Post #494: A little more focus on streetlights

Posted on December 13, 2019

Carlyle acorn

Which picture does not belong?  Source of images:  Dominion Energy website.

In my last post, I stumbled across the fact that the Town is going to put four of its “acorn” olde-tyme lamp posts along Wade Hampton, directly adjacent to the Sunrise assisted living facility.  Those aren’t shown in the architectural renderings (or at least, I didn’t see them).  You can only see them on the light plan (photometric plan).  And now that I look closely at the light plan, while the fixtures are shown, it sure doesn’t look like the estimated light levels on the ground include the output of those lights.  (At least, there no indication of any increase in light level around those light posts, which I’m fairly sure ought to occur.)

The upshot of all of that is that the existing photometric plan shows virtually no “light trespass” onto neighboring properties.  But … I’m not so sure that’s right.  And for sure, those Town-mandated lights are going to be annoyingly bright for whoever lives in the Sunrise rooms directly adjacent to those lights.

In any case, I had to learn to read light plans years ago, for a project I did at the church I then attended.  So I thought I’d look up the specs on those lamps, look at the light plan, and just generally take a closer look.  The light plan is the last page of this document (.pdf).

The plans currently call for four street lights running down Wade Hampton.  Each light is a single 26-watt LED fixture from Holophane, on a 14′ light pole.  So four fixtures total.  (But discussion at the last meeting suggested that the Town might require double acorns, or a total of eight such fixtures on Wade Hampton).

Looking on the Holophane website (e.g., here), that’s a very efficient LED, at about 125 lumens per watt.  That single LED bulb puts out about 3200 lumens.  For comparison, that’s about the same as two “100-watt” compact fluorescent light (CFL) bulbs, on each of four poles.And if the Town requires double-acorn fixtures, that would be the equivalent of four “100-watt” CFLs on each of four light poles.

I can’t tell whether that would over-illuminate the street or not.  And that’s because, at least by eye, the light plan does not appear to include the light from those Town fixtures.

A piece of the light plan is shown below.  It’s the Maple Avenue side of the building, with Maple at the bottom, and the parking garage entrance is at the left.  The big capital letters are light fixtures, and the little numbers are the light levels on the ground, in … foot-candles, I think (which is every bit as archiac as that sounds, equivalent to one lumen per square foot).

In this case, the Town’s single-fixture acorn lamp is A, circled in red.  Note that there’s no indication whatsoever that the acorn fixture raises the light level (green circle).  By contrast, a couple of slightly higher wattage downward facing fixtures at the garage entrance rise the light level to 10 foot-candles (blue circle).

Is the sequence of numbers in green plausible?  Just doing the raw math, a 3200 lumen source 14′ off the ground, throwing light equally in all directions, should produce about 5 foot-candles of illumination directly underneath the light (3200 / area of a sphere 7′ in diameter = ~ 5).  So no-apparent-impact doesn’t appear quite plausible to me.  It looks like the illumination levels don’t include those Town-mandated fixtures.

And so, as with so much of what I see with the Town of Vienna, I just end up confused.  As far as I can tell, from what I’ve read and heard:

  • The Town is going to require four of those olde-tyme lamp posts on Wade Hampton.
  • Those don’t appear in the pictures (architectural renderings).
  • Those are shown as single-fixture lights (one acorn per lamp post), but might be required to be double-fixture lights.
  • They are spec’d for 26 watt LEDs, and the manufacturer lists those as putting out 3200 lumens, about the same as two “100 watt” CFLs.
  • While those fixtures are shown on the photometric plan, it sure looks to me like the light from those fixtures is not shown.

My proximate point is that the light plan, as drawn, shows little in the way of “light trespass” on the neighboring properties.  But I don’t think that actually includes the light from the Town-mandated fixtures.  And for sure, it’s going to be an annoyance to at least some residents — those living in Sunrise — to have those bright lights there.

As a footnote, how brightly will these illuminate the side of the building?  These lights will sit about 10′ from the face of the building.  I can tell from the manufacturer’s specs that these fixtures are designed to cast most of their light downward.  About 70% of the light is below the horizontal plane, about 30% above it.  But just for a back-of-the-envelope, let me assume they broadcast light evenly in all directions.  Under that assumption, a window sitting directly across from such a light (assuming just one acorn fitting per pole) will be illuminated at 10 foot-candles.  Typical office lighting is 20 foot-candles.  So this will indeed be annoyingly bright for the bedrooms directly adjacent to the street lights.  Whether or not it will be annoying for the neighbors, I can’t say.

Just FYI, the modern LED streetlights pictured above are engineered not to scatter light willy-nilly like those olde-tyme acorn lamps.  Dark-sky-compliant lights broadcast no light above the horizontal plan.  And LED lamps as pictured can be set to broadcast little or no light beyond a line drawn a few feet behind the light pole.  Those acorn lights throw light in every direction, but modern lights put the light where you want it, and not where you don’t.