A NEW BASIS FOR LIGHTING PRACTICE

Christopher ‘Kit’ Cuttle

The ways in which we specify,measure, and calculate

illumination are in need of fundamental revision

From “A Guide to Designing Quality Lighting for People and Buildings”, Illuminating Engineering Society of North America, IES DG-18-08

Human Needs Served by Lighting

Task Visibility

Task visibility is essential to lighting design; lighting exists to enable vision. Recognition of this fact led to an emphasis on visibility above all other lighting design goals in the past, resulting in a high level of understanding of visibility and its importance.

From: IESNA Lighting Handbook, 9th Edition, 2000. Illuminating Engineering Society of North America, 10-13.

“Changes in visual performance as a function of task contrast and size, background reflectance and observer age can be calculated precisely.”

Orientation and navigation through a space …

“At a mean illuminance of 1.0 lux on the escape route people are able to move smoothly and steadily through the space at a speed very little different from that achieved under normal room lighting.”

Boyce PR, “Movement under emergency lighting: The effect of illuminance”, LR&T 1985, v17, 51-71

General lighting practice of 100 years ago showed substantial agreement with people’s visual performance needs, as indicated by modern research data.

It was in the middle of the last century that we made the transition from providing for people’s visual needs to meeting their expectations for an adequately lit space.

Can we specify illumination to provide “an adequately lit space”?

• The proposed criterion is Perceived Adequacy of Illumination (PAI)

• PAI is an assessment of whether or not the users of a space are likely to judge the ambient illumination to be adequate

• Ambient illumination refers to reflected light received at the eye

• How would adoption of PAI affect lighting practice?

The SLL Lighting Handbook, 2009: Office lighting,162.

“Unless specified otherwise, the recommended maintained illuminance is measured on a horizontal plane at desk height.”

Lighting standards specify minimum task illuminance (lux)

This is generally interpreted as illuminance on the horizontal work plane (HWP)

Energy standards specify maximum lighting power density (LPD) in watts per sq m (W/m2) or LPD per 100 lux

So “efficient” lighting requires:- High lumens per watt light sources- Luminaires that focus light onto the HWP

Mean Room Surface Exitance (MRSE)

An indicator of a typical assessment of the brightness of illumination in an indoor space

The average lm/m2 exiting (emerging from) all surrounding room surfaces: it is a measure of reflected light, excluding direct light

PAI specified in terms of MRSE?

Direct light (for presentations only)

A changed way of thinking about lighting ….

• The room surfaces are as much part of the lighting scheme as the luminaires

• The aim for the initial light distribution (by luminaires or fenestration) is to direct light onto reflecting (or translucent diffusing) surfaces

• The purpose is to deliver reflected light to the eye, treating the room as a second luminaire

• Move on from providing for human need (VP) to providing for human satisfaction (PAI)

• We need a lighting measure that relates to human satisfaction in order to assess lighting efficiency

http://kit-lightflow.blogspot.com