The position index of a glare source at the borderline between comfort and discomfort (BCD) in the whole visual field

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Abstract

The objective of this study was to propose a new position index for a source located in the whole visual field, including the lower visual field. The sensation of brightness was tested at various positions in the visual field. The Glare Tester especially designed for and used in this experiment had two sources: the standard source located on the line of sight and the test source located at a designated position in the visual field. The boundary of the visual field, BCD luminance on the line of sight, and BCD luminance at various positions in the visual field were measured. The visual field was measured, and the boundaries were approximately 90° temporally, 67° inferiorly, and 50° superiorly. From the measurement of BCD luminance on the line of sight, the BCD luminance was obtained at 2590 cd/m2 under the conditions of a source size of 0.0011 sr and background luminance of 34.3 cd/m2. In measuring BCD luminance at various positions, the standard source was set to the BCD luminance of each observer, with the luminance of the test source being increased. The observers were asked to stop when the sensation of the test source became equal to that of the standard source. From the results of these measurements, the position index Pi was derived. Finally, a chart for determining the position index Pi was proposed based on the three measurements.

Introduction

An interior working environment should enable occupants to perform visual tasks comfortably and safely; therefore, it is important to limit glare in order to avoid fatigue, errors, and accidents [1]. Several methods have been developed to control discomfort glare. The Unified Glare Rating (UGR) [2], the Visual Comfort Probability [3], and the Daylight Glare Index [4] are well-known methods. Specifically, the UGR has been recommended by the International Commission on Illumination (CIE) to evaluate discomfort glare in interior lighting.

The existing methods for evaluating discomfort glare are based on the four main factors influencing the degree of discomfort glare: the luminance of the glare source, the solid angle of the glare source, the background luminance, and the position of the glare source in the field of view [5].

The UGR formula has a limited application in terms of the position of the glare source in the field of view. Although the Guth Position Index is included as a main factor, the UGR formula does not define glare sources in the lower visual field. The Guth Position Index [6] was determined through an experiment to calculate the variation of the brightness sensation when a glare source is displaced from the line of sight. Based on this experiment, the Guth Position Index can be used for sources in the visual field; however, the Guth Position Index is defined only at the upper visual field. Therefore, the UGR formula cannot be used to evaluate discomfort glare from lights located below the line of sight.

In current lighting environments, lights are located not only in the upper visual field, but also in the lower visual field. For example, up light is often placed in the lower visual field. Furthermore, windows sometimes occupy part of the lower visual field.

To evaluate discomfort glare from sources located in the lower visual field, the position index for a source located below the line of sight was needed. Therefore, the Guth Position Index was extended to the lower visual field.

Because the glare sensation is different between the lower and upper visual fields [7], two positions that are symmetrical with respect to the horizontal line of sight cannot be given the same value in the Guth Position Index.

Rather, the position index for a source located in the lower visual field has to be calculated in order to enable the evaluation of discomfort glare from lighting environments. The objective of this study is to propose a position index for a source located in the whole visual field, including the lower visual field.

Section snippets

Guth Position Index

Luckiesh and Holladay [8] investigated the effects of a dazzle-glare source located above the line of sight. The results of their investigation show that the sensation of glare changes according to the position of the source in the visual field. For example, a 25-watt tungsten lamp 5° above the line of sight is as dazzling as a 900-watt lamp 30° above the line of sight.

The position index of a source expresses the change in discomfort glare experienced relative to the angular displacement

Apparatus

An apparatus called a Glare Tester was especially designed for this experiment (Fig. 1). It was devised so that the glare sources could be placed at various positions in the whole visual field.

The Glare Tester consists of a semi-spherical screen and two halogen lamps mounted on a round boom. The screen has a 1-m radius. The inside surface was painted white to produce diffuse light. The halogen lamps were viewing as circular sources in size 0.0011 sr. The possible range of source luminance is

Boundary of the visual field

The static positions of the test source were plotted, creating a boundary of the visual field. The average boundary of the visual field, based on the observers used in this study, is illustrated in Fig. 5. The boundary of the visual field is 90° temporally, 67° inferiorly (restricted by cheek), and 50° superiorly (restricted by the brow). This is slightly smaller than what Guth has proposed as the entire visual field: vertically upwards from the line of sight at 60°, vertically downwards at

Position index in the whole visual field

The BCD luminances at various positions in the visual field are illustrated in the Fig. 6. The position index Pi is represented in the Y-axis, where the position index Pi is the ratio of the BCD luminance to the luminance 2590 cd/m2 (the BCD luminance on the line of sight).

The regression lines in the figures below show that the variation of the position index Pi correlates to the angular distance. In Fig. 6a, the position index Pi increases more rapidly as the angular distance increases on the

Conclusion

The purpose of this study was to propose a position index for sources located in the whole visual field. The sensation of brightness was tested at various positions in the visual field. The Glare Tester used during the experiment had two sources: the standard source located on the line of sight and the test source located at a designated position in the visual field.

Three kinds of measurements were conducted: the boundary of visual field, BCD luminance on the line of sight, and BCD luminances

Acknowledgments

This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No. R11-2008-098-00000-0). The authors thank also Professor Yasuko Koga, Kyushu University, Japan for her advice during this study.

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