Why does ‘glare factor’ matter
Glare that is caused by a light fixture is described as either ‘discomfort’ or ‘disability’ glare. It means that someone working at a task finds it either difficult or impossible to see what they are doing. These days, a common cause of glare is a luminaire mounted in the ceiling in front of someone’s workstation.
We tend to think that glare is all about people working at computers, but it can be a problem for anyone who needs to focus on a task, be it an industrial process or an activity in the retail and leisure sectors. A bright light that stops you working is a problem.
Glare values have become an issue since modular LED panels became the luminaire of choice for commercial spaces. As these fixtures have increased in brightness, so their glare component has risen. Lighting designers need to ensure that luminaires are appropriate to purpose and location.
The lighting industry has addressed the issue of acceptable brightness by creating the unified glare rating (UGR). It’s a mathematical way of determining the ‘brightness’ of a luminaire in a given setting. And at first sight it’s a daunting equation. Without going into detail, these are the important factors:
- The luminance value of the luminaire, as reported by the photometric data
- The value of the background luminance, based on standard values for different surfaces
- The viewing angle from viewer to luminaire, measured from the horizontal
- A factor called the Guth Index, which is based on ‘visual comfort probability’
The important information to take away from this is that the UGR is based on a luminaire fitted within a space – it’s not just a luminaire in a box. A luminaire may be labelled as being ‘UGR19 compliant’, but that can only be taken as meaning that, in all likelihood, once the luminaire parameters are put into the UGR formula, that fixture will be compliant.
The UK health and safety regulations require that lighting at work should be suitable and sufficient; ‘suitable and sufficient’ is referenced back to the lighting design criteria developed by the Society of Light and Lighting (SLL). So UGR compliance is essentially a legal requirement.