The problem of glancing light

The problem of glancing light

Glancing light, simply put, is when sunlight or artificial light shines onto a surface at an acute angle.

moonThe effects of glancing light can transform what appears to be a perfectly flat, rendered surface during direct lighting, into a surface that appears seriously undulated.

This is caused by the light hitting any small, virtually invisible blemishes or undulations at an angle that subsequently casts a shadow highlighting the blemish or undulation. The more acute the angle, the greater the shadow that will be cast.

Both natural and artificial light should be taken into consideration when designing a rendered project and more importantly, when working with subcontractors. Depending on the orientation of the project, the effects of glancing light from the sun can often only be observed for an hour or less, but this is a very different story with artificial lighting,

The British Standards for rendering (BS EN 13914-1:2005) covers assessment of rendered finishes and lighting conditions in section NA.15 including general recommendations, glancing light and viewing conditions. Most importantly it states that “if the surface is to be assessed under glancing light conditions, this should be stated in the contract specification”.

Communication is always key when it comes to glancing light, and we would always recommend that if there is any chance of glancing light or that any facades will have artificial lighting shone against the wall, that all parties are made aware and details are included in the contract specification.

Quite often we are approached by architects, contractors and subcontractors after an elevation has been viewed under glancing light conditions, or external lighting has been installed that casts light onto an elevation at an acute angle. It is often apparent in this situation that little to no consideration was made for glancing light or details included in the contract for works.

If glancing light is overlooked and considerations not made to elevations that will experience glancing light, or included in any contract of works, we often feel it fair that no individual party is held responsible. In order to rectify the issue, the elevation in question is re-rendered with extra care and consideration to achieve as flat a surface as possible.

Issues caused by artificial lighting that were not taken in to account or communicated to all parties can be rectified more easily by adjusting the angle of the lighting so that the light is cast over the render at a more forgiving angle. We have seen on many occasions that adjusting the angle of the light can improve the appearance of the render drastically.

Before you start a project where glancing light may be a problem, or for more information on glancing light, please call our technical team on 01647 24620 or ask us to call you.

Posted by Adam Powell, Technical Consultant on 16 May 2017 | Comments