Given our large and modern urban populations, outdoor lighting is necessary. However, excessive illumination above and beyond adequate lighting is wasteful, frivolous, sometimes dangerous, and cities cannot ignore the significant consumption costs of dusk-to-dawn lighting. When private outdoor lighting is installed, there is exploitation involved by lighting contractors & installers.
Despite the many advances in the field of excellent, non-polluting lighting products and practices, the brightening of the night sky continues to expand over many regions. The color of the night sky is also shifting for the second time in 30 years; a third time in 60 years. The spectrum of sky-glow is constantly evolving. What does it all mean to a nebula hunter or astro-imager?
We know what sky-glow looks like over our gleaming cities, but what does the spectrum of light pollution look like? The kaleidoscopic spectrum for the top logo was easily obtained from an overcast sky over Montreal, Quebec, in 2017.
The January 1996 image was taken on a misty night while the 2019 image was shot on a drier evening. Can you spot any changes over the 23 years? The big changes are at street level.
Most amateur astronomers are aware of the basic causes of urban sky-glow. However, limited and very dated information on the spectrum of sky-glow has been available. Countless of amateurs have an incomplete knowledge of light-pollution compelling the need for an analysis such as this. That analysis is finally here.
When anyone asserts that some type of contamination exists, that claim is often backed up by a chemical analysis, which may also expose the cause. For any discussion of a "pollution of light", a spectral analysis is obligated. As in any field of spectroscopy, a spectral analysis of sky-glow will answer the question: What's it made from? An analysis of sky-glow is routinely done for professional observatories typically isolated from huge amounts of urban sky-glow. Having little knowledge of true urban sky-glow spectra, city stargazers are forced to accept these unique & eroneous spectrograms.
The difference between a clear night sky-glow spectrum and an overcast "up-light" spectrum over my back yard in (the year 2008) in Ville-Saint-Laurent, Quebec, a suburb of Montreal.
Shot in 2008 using a modified digital camera, the two spectra above were obtained less than 24 hours apart; the top part on a clear night, the bottom part on the following overcast evening. The top spectrum approaches the classic Rayleigh scattering law (defined as Iscattering ∝ 1/λ4). The bottom part is simple diffuse reflection; the scattered intensities are essentially independent of any wavelength (Iscattering ∝ λ0). Such spectra are a depiction of the local up-light.
From the data written on the images on the right, you might calculate that the top exposure was 10x longer than the bottom overcast exposure. Clear-night spectra are difficult to obtain but not impossible. On the other hand, urban & suburban overcast spectra are very easy to obtain.
All clear nights are NOT equal. Notice the color of the sky between the warm night and the cooler night using exactly the same digital camera settings.
Taken over my back yard in 2018, spectra acquired for the above two clear nights show how the air temperature can shift the spectral features. Remember that we're talking about clear nights here.
