Determining street light wavelengths?

[chrisp]

Is it possible to take a photo of a streetlight using a DSLR and then using photoshop or something to determine the wavelengths it transmits?

You would then know which light pollution filter to buy...

Sorry if this is a daft question

[lukebl]

...Sorry if this is a daft question

Sorry, can't answer the question but it's certainly not a daft one. I'd like to know the answer myself.

[Bizibilder]

Would this help?

Street Lamp Spectra

(I just googled "streetlight spectrum")

[dlp]

You could be scientific and build a spectrograph using a diffraction grating - Chapter 7: Light and Optics -- Build a lab quality spectrograph in 15 minutes for less than $20.

[chrisp]

Thanks for the replies. The spectrograph idea is tempting. Yes I do want to be scientific and find out what spectrum my streetlights are emitting.

I wonder if UK streetlights emit different wavelengths to US ones. Therefore should I buy filters that work for the UK...

[Photosbykev]

Very little, or nothing, is directly made for the UK market, it is too small to even be considered in a global market so we get what the Americans have lol

[Ouroboros]

I found it quite informative to look at the diffracted light from our local street lights using an old CD as a makeshift diffraction grating. It showed, as I expected, that these high pressure sodium lamps emit lines in the blue, green and red regions of the visible spectrum. I bought a Astromik CLS light pollution EOS clip filter for my Canon 450D. It certainly cuts out the red and blue ends but inevitably lets the green through since it has high transmission between 450 - 540nm. I have so far only used the filter a little. I found the green cast it gives to images quite difficult to get rid of using PS, but that could be down to my inexperience. I need to do some more work with it really. It certainly wasn't the magic bullet I rather naively hoped it would be.

[chrisp]

Great idea to use a CD. I will try this tonight!

I've since bought a Skywatcher Light Pollution Filter but with the constant cloud I haven't been able to try it out yet.

I looked at the Astronomic clip filters but dont think they produce one for my model of camera.

[Psychobilly]

You might find the information here useful...

Street Lamp Spectra

http://www.asdi.com/documents/spectral-signatures-of-nighttime-lights.pdf

Billy...

[PeterW]

A CLS filter will get rid of most stuff that is not nebula emission, physics being the same in the UK and US the colours emitted will be the same for the same types of light. There are a few different flavours of streetlight, the low and high pressure sodium being the common ones, though the mercury/metal halide are about. Also near my parents there are some LED ones... not seen them at night, but they will put out a continuous spectra (unlike the other mentioned above), IE filters will have much less effect on them.

Good paper by Elvidge (a big name in measuring nightime earth light emissions), pity the plots go out to 2500nm...make it hard for us to see exactly what they do in the visible.

The best defence is to do pure narrowband imaging, but then you limit yourself to emission nebulae...galaxies and the like will just disappear as their light output is spread across the colours. All we can hope for is better use of night light. Less light put in the places it is needed, with minimal spill, hence minimum amount that can go into light pollution. OUR light of choice being the low pressure sodium (easy to filter out), but it looks like the industry is moving to bluer LEDs as the colour rendering is better and they give greater efficiency as the human eye response shifts bluewards in the dark.

Cheers

PEterW

[Merlin66]

An easy, and obvious, first check is to look at the offending light through the filter....if the light appears to blank out then your filter is good for that type of lamp/ light.

A small grating spectroscope will quickly show the emission spectra of the lamps and you'll immediately see what is being blocked and what is not.