Letter submitted to Albany Times Union, 21-Jan-02
In your 1/21/02 issue you printed a letter from Michele McColgan of the Lighting Research Center of RPI. The letter claimed that more light, not less, might go into the sky if full-cutoff lighting fixtures were to replace the current non-cutoff design which allow light to directly enter the sky. The reason given was that you would need more full-cutoff to "achieve satisfactory lighting objectives."
McColgan's claim is pure baloney. Currently about 25% of typical non-cutoff lighting we see all over our environment directly enters the sky while about 7% of the remaining 75% striking the earth is reflected back up into sky. The bottom line is that about 30% of the light goes into space or for a 200 watt fixture, about 60 watts into the sky.
Conversely, a full-cutoff luminaire sends light only to the ground. It is not known if you would need more of them to replace the non-cutoff, but supposing that you did, and that you needed 50% more of them, and did not change the wattage of the bulbs (which with cutoff you generally reduce because no light strays directly up), then the number of units of light hitting the ground and reflecting back up into space would be 1.50 x 200 watts x .07 = 21 watts into the sky. That is, full cut-offs would send 65% LESS light into the sky than non-cutoff based on increasing the number of full-cutoffs by 50% and NOT reducing the wattage. If the full-cutoff wattage is reduced, as it should be to say 140 watts, then the reduction in light entering the sky would be 75% at about the same cost in energy.
But if we suppose that full-cutoff does not force more luminaires (this is still controversial), the decrease in skyglow could be as high as 85% AND at LOWER energy use. It is one of the great scare tactics of lighting suppliers and manufacturers and those 'research' bodies they fund to raise the specter of more luminaires, higher capital cost, etc when full-cutoff lighting rears its unpleasant head. The LRC seems to be kind of slow getting the message already out there and well proven by any number of reputable outfits; full-cutoff reduces glare and reduces energy consumption.
George D. Nickas, Chair of the Department of Physics and Astronomy
Hanover, Indiana 47250