Target illumination: Visual emphasis and target/ambient illuminance ratio

The lighting design objectives procedure based on ambient and target illumination has been proposed by Cuttle and supported by notable industry experts as part of a Lighting Manifesto. It uses two new metrics. Mean room surface exitance (MRSE) has been tested and proven to be a reliable metric in assessing perceived adequacy of illumination and spatial brightness. The second metric, target/ambient illuminance ratio (TAIR), has not been tested with no published research on it. TAIR is the ratio of target illumination at any relevant points of emphasis to background MRSE. It may be used to design for visual performance or for visual acuity, or perhaps to provide interest, sparkle or modelling in an interior. This paper examines TAIR potential as a suitable metric to design and evaluate target illumination. It does so by using a case study with two cohorts of 32 participants assessing paintings in a small room, focusing on visual emphasis. The results suggest that TAIR could be measured and assessed reliably for design in general but proved somewhat unreliable where reflectances varied. An improvement on TAIR is proposed. A visual emphasis scale that varies from the original scale proposed by Cuttle is established from the data.