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New 2020 Global Light Pollution Atlas

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#1 DaveL

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Posted 20 September 2021 - 01:57 AM

I've just calculated a new light pollution atlas for 2020 from the VIIRS nighttime lights satellite data. This latest version of the VIIRS satellite data has the additional advantage that previous years were reprocessed so that the satellite record should be more consistent. Therefore, I've also calculated trends over the years 2014-2020.

 

The new atlas is here:

https://djlorenz.git...tronomy/lp2020/

 

The trends are only available when viewing in openstreetmaps (go to the control panel on the left):

https://djlorenz.git...erlay/dark.html

 

The usual caveats apply, for example see:

https://www.cloudyni...atlas-for-2016/

 

Clear skies,

Dave


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#2 Loren Gibson

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Posted 20 September 2021 - 07:18 AM

Very nice. Is there a legend or key for the colors shown in the 2014-2020 trend visualization?

 

Loren



#3 DaveL

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Posted 20 September 2021 - 08:33 AM

Yes, in the control panel click the title (which is a hyperlink) instead of the check box. Maybe I should figure out a better way to display this?

#4 jdupton

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Posted 20 September 2021 - 08:43 AM

Dave,

 

   Thanks for the update! The Trends Map is especially useful. I have already added it to my local Google Earth views.

 

 

John



#5 psy_zju

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Posted 20 September 2021 - 09:56 AM

Thank you for the update.

#6 Loren Gibson

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Posted 20 September 2021 - 02:36 PM

Yes, in the control panel click the title (which is a hyperlink) instead of the check box. Maybe I should figure out a better way to display this?

Got it! Thank you. smile.gif

 

Loren



#7 Illinois

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Posted 21 September 2021 - 05:33 AM

I looked closer and it’s pretty accurate!  My backyard is not dark as 10 years ago! That’s sad! Dark sky getting smaller and smaller! Wow! Florida look pretty bad! South of Orlando/Kissimmee is no Blue zone.  More white and dark red in Florida! Good job for update! 


Edited by Illinois, 21 September 2021 - 05:37 AM.


#8 csrlice12

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Posted 21 September 2021 - 09:47 AM

Nice....looks like our dark site is now bordering light blue rather than solid dark blue....this fits with my experience over the years.



#9 BrooksObs

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Posted 21 September 2021 - 12:35 PM

Amusing, but not overly helpful in my opinion if you live anywhere near any significant population center, as the atlas essentially must be based on measurements of illumination looking down at the ground from space and not up at the sky from a given location (which is an entirely different situation).

 

As an example, in the area surrounding NYC out to at least 75 miles the map is not reflective of the actual sky brightness situation at all. For one, there are several supposed 'darker' zones, or fingers of darker regions in Dutchess, Putnam and Westchester Counties that simply don't exist in any form, as well as the overall sky illumination likely being consistently underestimated throughout the area by at least one full brightness class (perhaps more) relative from that proposed by the atlas.

 

Given the above, I would  pretty much assume the same to be true surrounding any truly significant center of population, even modest ones, not just in the NYC area. Unless consisting of actual measurement of the sky brightness from given locations atlases such as these can, in my opinion, be more misleading than truly helpful to the would-be observer looking for truly better skies than those at home. My advice is once again 'use with caution'.

 

BrooksObs


Edited by BrooksObs, 21 September 2021 - 12:36 PM.


#10 DaveL

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Posted 21 September 2021 - 08:16 PM

Hi BrookObs,

 

Yes, the atlas is based on upward propagating light detected by satellite. But this is only the first step in making the atlas. The upward light just the input data to a model of the propagation, extinction and scattering of light in the atmosphere. The quantity I calculate is the artificial brightness at zenith. Because a single localized light source pollutes a wide region with downward scattered light, my maps are much smoother than the raw upward light images/maps that you see on the internet.

 

Do you have SQM data to back up your assertion that fingers of relatively darker skies to not exist in the New York area, or that the atlas is not bright enough?

