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This paper describes a project to reduce the excessive daylight exposure of an oil painting, Hambletonian, Rubbing Down, displayed at Mount Stewart, Northern Ireland. Climate-based daylight modelling (CBDM) was used to understand the light exposure of Hambletonian and to assess the impact of control measures on the annual light exposure and viewing condition of the painting in the winter months. The computer model was used in conjunction with measured lux data to establish the base case light exposure and the effect of the control measures. Light control was implemented through the use of darker paint finishes on the walls and ceiling, which reduced the amount of reflected light reaching Hambletonian;; and the addition of a mesh screen to the outside of the glazed dome above the painting. These interventions were cost-effective and straightforward to implement and manage. CBDM suggests the interventions reduced Hambletonian's annual daylight exposure from 3.5 mlxhr to 0.63 mlxhr.

Conservation-based restrictions on light levels and cumulative exposure in museum galleries are now well established. These have resulted in restrictions on the use of daylight and prolonged development of systems to deliver and control its admittance. Attempts to control daylight and deliver fixed illuminance levels have proved difficult to sustain. Recent installations, in the 1980s and 1990s, have become reliant on a proportion of electric lighting during most opening hours and an acceptance of limited daylight variability within dosage recommendations. Given the difficulties in estimating and measuring dosage, most museums have limited data on their actual lighting performance, and guidance is sparse on how such targets can be met. This paper reviews the limited goals widely referred to in guidance to designers and presents typical data drawn from monitoring in the Frick Collection, New York. The balance between electric light and daylight is discussed in the context of an interior's daylit sensibility, and this provides the basis for a proposed widening of criteria for lighting performance in museum galleries.

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A novel high-dynamic range camera-based approach to measure the cumulative daylight dose in historic/conservation spaces has been devised and tested in a real-world setting under normal viewing conditions. The technique derives illuminance from the high-dynamic range-acquired luminance at numerous patches of the wall surfaces visible in the high-dynamic range image. These patches serve as proxy illuminance meters, thereby allowing the prevailing illumination field across the entire visible wall to be derived using a Kriging algorithm. The technique was validated under both controlled and in-situ conditions over periods of several months and shown to have comparable accuracy to the illuminance logging devices commonly used in historic/conservation settings. The set-up comprising a consumer digital camera and tethered computer proved remarkably resilient and allowed for continuous monitoring periods of six months or more. Application in a real-world setting revealed operational practicalities that would not have been found in a controlled environment.