Further work is necessary, particularly in the areas of defining nighttime visual tasks, to determine whether more or larger multipliers can and should be developed to represent additional factors related to visual performance of off-axis tasks. It is beyond the scope of this paper to recommend actual LEM values which should be used for different sources under different conditions. However, this paper provides an overview of methods and a suggested framework for evaluation by IESNA committees, and example tables of values. Overseas, where this work has been reviewed, rather than a table of LEM's, a value of LEM = 2.0 has been suggested for use when white HID sources are used for roadway lighting design. While this seems to be an oversimplification, the work of Lewis, Bullough et al suggests that different, and perhaps considerably greater, values may be justified in the future for certain types of tasks. Perhaps we are opening the door to increased nighttime safety and security by developing a methodology to allow the designer to account for light source spectral effects. An associated direct benefit may be a savings in lighting energy. The task of reviewing this work appears potentially worthwhile.
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