 

-Dave


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#11 BrooksObs

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Posted 22 September 2021 - 10:29 AM

Hi BrookObs,

 

Yes, the atlas is based on upward propagating light detected by satellite. But this is only the first step in making the atlas. The upward light just the input data to a model of the propagation, extinction and scattering of light in the atmosphere. The quantity I calculate is the artificial brightness at zenith. Because a single localized light source pollutes a wide region with downward scattered light, my maps are much smoother than the raw upward light images/maps that you see on the internet.

 

Do you have SQM data to back up your assertion that fingers of relatively darker skies to not exist in the New York area, or that the atlas is not bright enough?

 

-Dave

 

Dave - I am the creator of the Dark Sky Scale, the most widely used scale in the world of the past 20 years for evaluating light pollution's impact on observers skies. It considers the night sky's overall brightness, something far more useful to actual observers than is a SQM reading taken simply at the zenith. Likewise, the DSS, when used properly, indicates far more about a given observing site than possibly could be gained from that assumed by use of  some correcting formulae used to calculate the degree of light scattered in the atmosphere judging from measures of upward illumination levels measured from satellite data.

 

While Light Pollution maps may prove useful to city and development planners, or those studying the spread of the problem from urban sprawl, they are only vaguely applicable to what actual observers find on visiting a potential site. According to several Light Pollution maps I've consulted, my home is supposed to be in a DSS level 4 sky zone. In fact, I rate my skies here currently no better than a level 5 bordering on a level 6 in brightness. Worse yet, my entire county of residence (outside a couple of urban centers) is rated as a level 4 across large low population density areas according to Light Pollution maps. This in spite of the fact that zenith SQM readings can range from approximately 19.7 to 21.7 !  At the same time, nowhere in Putnam County, located to my south and thus closer to NYC, is any better than what exists at my home and many areas are worse. Nevertheless, there is an assumed finger of darker area indicated on your map that seems to extend through that county even as far as northern Westchester County, which is simply impossible. 

 

I have been pointing out for several years now that all the Light Pollution maps currently being employed by hobbyists are, at a minimum, one full DSS level too optimistic in their ratings for nearly any given region in the eastern United States. Some of those maps even suggest that the skies over regions surrounding urban centers have grown slightly darker over the past few years, which is utter nonsense. 

 

I do applaud you for your efforts that must have gone into compiling your maps, but I maintain that they are misleading to those of us in the astronomical community attempting to employ them to find significantly darker sites to use for observing.

 

BrooksObs  


Edited by BrooksObs, 22 September 2021 - 10:31 AM.


#12 photobiker

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Posted 22 September 2021 - 12:14 PM

John what did you use for your east, west, north and south location.  I kinda slid it around until if fit, but the older map I have provided degrees, minutes and sec.



#13 DaveL

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Posted 22 September 2021 - 11:20 PM

BrooksObs,

You are the creator of the Bortle Scale! It's an honor to "talk" to you. Sorry it took me another post to see where you are coming from.

 

As you say, the zenith sky brightness (i.e. light pollution maps) is a single metric of sky brightness and the Bortle scale is a multivariate metric of sky quality (i.e. takes into account zenith, close to the horizon and transparency as well). If I had to choose between a good Bortle site and a good light pollution atlas site, I would choose the Bortle because (1) it is multivariate index and (2) it is based on "on the ground" observations.

 

There are some important comments I'd like to make that may help clear up or at least help you better understand your issues with the light pollution atlases.

 

1. You are correct that the most widely used light pollution map is biased. The https://www.lightpollutionmap.info map, which is not my work, is systematically biased toward dark skies (i.e. it says the sky is darker than it really is). For one of my favorite observing sites in Wisconsin, lightpollutionmap.info say the SQM is 21.74. In reality, on excellent nights, I get a SQM of 21.5. The 21.5 is consistent with my maps, which show my site on the boundary between light and dark green. The original 2001 light pollution atlas (Cinzano et al 2001, not my work either) did not have this huge bias, and my model is based on that work. Lightpollutionmap.info, on the other hand, shows a newer 2015 atlas described in this paper:

 

Falchi, F., Cinzano, P., Duriscoe, D., Kyba, C. C., Elvidge, C. D., Baugh, K., ... & Furgoni, R. (2016). The new world atlas of artificial night sky brightness. Science advances, 2(6), e1600377.

