be modelled before anything is built.
Inside buildings the concern is usually discomfort glare from bright luminaires, windows and reflections in screens and shiny surfaces. The CIE's Unified Glare Rating, set out in CIE 117-1995, estimates discomfort from the luminance of light sources in view, their size and position, and the background luminance. Surface colour enters through the background: dark walls and ceilings make the same luminaire more glaring than light ones, because the contrast is higher. That is one reason lighting designers ask for reasonably light room surfaces, and why the same fitting can be comfortable in one room and harsh in another.
Very high-reflectance outdoor surfaces — white paving, bright render, reflective roofs — can make streets and courtyards uncomfortably bright in strong sun and reflect heat onto neighbouring walls and people, even if they keep their own surface cooler. The trade-off between cooling a surface and glare or heat on those around it is real and site-specific. Designers manage it with matt finishes, texture that breaks up reflections, vegetation and shading, and by modelling where reflections will fall. None of those tools changes a colour's hue; they change how light leaves the surface.
Why: Specular reflection of low sun from angled glazing towards a road.
Fix: Model solar reflections at design stage; adjust glazing angle, use lower-reflectance glass or add shading.
Why: High-reflectance paving and walls return much of the sunlight into the space.
Fix: Use mid-reflectance, textured or planted surfaces, and shade where people sit.
Why: Darker walls raise the contrast between luminaires and their background.
Fix: Keep ceilings and upper walls light, or choose luminaires with lower luminance in view.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
During construction of 20 Fenchurch Street in 2013, its concave south façade focused reflected sunlight onto streets to the south for about two hours a day, with a media-reported spot reading of 91.3 °C and damage to a parked car.
Caveat: Temperatures are spot readings reported by media rather than controlled measurements.
Source: London skyscraper 'melts' car (and follow-up reporting, 2–3 September 2013); 20 Fenchurch Street — solar glare problem (Wikipedia)
FactStrong evidence
CIE 117-1995 defines the Unified Glare Rating, a practical system for evaluating discomfort glare in interior lighting.
FactStrong evidence
Light reflectance value measures the total visible light a surface reflects in all directions, so it does not distinguish a matt surface from a glossy one of similar reflectance.
Colourwise interpretationModerate evidence
Façade glare risk depends more on surface finish and geometry than on colour, which mainly sets how much light is available to be redirected.
Based on: Follows from the difference between specular and diffuse reflection and from the Fenchurch Street case, where geometry concentrated reflected sunlight.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.