Evan Applegate

Illuminated maps, low-pressure sodium style

To me the low-pressure sodium bulb, a monochromatic 589nm emitter, means an inviting L.A. gloom…a sidewalk paver held at 40deg by a fig root…january mist tumbling through a magnolia tree…when you’ve stood on that still-hot asphalt on a cool night and felt the breeze under those humming lamps, you’ll get it.

I bought a retired Escondido, CA low-pressure sodium streetlamp off ebay, replaced the bulb, removed the solar sensor, and four years after I started: I have the correct glow for my illuminated maps.

Escondido used these lamps out of consideration for the 200″ Hale Reflecting Telescope on nearby Mt. Palomar. Narrowband emitters are much easier to filter out of your astronomical observations than a broad-spectrum source (e.g. the LED lamp that replaced this fixture).

Here’s how I wired it, ymmv (I don’t advise using my particular power source, a cheap extension cord with the receptacle end ripped off).
When you first plug it in the lamp glows shocking pink; the sodium on the electrodes hasn’t yet reached vaporization temp, so all flux is coming from the ionized neon in the tube.
Now we’re cookin: 90 seconds later the sodium has vaporized, we’re glowin at 589nm. A sickly yellow or a great yellow, depends on who you ask.
Monochromaticity (sic.) demo: narrow-spectrum low-pressure sodium vs. a wide-spectrum 1800k LED meant to replace it. Notice how the green of the cutting mat is only visible under the LED.
Using the lamp to backlight a chunk of my 3×5′ world map.

I first experimented with backlit maps in 2019 when I lived in Silver Lake; the color range of the LPS lamp, hot pink and yellow, cover most of what you’ll see of SoCal emissions.

A sidewalk buckled by fig roots, pink and yellow pools…I tried to get that low-pressure sodium flux with gels, inkjet-polyester film, tunable RGB LEDs. But there’s nothing like the real thing. I think once the last LPS bulb breaks, that’s the end, no one’s manufacturing any more. But I’ll enjoy my time with my favorite glow while it lasts :^)

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