 

The 2015 atlas changed the ratio of upward propagating to (nearly) horizontally propagating (increased upward propagating too much). Since horizontal propagating light is the main culprit, especially more than a couple kilometers from a light source, they biased the map dark. For some reason, this atlas became the most popular. Perhaps because it has the most "information", which gets me to my next point below.

 

2. When you click on the lightpollutionmap.info maps, you get the coordinates, the elevation, the SQM reading to the hundredths place!, and the Bortle scale. The site assumes that there is a one-to-one correspondence between zenith brightness and Bortle. I do not claim my maps are the Bortle scale and I intentionally do not mention the Bortle scale on my site. When I introduced the previous version of my maps (2016 version), I listed out 5 important points, the first one was short and sweet: "These maps are not the Bortle Scale". I wrote this because the widespread conflation of the two. I think this conflation started innocently with a local study by the Northern Virginia Astronomy Club (NOVAC). They tried to find the best correspondence between the Bortle Scale and the recently published 2001 light pollution atlas (Cinzano et al 2001). For their region, with its particular blend of cities, town and rural lights, the correspondence might have been pretty good. But this would not necessarily extrapolate to a different region where light pollution is dominated by a single large city, or where light pollution is the result of a more or less spatially uniform light sources.

 

For lightpollutionmap.info, the even bigger problem is that they took the 2001 maps/Bortle scale comparison and used it on the 2015 maps, which are biased. Hence, my favorite observing site above is "Bortle 3" according to lightpollutionmap.info, but the site is Bortle 4 on excellent nights. On the other hand, if you use the zenith brightness on my maps, then the NOVAC relationship says my site is Bortle 4, which is consistent. Nevertheless, the correspondence can never be expected to hold in general because the Bortle scale is more than just the zenith, so I do not even mention Bortle on my site. Lightpollutionmap.info, on the other hand, gives this extra "information", never mind that such a one-to-one correspondence does not exist.

 

3. Regarding the finger of dark skies in my maps: I think this is a consequence of the fact that horizons are more affected by nonlocal light sources than zenith. While my zenith maps are a smoothed version of the upward light sources, a map based the sky brightness closer to the horizon would be even more smoothed. So the dark finger may exist for zenith brightness but things are too smeared out for lower elevation brightness. Since the Bortle scale is affected my much more than zenith, the dark finger does not exist for a Bortle scale "map". Since many interesting objects do not rise to zenith, I agree that the Bortle scale is better than zenith brightness. I've thought about making a separate set of maps showing the brightness closer to the horizon, especially the southern horizon (for us northern hemisphere people). So far I've avoided the trouble of modifying my calculations because I assumed I could guess the brightness of the southern horizon based on the zenith brightness further to the south. Based on your observations this is not a good idea because the lower elevations are more sensitive to distance sources so you can't just "shift my maps north" to get a good idea of the southern horizon. I think I will be adding maps based on lower elevation brightness at some point in the next year (this is not my day job). While I'm at it, I guess I'll also do some northern horizon brightness maps for those lucky southern hemisphere people with all the good stuff and dark skies.

 

4. The darkening trend seen in the maps is almost certainly due to the change over to LED fixtures, which (1) tend to be much better shielded and (2) have a different wavelength spectrum. This could be a real thing, or it could artificial. After I updated previous maps, I've had people share opinions both ways as to whether there has been darkening due to LEDs or not (both private messages and on the forum). I do not live in a region with the "darkening skies" so I do not have an opinion. For sure, LEDs are better shielded. On the other hand, the detectors on the satellites are more sensitive to the spectrum of sodium lights than the spectrum of LEDs. So an LED could appear dimmer when it is actually not. Mostly likely, both of these competing effects are operating, but I'm not sure which effect "wins".

 

Dark skies,
Dave


Edited by DaveL, 22 September 2021 - 11:22 PM.

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#14 George N

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Posted 23 September 2021 - 04:49 PM

BrooksObs,

You are the creator of the Bortle Scale! It's an honor to "talk" to you. Sorry it took me another post to see where you are coming from.

