Ann Arbor MI is converting to LED streetlights to save of money over the
long run. LED street lights I have seen were not any more brighter than a
porch light. Up to 1200 lumens. This has me concerned about safety. I'm all
for saving the environment, but does this make sense?
1. The light may be better directed, so for same illumination of areas
where the light needs to go less light output in lumens is required.
2. At streetlighting illumintion levels, in my experience human vision
is largely "mesopic". Although one sees things in focus and with color,
scotopic vision makes a significant contribution to "sensation of
illumination".
For example, look at two similar nighttime scenes illuminated equally
(for example, at 5 or 10 lux) but one with HPS and the other with ~4100K
metal halide that has scandium for greenish-bluish spectral contribution
(usual USA formulation, probably usual North America formulation). See
how the MH scene apears more-brightly-illuminated than the HPS scene does.
Most white LEDs have scotopic/photopic ratio similar to or greater than
that of the above metal halides.
3. HPS has very low color rendering index around 21-25, and with the
main color distortions being darkening/duling of reds and greens. The low
color temperature darkens/dulls blues. A higher color temperature, higher
color rendering index light source will achieve brighter overall
illumination where these colors have high presence even at same amount of
illumination by both photopic and scotopic measures.
4. I hear plenty that a lot of streetlighting is brighter than it needs
to be.
Now, if one were to retrofit some hypothetical HPS streetlighting
installation with metal halides achieving an illumination level towards
the lower end of an acceptable range, this might involve maybe 39 watt MH
lamps. Those have less life expectancy than higher wattage MH, and MH
tends to have less life expectancy than same wattage HPS. And 39 watt MH
lamps cost more than 175 watt ones, which cost more than 150 watt HPS
ones. Add to this labor costs of relamping, and at lower wattage per
fixture LEDs may sound only a little outrageous to maybe close to par in
overall cost over the life expectancy of the LEDs, even if the LEDs have
luminous efficacy only about the same as that of 39 watt metal halide or
even a bit less.
Furthermore, the politicians could gain votes for making the
streetlights more "modern" or "advanced" or "futuristic" even if the cost
is increased (preferably not by much) over the life expectancy of the
LEDs.
- Don Klipstein (
[email protected])
