The documentation for RADIANCE Visual Comfort Calculation (http://radsite.lbl.gov/radiance/refer/Notes/glare.html) states:
The Guth position index is used by this formula as well, and its calculation is described in [Levin75]. This is true for IESNA Visual Comfort Probability (VCP) calculations, but it is *not* true for CIE UGR calculations. Quoting from CIE 117-1995, "Discomfort Glare in Interior Lighting," Section 4.5: "The position index is found by interpolating the data of Table 4.1 [Table of position indices]." The equation of Levin, R. E. 1975, "Position Index in VCP Calculations," Journal of the IES, pp. 99-105, is based on: Committee on Recommendations of Quality and Quantity of Illumination of the IES, "Outline of a Standard Procedure for Computing Visual Comfort Ratings for Interior Lighting, Report No. 2 (1966), Illuminating Engineering, Vol. 61, No. 10, October 1966, p. 643. which shows a plot of the Guth position index. As noted by Levin, "The maximum difference between the basic Guth plot and Equation (6) is about 12 percent." An error plot shows that errors in excess of 5 percent occur over most of the range of the Guth position plot. This issue aside, the RADIANCE documentation states: If a single view direction is selected, findglare samples uniformly on the projected hemisphere. Unfortunately, CIE 117-1995, Section 4.5, states: "It is recommended that luminaires with T/R values outside the range of the table (0 to 3) be ignored. It is further noted that some positions in the table at large H/R values are void. These correspond to positions hidden from observers by eyebrows and foreheads, and accordingly to luminaires which do not add to the UGR." The entries of Table 4.1 define an elliptical field of view measuring approximately 62 degrees vertical by 73 degrees horizontal. The problem is that by ignoring the shielding effect of eyebrows and forehead in the Guth position index (which regrettably assumes a Caucasian physiognomy), the UGR values for narrow-beam luminaires with small luminous areas seen at high viewing angles can be grossly overestimated. In one study, the difference was 23 versus 14. Such errors typically occur within a range of less than 30 cm [12 inches] for the observer position, but it remains that findglare does not appear to be in full compliance with the requirements and recommendations of CIE 117-1995. To do so would require replacing the Levin analytic equation with an interpolation of the Guth position indices table published therein. (This analysis is based on the above-referenced Visual Comfort Calculation document, dated 1992. I have not examined the current source code for findglare to see whether it still applies.) Ian Ashdown, P. Eng., FIES President byHeart Consultants Limited http://www.helios32.com <http://www.helios32.com/> CONFIDENTIALITY NOTICE: This entire communication, including without limitation any attachments, is intended for the use of the recipient to which or whom it is addressed, and may contain confidential, personal, and/or privileged information. Please contact us immediately if you are not the intended recipient of this communication, and do not copy, distribute, or take action relying on it. Any communication received in error, or subsequent reply, should be deleted or destroyed.
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