The spectrum near that downtown hot-spot (top picture) is shown below. This is easily obtained with today's DSLR cameras - as 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.
Without getting into the details of the spectrographic device I constructed and DSLR body modifications I made to obtain my up-light 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.
I can get plenty of detail with my spectrographic setup..the runnup of a once very common lamp, that of HPS, from the xenon-mercury starters on through to the pressure-broadening of the sodium is shown below.
The values for the mercury and sodium spectral lines are well known, in Angstroms or in nanometers. These can be found in many published papers or online documents.
Spectral Analysis Of My Sky-Glow:
First, a comparison of my clear-night spectrum (top portion) obtained in 2008, with a DX phosphor-coated mercury-vapor lamp (bottom portion):
I get the mercury (Hg) lines alright, but the bands due to the DX phosphor do not match anything in my sky-glow.
Comparison of my 2008 clear night spectrum (top) with a high-pressure-sodium lamp (bottom):
Ok good, I get many sodium lines, however, there are blue and blue-green lines unaccounted for.
Comparison of my 2008 clear night spectrum (top) with a standard North American metal-halide lamp (bottom part):
This 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 interesting. Moreover, the peak inside the sodium self-reversal band can now be explained. The white arrows indicate the spectral lines of just one element. What's this element?
Going further, using an overcast 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 - the so-called tri-phosphors.
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:
The up-light spectrum of any region can now be completely known. Even minor lines and features can be attributed. It turns out that both up-light and clear-night skyglow are quite variable in time or by location.