Quantifying Visual Discomfort in the Daylit Workplace using a High Dynamic Range (HDR) Imaging Approach

Reducing the electrical energy consumption of non-domestic buildings is a major part of the carbon reduction strategy of all developed nations. In offices, approximately 30 % of electrical energy is used for lighting. Increased use of daylight in buildings is a recognised means to reduce lighting energy requirements. Additionally, a good provision of daylight is considered to be highly desirable for building occupants' well-being and productivity. Daylight however can cause visual discomfort by inducing glare and veiling reflections. With increased daylight provision, the hoped-for reduction in electrical lighting use is often not achieved because the devices that occupants use to control visual comfort (e.g. blinds) invariably reduce the overall daylight provision. It is recognised that the visual conditions (e.g. daylight glare) that provoke the use of shading devices are poorly understood. Thus, at the design stage, it is impossible to reliably predict the degree of visual discomfort that may occur in a well daylit building, and to be certain of what energy savings can be achieved.Numerous laboratory studies have been carried out in order to investigate visual discomfort from daylight through windows. It is acknowledged however that the findings from these studies have had limited applicability for predicting glare conditions experienced in real buildings, due to restricting factors such as:- Subjects were re-located from their usual place of work to an unfamiliar environment.- Only limited measurements were made of the visual field or luminous environment.- The full range of potential glare scenarios was not studied.- The monitoring period was limited, i.e. not all of the naturally occurring daylight conditions that may cause glare were captured.Considering that the natural variability in daylight contains seasonal, daily and short-term components and that discomfort glare is known to depend strongly on directional factors and the variability of the luminous field, long-term monitoring is needed to capture the full range in experienced conditions. Moreover, occupants' value judgements need to be recorded together with physical measurements in order to assess the effect of daylight glare sources on visual comfort.This project will be the first to carry out comprehensive long-term monitoring (12 months) of the visual field linked with simultaneous recording of the occupants' response to the perceived environment. The subjects will be monitored in their usual workplaces with a minimum of interference. A technique called high dynamic range (HDR) image synthesis will be used to quantify the field-of-view luminance at occupants' workstations in an open-plan well daylit building. HDR image synthesis is a process whereby a sequence of digital images are combined to produce one image with contains an accurate measure of the scene luminance (i.e. brightness) at every pixel. Each HDR image provides a complete record of the magnitude and spatial variation of the luminance in the field-of-view, which represents a significant improvement over the traditionally used spot-measurement methods. Simultaneous with the monitoring of physical conditions, there will be an on-screen survey of the occupants' perception of the visual comfort. At regular intervals, or whenever visual discomfort is perceived, each subject will have the opportunity to complete a short on-screen form to indicate the degree of discomfort experienced and to identify the region(s) in the field-of-view that are the source of the discomfort, using an image of their field-of-view supplied on the form. Thus the magnitude of the glare source and its position can be identified for the HDR image taken at that instant. The acquired data set will thus facilitate the development of a behavioural model, founded on measurements taken in the work-place, which is designed to be used with simulation tools for the design stage evaluation of low-energy, well-daylit buildings