 

.......

 

4. The darkening trend seen in the maps is almost certainly due to the change over to LED fixtures, which (1) tend to be much better shielded and (2) have a different wavelength spectrum. This could be a real thing, or it could artificial. After I updated previous maps, I've had people share opinions both ways as to whether there has been darkening due to LEDs or not (both private messages and on the forum). I do not live in a region with the "darkening skies" so I do not have an opinion. For sure, LEDs are better shielded. On the other hand, the detectors on the satellites are more sensitive to the spectrum of sodium lights than the spectrum of LEDs. So an LED could appear dimmer when it is actually not. Mostly likely, both of these competing effects are operating, but I'm not sure which effect "wins".

 

Dark skies,
Dave

Here's thanks to John and his "scale" and Dave and his "map".

 

As for darkening skies in some areas - particularly parts of the Northeast - I don't think we have a real idea. The LEDs change-over? Maybe.

 

In the area I live about 2/3 of the time NY/PA border area -- the end (for the time being) of gas fracking makes for a big difference. Some of the earlier 'dark sky maps' show a giant light source - bigger than any area city - in the middle of nowhere PA mountains - and now gone!! - it was from fracking! Altho I often observe in NY where frack'ing is banded - it is just across the PA border. During the Winter friends and I could easlily see flickering orange light moving across the snow fields -- from towering gas flames higher than a 10 story building. As I said - certainly by 2020 it is all gone - for now.

 

Also, much of interior New England and large swaths of Upstate NY are both declining in population and economic strength. For me - that includes all of the NY/PA Southern Tier Counties and cities - all with declining population. My other County - Hamilton in the central Adirondacks - also continues to lose population and business - as do all the surounding counties -- but forest critters like moose and lynx are making a big comeback. That decline may well have some impact on Light Pollution - especially that driven by business ( a number of big-box retail stores have closed - parking lots now dark ).


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#15 DaveL

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Posted 23 September 2021 - 07:59 PM

Hi George, you’re right I was way too simplistic. I agree with your post. Thanks, Dave
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#16 Tony Flanders

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Posted 24 September 2021 - 06:50 AM

As an example, in the area surrounding NYC out to at least 75 miles the map is not reflective of the actual sky brightness situation at all. For one, there are several supposed 'darker' zones, or fingers of darker regions in Dutchess, Putnam and Westchester Counties that simply don't exist in any form, as well as the overall sky illumination likely being consistently underestimated throughout the area by at least one full brightness class (perhaps more) relative from that proposed by the atlas.


As I remember, Zane Landers (Cloudy Nights user Augustus) rates Ward Pound Ridge Reservation as Bortle 4. I doubt that I would find it as good as that, but he is an experienced observer, and I have no doubt that it's Bortle-4 for him.

 

There are certainly plenty of locations in northern Dutchess County that are Bortle 4 on a good night by anybody's standards, and possibly even Bortle 3, such as Taconic State Park. Considerably darker than my own backyard some 50 miles to the north.


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#17 BrooksObs

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Posted 24 September 2021 - 11:00 AM

As I remember, Zane Landers (Cloudy Nights user Augustus) rates Ward Pound Ridge Reservation as Bortle 4. I doubt that I would find it as good as that, but he is an experienced observer, and I have no doubt that it's Bortle-4 for him.

 

There are certainly plenty of locations in northern Dutchess County that are Bortle 4 on a good night by anybody's standards, and possibly even Bortle 3, such as Taconic State Park. Considerably darker than my own backyard some 50 miles to the north.

 

Having been to the Ward Ridge Reservation area at night on numerous occasions, I can state without reservation that its skies are nowhere near a DSS level 4, nor even are my own any longer much farther to the north of there.

