Downtown Montreal Hot-Spot

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Analysis of Sky-Glow:

The spectrum near that downtown hot-spot (top picture) is shown below. This is easily obtained with today's DSLR or mirrorless cameras, long as you're aware of the capabilities of your equipment and know how to modify and tweak all the parts to make them work.

Up-Light Spectrum of Downtown Montreal

Currently, all cities and towns, even those with LED systems, have the same elements & phosphors appearing in their sky-glow spectrum but in varying strengths. Precise data will be found in "Your Light-Pollution - The New Sky-Glow Spectra" PDF.

Without getting into the details of the spectrographic device I constructed and DSLR body I modified to obtain spectrographs, the results need to be analyzed and referenced, otherwise they are meaningless. Any digital camera can capture spectra with some preparation. Using exactly the same setup, obtaining different lamp spectra as comparisons is relatively straightforward. The lamps are bright and require fractions of a second to get each spectrum as compared to the few minutes required for overcast spectra or 20 minutes and more to get good clear-night spectrum.

Coated Lamps

I can get plenty of detail with my spectrographic setup, such as.. the runnup of a once very common lamp, HPS. Shown below is the sequence from the xenon-mercury starters on through to the pressure-broadening of sodium.

HPS-start to stable

Spectral values for the mercury and sodium lines are well known, in Angstroms or in nanometers. These can be found in many published papers or in documents online. Spectral features of other elements are harder to find and need some searching to be unearthed.


First, a comparison of my clear-night spectrum (top part) obtained in 2008, with a once common DX phosphor-coated mercury-vapor lamp (bottom part):

DX Phosphored Mercury-Vapor Lamp

I get the mercury (Hg) lines alright (white arrows), but the bands due to the DX phosphor do not match anything in my sky-glow. Is it possible for the mercury spectral lines in sky-glow to not originate from mercury lamps? Yes of course.

Comparing my 2008 clear night spectrum (top) with a high-pressure-sodium (HPS) lamp (bottom):

HPS comparison

Great, I get many sodium lines, however, there are blue and blue-green lines unaccounted for.

Comparing my clear night spectrum (top) with that for a standard North American metal-halide lamp (bottom part):

Metal-Halide Comparison

The comparison not only gives me the significant contributions from one element, but the mercury lines as well. Most of the lines can now be attributed; the similarity in the violet and blue parts of the spectrum is striking. Moreover, the peak inside the sodium self-reversal band (near 590 nm) can now be explained. The white arrows indicate the spectral lines of just one element.

Going further, using an overcast "up-light" spectrum to bring out red features, a more recent up-light spectrum is compared to the spectrum of an induction-lamp. This type of outdoor fluorescent lamp has the same modern phosphors as any compact-fluorescent-lamp.

Tri-Phosphor Lamp Comparison

The 3 phosphor bands of the "tri-phosphor" lamp are marked with heavy arrows. I can now attribute that extra red line (to the left of the 616 nm sodium line) to induction lighting. No, that line can't be due to porch lights. The very strong Hg lines in contemporary urban sky-glow with this extra red line infers widespread fluorescent lighting. The spectral signature of such a lamp:

Tri-Phosphor Fluorescent Lamp

The up-light spectrum of any region can now be confidently known. Even minor lines and features can be attributed. It turns out that both up-light and clear-night skyglow can be variable in time or by location.

You'll find precise details and data in the PDF for the New Sky-Glow Spectra.

The history of outdoor lighting and the evolution of sky-glow can be found in these pages.

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