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Using photographic meter to measure sky glow?

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#1 ih8usrnames

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Posted 13 February 2013 - 10:30 AM

I have a light meter I use when shooting photos with large format film. I set the ISO of the film, and measure the light, it returns an f-stop/shutter speed for the given light situation.

I wonder if anybody has created a "rule of thumb" for using an ambient light meter to measure sky glow?

Seems like it would be straight forward:
Set it to ISO 100 and measure ambient light.
With F-Ratio set to 1.0 how many seconds does it say for an exposure?

The longer the suggested exposure the lower the light pollution. Just an idea.

#2 Tony Flanders

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Posted 13 February 2013 - 10:59 AM

Normal photographic light meters aren't sensitive enough to measure skyglow.

#3 Tonk

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Posted 13 February 2013 - 11:53 AM

You use a Unihedron SQM (Sky Quality Meter). This *is* designed to measure sky glow

http://www.unihedron...ojects/darksky/

#4 ih8usrnames

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Posted 14 February 2013 - 04:24 PM

I know there are sky meters, I am not about to buy one however.

My idea being;
- ISO ratings are standards (100,200,400,800,1600,3200).
- f/stops are standard ratios.
- time is a standard.

With all these standards it may be possible to figure out a chart for sky glow. Tony already said they are not sensitive enough, I am not debating that. I am only continuing this conversation to fully express my initial idea.

Potentially one could do the following:
Set meter to ISO 3200
Set f/stop to 1
Measure
The meter supplies the number of seconds to create an exposure based on 18% grey.
Create a chart that correlates # seconds with Limiting Magnitude.
30s = lm6
25s = lm5.5
20s = lm5
etc etc.

It would be very cool if it were possible because there are many more photographers in this world with Sekonic LM-358 light meters than there are people with SkyGlow meters.

Apparently it is unrealistic but it was an idea.

#5 GlennLeDrew

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Posted 14 February 2013 - 05:36 PM

Have you pointed your meter at the night sky to see if you are getting a definite signal to read? No photographic meter I've used could do this.

#6 phreon

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Posted 14 February 2013 - 07:10 PM

I have an ancient Gossen Luna Pro that will indeed twitch a great deal from the sky glow in my area. Most meters will give raw readings in foot candles, LUX, etc. You have to consider how big of a swath your meter is seeing though. How many degrees wide? Averaged, center weight, spot or other?

Doug

#7 derangedhermit

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Posted 14 February 2013 - 07:40 PM

ISO is a standard, but manufacturers of digital cameras don't implement it in comparable ways. I also doubt the consistency across light meters would allow comparison in a significant way.

#8 GlennLeDrew

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Posted 14 February 2013 - 11:24 PM

To be meaningful, sky brightness measurements should resolve to 0.1 magnitude, 0.2 at worst, or 10-20%. For comparison, 1/3 of an f/stop, which some meters only reliably--or are designed to--resolve to, is a 25% difference.

#9 Tony Flanders

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Posted 15 February 2013 - 04:30 AM

ISO is a standard, but manufacturers of digital cameras don't implement it in comparable ways.


Indeed! I have taken photos of the sky using different cameras at exactly the same ISO setting, f/stop, and time, an the brightness varies by a factor of two from the most sensitive to the least sensitive.

The variation in films rated at identical ISO was probably just as great.

There's also the huge question of what wavelength you're measuring. Some cameras/films are more sensitive to red, others to blue.

#10 Boy of wind

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Posted 05 December 2015 - 03:43 AM

Hi every body

I have a question about measuring. Has anyone any experience on measuring light pollution with digital camera?

While measuring the Tehran sky glow, I encountered the following problem. pleas advise me on it.
How to change of raw image format to different colors of images that shows level of sky glow(wite,pink,red,orange, yellow,green,...?
Is there any relationship between these colors and lumen per wattage or luminescence or some things like this?

 

thank you
Hamed



#11 MCovington

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Posted 05 December 2015 - 05:59 AM

 

ISO is a standard, but manufacturers of digital cameras don't implement it in comparable ways.


Indeed! I have taken photos of the sky using different cameras at exactly the same ISO setting, f/stop, and time, an the brightness varies by a factor of two from the most sensitive to the least sensitive.

The variation in films rated at identical ISO was probably just as great.

There's also the huge question of what wavelength you're measuring. Some cameras/films are more sensitive to red, others to blue.

 

 

With film there is huge variation.  With a digital camera you should get consistent results, especially if you analyze the raw image rather than just looking at how bright something appears on the screen.  The analysis must be done with software that gives you the numerical values of the pixels, such as PixInsight.


Edited by MCovington, 05 December 2015 - 06:00 AM.