 

In point of fact, since the growing takeover of LED streetlights in NYC and a few of its immediately surrounding inner suburbs, my area's skies have grown very noticeably brighter, to the degree of becoming DSS level 5-6. Undoubtedly, some areas of southern Dutchess County likely having become even worse than that, in spite of being situated 75 airmiles outside NYC. And that was exactly my point when I stated that Dutchess County is certainly not of a DSS level 4 universally today. Light Pollution Maps are all essentially  too optimistic in their assignment of true DSS darkness levels to regions, given that they are not based on actual ground level data. This is similarly true of even those locations  locations well removed from lesser population centers throughout the eastern U.S., as I've witnessed during my own journeys. 

 

BrooksObs


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#18 DaveL

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Posted 24 September 2021 - 08:41 PM

...Light Pollution Maps are all essentially  too optimistic in their assignment of true DSS darkness levels to regions...

 

BrooksObs

Yes, lightpollutionmap.info, which is the most popular map, is definitely biased too optimistic. But my maps are brighter and, based on my comparisons over the US, not systematically biased. For the New York City area, here's my 2016 map on the left, and the lightpollutionmap.info map on the right (with the color scale altered so that it is equivalent to my map). Especially outside of the cities, lightpollutionmap.info is systematically darker than my map.

 

-Dave

 

compare_2015.png


Edited by DaveL, 24 September 2021 - 08:43 PM.


#19 Tom Masterson

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Posted 10 October 2021 - 06:21 PM

One thing I've noticed with LED lighting is more scatter from the shorter wavelengths, especially if there's humidity or dust in the air. I really became aware of it when my town switched many of the streetlights over to LEDs. You could easily see that they had a better horizontal cut-off because you could actually see the light cone in the air as you approached because of the additional scatter. Another thing I noticed is when flying into cities that were changing over to LEDs. Again, better shielding, but more lumens bouncing off paved surfaces like parking lots. A lot of time the air around these lights appeared slightly hazy, again from scattering especially when compared to sodium lighting nearby. 

 

I feel LED lighting is one step forward - better shielding, but two steps back, more scatter and more lumens per fixture. The institution I worked at until 2016 started moving to LED lighting but one thing that we used to save costs were retrofit kits for wall packs. Remove the ballast and socket from the fixture, install the driver and stick a magnet backed LED board in and you are good to go, except you now have a the same unshielded fixture throwing out twice the lumens.

 

I can also understand why there seems to be disagreement in these threads as to whether the switch to LEDs has made things better or worse. If you live in an area where there a lot of poor shielded lighting such as wall packs, and the switch is made to shielded LED fixtures, your area would improve. In areas where a switch to higher lumen fixtures reflecting off pavement or with intensely bright flood lighting like some car dealers use, your skies would be worse. As mentioned shifts in population, retail or industry would have an impact too.

 

As a long time amateur who uses these types of maps and Bortle info, especially now that I'm traveling full time and am sometimes on the look out for dark places to stay, thanks for the work. One thing I did notice when I did stay in areas that should have been quite dark, is the skies seemed to be more gray than  I expected. This thread helps explain what I found.


Edited by Tom Masterson, 10 October 2021 - 06:49 PM.


#20 DaveL

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Posted 10 October 2021 - 09:34 PM

Tom,

Thanks for your perspective. Just in the last week I noticed a paper that measured sky brightness before and after Chelan County (Washington state) switched over to LED lights. They found that the skies brightened, which is consistent with your observations. They used the high quality National Park Service CCD set-up to measure the sky brightness. Here's a link to the paper:

 

https://www.scienced...301479721008380

 

Furthermore the VIIRS satellite, which is less sensitive to blue light, failed to notice the brightening. Instead the satellite saw a darkening. VIIRS is the satellite I and the Falchi/Cinzano group use to make the light pollution atlases. So without alternative backup data, one should probably assume that the decreases in light pollution seen on the atlases in the U.S., U.K. and France are likely artificial.

 

-Dave



#21 ghayduke

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Posted 11 October 2021 - 12:18 PM

Hi Dave,

I have seen your work over the years, are your exact methods for the model published? A quick search on Google Scholar did not find any papers.

