Light Pollution Filters

The following simple spectra were taken of different light sources to show the photographic effects of various light pollution filters on street lamps. It is unusual for an astronomer to deliberately photograph street lamps, but it provided the easiest way to illustrate the main effects of using different filters. These spectra were taken with a Kodak DC-120 camera. The slightly odd colours in the solar spectrum reflect the difficulty a standard tricolour camera has in representing the colours of monochromatic light especially in the green-blue transition region.

I admit it is pretty odd to deliberately photograph street lamps, but it helps to give some impression of what effects each of the various filters will have on different forms of light pollution.

I have photographed each light source unfiltered and then with different filters interposed using a simple diffraction grating to display the zero and first order spectra. I choose the Meade Broadband Nebula Filter 911B and Lumicon OIII filter as the representative narrow band filter because it shows such a stunning rejection of all extraneous green light except for a narrow green/turquoise centred on the OIII nebula line.

I added the Orion(UK) visual Sodium light filter that specifically blocks the main HPS emissions and the Orion(US) UltraBlock narrowband filter to the spectrum photographs in October 2003. They are taken with the same camera and grating, but due to having moved the streetlamps are different. I have tried to find very similar ones to those used in the original series. I am still hunting for a suitable LPS lamp to use.

Unfiltered Nonad Meade Nebula Lumicon OIII Orion(UK) Sodium Orion(US) UltraBlock

Sunlight

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The sun is a reliable continuum source of white light which is useful as a reference standard.

Clearly Nonad is a broadband notch reject filter affecting mainly Sodium D-line yellow light with a weaker dark line in the mid greens. The broadband Meade Nebula filter eliminates orange/yellow and most green up to the mercury line, and has a stronger attenuation of continuum starlight. The Orion(UK) visual Sodium filter passes shorter wavelengths and removes red through to mid green. This contrasts with the excellent Lumicon OIII filter which lets only a narrow pass band of nebula emission together with small leaks in the red and far purple ends of the spectrum. And the Orion(US) UltraBlock which lets though a narrow band on the cyan blue boundary.

The solar spectra were obtained by photographing the reflection of the sun from a chrome metal bar placed on a black velvet cloth. I would welcome contributions showing other filters characteristic spectra. Note that the first order spectrum is in the top half of the image and the zero order image shows the undispersed image at the bottom. These results should be considered qualitative and representative only the specimens tested other filters may give slightly different results.

Unfiltered Nonad Lumicon OIII Nonad dense

Low pressure Sodium

The light from a low pressure sodium street lamp is predominantly monochromatic in the yellow orange D-lines.

Nonad filters cut the intensity of this line by about 300x whilst leaving other colours unchanged. It is good for photography in areas with mainly LPS lighting. The broadband Meade filter is also useful for photography but it cuts out more continuum light and exposures are increased accordingly. The Lumicon OIII filter provides better attenuation of the D-lines and is excellent for visual use, but also cuts out most of the spectrum and so is not suitable for photographic use. I also have a prototype Nonad dense filter which gives even more attenuation on the D-line than any of the others whilst still maintaining reasonable continuum transmission. This cuts out all the yellow light even when standing underneath a street lamp! More of a novelty than a serious astronomy filter.

The biggest problem with UK street lighting is that very little effort is made to direct the bulk of it downwards, and as much as 30% goes straight up! Belgium manages to have full cutoff LPS luminaires, but I have never seen any in the UK.

Unfiltered Nonad Meade Nebula Lumicon OIII Orion(UK) Sodium Orion(US) UltraBlock

High pressure Sodium

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The light from a high pressure sodium street lamp is a peachy white with considerable pressure broadened continuum emission in the red to green band and a few sharp lines in the green and blue. Commonly used in city centres where colour vision is required at night.

High pressure sodium lamps already have an absorption line where the D-line would be and the Nonad filter is unable to remove the broad continuum.

The standard nebula filter takes out a larger chunk of emission from orange out to the start of the nebula emission lines. In both cases HPS lamps will add a pinkish cast to the sky photographed through the filters.

The Orion(UK) visual sodium filter is fairly effective against HPS and removes most of the wavelengths from red through to green.

Unfortunately HPS lamps emit some light right into the passband of the narrowest band Lumicon OIII and Orion(US) UltraBlock.

The only good thing about HPS lamps is that they are physically smaller than LPS and can be mounted in full cutoff luminaires.

Unfiltered Nonad Meade Nebula Lumicon OIII Orion(UK) Sodium Orion(US) UltraBlock

Mercury light

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The light from a mercury street lamp is a harsh blue white with strong sharp line emission in the green and violet.

Many mercury lamps have a small amount of sodium emission in their spectra, and Nonad is easily able to quench it, but leaves the other mercury lines unaffected. The Orion(UK) Sodium filter is equally ineffective against mercury light.

By contrast both the broadband Meade, Lumicon OIII and Orion(US) UltraBlock all greatly attenuate mercury emission lines apart from a slight leak in the deep red on the Lumicon (which is insignificant). The deep red light leak of the OIII does not seem to affect it's utility and may actually be helpful in seeing H-alpha emissions.

The interference filters are clearly the best option for handling pollution from mercury street lighting. This sort of lighting is most common in the USA.

Other narrowband interference filters have similar behaviour to the OIII but with their peak transmission shifted. The broadband interference filters have less transmission than Nonad, but will work effectively against a wider variety of light pollution (both mercury and sodium). Most popular filters are optimised for mercury lighting which is the predominant form of street lighting in America. Eliminating the powerful mercury green line at 546nm being the single most important effect that they have. Narrow bandpass filters also take out sky glow emission lines which also helps to improve the visual contrast of very faint nebulae. There is also the Orion(UK) visual Sodium filter that sits somewhere between the generic nebula filter and the narrow bandpass types. It gives much better visual performance than any other broadband filter I have tried in a mixed HPS and LPS environment.

The most important thing is to know which sort of street lighting is predominant in your area and choose your filters accordingly. You can easily examine the spectrum of a streetlamp by looking at it's grating reflection in a free CD. Another simple rule of thumb is to watch them switch on. Decorative filament lamps and quartz halogen lamps give continuum emission from a hot filament and warm up almost instantly. They are rarely used as street lamps. Low pressure sodium lamps initially strike as a dull red and then over a warmup period of a few minutes the sodium takes over and bright yellow light appears. High pressure sodium lamps strike and immediately go yellow and then become whiter as they heat up and pressure builds. Mercury lamps strike and gradually brighten as they warm up - they have a distinctive cold blue light.

Nonad is only suitable if the majority of your light pollution is from orange-yellow low pressure sodium lamps which are more common in urban areas of the UK and Europe.


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Last modified 6th October 2003