Alliance for Lighting InformationAlliance for Lighting InformationThe 'Glare Ratio' described in this report is a new and made-up metric with no scientific or mathematical basis. This metric has never been presented to or accepted by any technical society involved with lighting. Neither the Illuminating Engineering Society of North America (IESNA) nor the Commission Internationale de L'Eclairage (CIE) has accepted this "Glare Ratio" as a valid metric. When introducing a new metric it is common practice to fully describe the method by which the data is collected. There is no description of the data collection process for this metric except that an illuminance measurement is taken pointing up at the bottom of a luminaire and then taking a measurement with the meter pointed down. There is no mention as to the height of the measurements or exactly what is being measured when the illuminance meter is pointed downward. Is the measurement take of the umbra or pre-umbra of the shadow cast by the illuminance meter or does the measurement include the direct light from the luminaire? There is reference to a 'special snoot type attachment' to the illuminance meter used for the data collection. There is no information describing the physical nature or dimensions of this 'snoot'. Is the 'snoot' a toilet paper roll or a paper towel roll, is the interior of the snoot white, black or some color? Without this information the collected data is not reproducible and therefore not reliable science.
The glare metric described here has nothing to do with glare. The measurement and ratio described is the inverse of the definition of surface reflectance. Glare is a luminance (brightness) based phenomenon and is not based on what was measured - illuminance. Direct glare from a luminaire is directionally dependent - looking at the top of a flat glass luminaire does not produce the same glare phenomenon as looking at the bottom. The brightness of a fixture can change significantly with the direction of view, so measuring the illuminance directly below the luminaire says nothing about it's glare potential when viewed from a different direction. Additionally, glare is related among other things to contrast - the brightness of the target versus the brightness of the background. As an example, car headlights in the daytime produce almost no glare compared to the same headlights at night on an unlighted street. The only thing this "Glare Ratio" metric reveals is that as the 'glare reading' increases, the reflectance of the surface beneath the luminaire decreases or that the overlap from adjacent luminaires is low. The higher 'glare readings are probably associated with very dark asphalt paving materials in systems with little overlap while the lower readings are associated with higher reflectance concrete surfaces and better beam overlap. There is no way to verify this as the data sets did not include any information about the paving materials, which significantly alter the perceived brightness of the surroundings depending on the amount of light reflecting onto other surrounding surfaces.
Since the data sets are presented in such a scattered fashion, only data for sites with luminaires producing glare that were 1000 watts or more was reconstructed for this review. This represents 9 sites of the total. This limited review of the data set reveals the problems associated with the metric.
There is a problem either in the way the measurements are taken or in the data entry of the values. Site number 664 has a 1000 watt metal halide flat lens shoebox luminaire mounted at 24' and the illuminance reading at nadir is reported as 0.135 footcandles. This is a physical impossibility given the luminaire type and wattage unless the lens was so dirty that it occluded 90% or more of the flux at nadir. Sites 1067, 177, 513, and 1452 have similar problems.
The subjective evaluations of the glare produced by the luminaire were not very consistent with the numeric glare ratios. Site 1769 is an interesting example with a 1000 watt high pressure sodium sport light flood mounted at 24' to illuminate a parking area and sidewalk. The subjective analysis of glare from this fixture was rated a 4, but the glare ratio calculated for this luminaire is 1.18. This is the fifth lowest glare ratio calculated by this metric for all 241 'glary' luminaires. This metric would indicate that this 1000 watt sportslighters is less of a glare source than almost all of the cutoff luminaires that were measured regardless of wattage.
If we further follow the logic of this metric it would appear that the illuminance reading with the meter pointed down is measuring the reflected light within the umbra of the shadow cast by the illuminance meter (and measurer). This would theoretically measure only the light reflected by adjacent luminaires at the point directly under the 'offending' luminaire. If this is the case, then any single luminaire, regardless of wattage or cutoff designation, by itself and not near another light sources will have an infinitely high glare ratio as the measurement with the meter pointed down should be zero.
The presentation of the glare ratio data is also odd. For instance, Table 56 reports the percentage of sites of a particular building type that have glare ratios within 4 point data ranges. The table also reports the number of each type of building included in the population. The discrepancy is that the percentages listed do not reflect the possible percentages given the sample sizes. For instance, the grocery, hotel, and large school categories each have two sites represented yet the percentage of sites within the glare ratio categories are not 50% as would be expected, but instead range from 37.8% to 62.2%. There is further inconsistency in that Table 67 reports that 2 of the 3 grocery stores visited were 'not glary' yet Table 56 indicates that 2 sites were glary enough to be included in the glare assessment.
In addition the data presented in Table 70 is very misleading. The table employs a two-valued logic system that classifies fixtures as either cut-off or non-cutoff. The IESNA defines the photometric requirements that classify luminaires into Full Cut-off, Cut-off, Semi Cut-off, and Non Cut-off categories. Full Cut-off, Cut-off and Semi Cut-off are the classes of luminaires typically used for area lighting and the Non Cut-off class of luminaires is a catchall category that includes floodlights, spotlights, wall packs, luminous decorative luminaires, and any other luminaire that does not meet the requirements of the Semi Cut-off class. Fixture types A and B in this report represent the Full Cut-Off and Cut-off luminaires most typically used for area lighting while types C, D, and G represent the Semi Cut-off luminaires typically used for area lighting.
The data presented in the report is used to support the notion that using Cut-off luminaires will decrease glare in exterior lighting, however the raw data does not support this conclusion. The raw data shows that approximately 50% (59 out of 119) of the sites utilizing Type A luminaires (Cut-off Shoeboxes) have a glare problem, whereas only 12.5% of the sites with Type C luminaires (Vertical Lamp Sag Lens) have a glare problem, 33% of the sites with Type D luminaires (Vertical Lamp Cubes) have a glare problem and 20% of the sites with Type G luminaires (Drop Lens Cobra Heads) have a glare problem. These numbers would indicate that Semi Cut-off luminaires are less of a glare source than are Cut-off luminaires. The method of data presentation in this report is skewed in that it misrepresents the qualities of Semi Cut-off luminaires by including them in the same group as sports field lighting equipment.
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