In reply so some of the comments, the most infuriating thing about blue-rich LED is that in the natural night sky, there are almost no sources of sky background brightness bluer than the 557.7 nm Oxygen airglow. So the increase in blue sky glow from LED lights is almost infinite over natural conditions. Blue light also harms human night vision much more severely than red light. As noted, the VIIRS DNB sensor is blind to light bluer than 500 nm. Incidentally, it is also quite sensitive to deep red and infrared, so gas flares and other types of fires result in an over-representation of upward radiance when compared to the human eye's sensitivity range. Blue light scatters more readily in the atmosphere, even under clear air conditions, than light from other sources such as HPS. This results in it being extinguished more readily at greater distances and enhanced at short distances from the source. Remote observing locations may benefit, while if you are used to observing from your backyard in suburbia and your local high school becomes a blue-white LED prison at night, you are screwed no matter how well shielded the lights are.

Thanks,

-Dan Duriscoe



#22 DaveL

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Posted 11 October 2021 - 01:17 PM

Hi Dan,

 

That's interesting about the increased scattering of blue light may make remote observing locations darker. I'll have to put some numbers in the model and see how big the effect is.

 

Also, I want to mention that your wide field CCD data from the National Parks is an awesome resource for light pollution research. I've downloaded data from all 397 nights and have used it to compare to the light pollution atlas. Lately I've been comparing the Bortle observations in the NPS dataset to the sky brightness measurements to look for patterns.

 

Also, I'm just repeating the Garstang methods as performed by the original Cinzano et al (2001) paper, so I haven't published anything related to this.

 

-Dave



#23 ghayduke

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Posted 11 October 2021 - 04:47 PM

Thanks Dave. You must have a supercomputer available to do the whole world!

For fans of the Bortle Scale, see this rather obscure paper, not peer reviewed .

https://artificialli...2015_bortle.pdf

Especially pages 18-22 which show that Borle Class correlates pretty well with NELM and the BRIGHTEST part of the sky (which is always near the horizon), and quite poorly with Zenith Sky Brightness, especially in classes 1-4 where the curve is almost flat.

Also Li-Wei's latest paper from NPS.

https://academic.oup...tab2662/6374878

They hope to make the 1000+ nights of data from the all sky camera work available publicly, someday.

Dan


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#24 ghayduke

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Posted 11 October 2021 - 05:15 PM

See also

https://www.energy.g...ct-sky-glow.pdf

a very comprehensive modeling study. There are some key takeaways in the executive summary:

 

"While increasing the short wavelength content of exterior lighting sources increases the potential for sky glow,
other characteristics of LED street lighting luminaires can reduce or completely offset these effects. The three
main characteristics of luminaires that influence sky glow are spectral power distribution (SPD), total lumen
output, and luminaire light distribution (and, most importantly, the amount of that distribution emitted as
uplight). Each of these characteristics can be specified through the selection of luminaires and should therefore
be carefully evaluated as part of the system design.

 

Distant from the city, the sky glow contribution from the typical LED street lighting conversion appears to be
negligible, at all SPDs, for human observers (e.g., scotopically weighted results) as well as astronomical
equipment (e.g., unweighted results). Although the reduction is striking even at this relatively short distance of
40 km, street lighting must again be considered in the context of accounting for only one (now former) source of
sky glow from an urban area, as the others remain unaffected."

 

The first paragraph is quite accurate, but, unfortunately, most installations do not "carefully evaluate" much of anything.

The second paragraph seems overly optimistic, even if applied ONLY to street lighting. The Chelan County study seems to refute this assertion, but I believe the distance to the sources was only 13 km.



#25 DaveL

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Posted 11 October 2021 - 08:00 PM

Hi Dan,
Thanks for the links, I will definitely look them over.

Also, I didn’t need a supercomputer. When you assume sea level everywhere the light pollution only depends on distance to the sources. I make a “table” of light pollution versus distance for a range of distances just once beforehand and then look up from the table while doing the map(use linear interpolation). You can also limit the number of distance calculations by a factor of the number of longitudes times 4: the distance between the source and a particular point is the same for all points on that same latitude circle with the same latitude-longitude offset, the longitude offset can be plus or minus, and finally repeat for Southern Hemisphere.

Dave

Edited by DaveL, 11 October 2021 - 08:06 PM.



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