#12 Tony Flanders

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Posted 05 December 2015 - 06:42 AM

 

 

ISO is a standard, but manufacturers of digital cameras don't implement it in comparable ways.


Indeed! I have taken photos of the sky using different cameras at exactly the same ISO setting, f/stop, and time, an the brightness varies by a factor of two from the most sensitive to the least sensitive.

The variation in films rated at identical ISO was probably just as great.

There's also the huge question of what wavelength you're measuring. Some cameras/films are more sensitive to red, others to blue.

 

 

With film there is huge variation.  With a digital camera you should get consistent results, especially if you analyze the raw image rather than just looking at how bright something appears on the screen.  The analysis must be done with software that gives you the numerical values of the pixels, such as PixInsight.

 

Any given digital camera is extremely consistent, in my experience. However, two different cameras, both rated at (say) ISO 100, will almost certainly give different pixel values for identical exposures. Sometimes very different.

 

For what it's worth, Photoshop gives numerical pixel values quite nicely.



#13 Tony Flanders

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Posted 05 December 2015 - 06:55 AM

Hi every body

I have a question about measuring. Has anyone any experience on measuring light pollution with digital camera?

While measuring the Tehran sky glow, I encountered the following problem. pleas advise me on it.
How to change of raw image format to different colors of images that shows level of sky glow(wite,pink,red,orange, yellow,green,...?
Is there any relationship between these colors and lumen per wattage or luminescence or some things like this?

 

thank you
Hamed

The well-known color zones were established by the World Atlas of Artificial Night Sky Brightness, by Cinzano et. al. They are based on satellite measurements and don't have a direct relationship with actual skyglow. The team did hypothesize such a relationship, as you can see by clicking "Our scientific papers" in the left-hand column. But it has been disputed.

 

For what it's worth, all of Tehran is well inside the white zone, as one would expect.

 

The standard measure of skyglow is magnitude per square arcsecond, rather than units such as lumens. If your digital camera can take an exposure at a known ISO rating, time, and f/stop, and then measure the level of individual pixels with Photoshop or a similar tool, it is possible to translate this into magnitude per square arcsecond within a factor of 1.5 or thereabouts.



#14 Boy of wind

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Posted 06 December 2015 - 11:22 PM

 

Hi every body

I have a question about measuring. Has anyone any experience on measuring light pollution with digital camera?

While measuring the Tehran sky glow, I encountered the following problem. pleas advise me on it.
How to change of raw image format to different colors of images that shows level of sky glow(wite,pink,red,orange, yellow,green,...?
Is there any relationship between these colors and lumen per wattage or luminescence or some things like this?

 

thank you
Hamed

The well-known color zones were established by the World Atlas of Artificial Night Sky Brightness, by Cinzano et. al. They are based on satellite measurements and don't have a direct relationship with actual skyglow. The team did hypothesize such a relationship, as you can see by clicking "Our scientific papers" in the left-hand column. But it has been disputed.

 

For what it's worth, all of Tehran is well inside the white zone, as one would expect.

 

The standard measure of skyglow is magnitude per square arcsecond, rather than units such as lumens. If your digital camera can take an exposure at a known ISO rating, time, and f/stop, and then measure the level of individual pixels with Photoshop or a similar tool, it is possible to translate this into magnitude per square arcsecond within a factor of 1.5 or thereabouts.

 

Thanks Tony for your reply

   as you maybe know,  the thing that I meant was how the photo with raw format can be changed to the photo I have linked below. I took a photo from Tehran sky glow in raw format and I should change it to something like link below.

    

    http://www.nps.gov/j...htPollution.jpg


Edited by Boy of wind, 06 December 2015 - 11:32 PM.


#15 Tony Flanders

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Posted 08 December 2015 - 05:48 AM

Thanks Tony for your reply
   as you maybe know,  the thing that I meant was how the photo with raw format can be changed to the photo I have linked below. I took a photo from Tehran sky glow in raw format and I should change it to something like link below.
    
    http://www.nps.gov/j...htPollution.jpg

As I said in a different thread, I'm not aware of any commercial software that can do this.
 
It's a pretty thorny problem. First of all, the National Park Service Night Sky Team photos -- and the ones that I published in Sky & Telescope are full-sky panoramas composed of many individual shots. That requires correcting for vignetting as you move away from the center of each frame and also correcting for distortion, which all lenses exhibit to some extent. My software doesn't do a great job of this; you can see that in the octagonal patterns of my measurements, which are presumably artifacts of imperfect correction.

 

Then you have to calibrate the brightness of each pixel with some objective measure of sky brightness such as magnitude per square arcsecond. And finally, the easy part, you have to color-code that sky brightness as a new pixel.


Edited by Tony Flanders, 08 December 2015 - 05:50 AM.

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