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Spectrum of moon light

Posted by Marcel van der Steen in Explanation
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005_moonWhen I was looking for a spectrum of moonlight, I could not find it on the internet. As I do a lot of lamp measurements myself (see the category lamp measurements), I decided to measure the moon spectrum myself and put that info available on the internet.

Measurement circumstances

The measurement has been done on 14th of April 2014. It was an evening with few clouds and we had a full moon, well visible. Taking measurements with the spectrometer SpecBos 1211 can then be done in two ways.

1) measurement of the illuminance (in lux). However the expected illuminance is only 1 lux and that results in a very faint spectrum with signals for each wavelength too close to the noise limit to have a nice signal.

2) measurement of luminance (in Cd/mˆ2), and that requires a very well alignment of the measurement device in the direction of the moon, as the sensitivity area of the device in this mode is only two degrees wide.

I mounted the spectrometer on a tripod and kept on aligning and measuring until I had an output with a high signal.

Measurement results

Here the spectrum and derived parameters.

001_MoonSpectrum_177cdm-2

002_MoonSpectrum177cdm-2_24measurements

003_CRI_moonspectrumMacLv

Latsly an csv-file with the measurement data, which is an average of 24 measurements.

file-icon-csv-300px

11 Responses to “Spectrum of moon light”

  1. Ur-1el Says:

    Great job. I was very helpful. I want to thank you very much. Can I repay you somehow?

  2. George Felix Says:

    Great work, I’m thinking of doing my PhD on the cooling effects of the moonlight.

    Do you have the Latitude and Longitude of the experiment location?

    If I understand correctly, the experiment was done on a full moon. Have you run the experiment on a new moon (Black No Moon) in order to compare the results between the two?

    Can I use your information in my dissertation and if so, please send me your titles.

  3. mvdsteen Says:

    Dear George. I measured the moon from my address. In the measurement file you can find the time and date. The address coordinates are: latitude: 51.42359906, longitude: 5.40894538. I did not redo the measurement at other moon cycles. It was already hard enough to get a nice measurement with the full moon (I had to point it very precisely as the spectrometer had to be in radiance mode and has then only an acceptance window of 1.8 degrees).
    You can use the data, my info is found here: http://www.olino.org/us/contributors/marcel-van-der-steen

  4. Richard Davis Says:

    Marcel…
    This is very well done, original observation. It’s very much appreciated.

    Do you have any ideas about what might be causing the notch in the lunar light spectrum at about 760 nm? That is an interesting spot on the wavelength line because many IR filters have that as a pass band. I’m hoping there is something to pass.

  5. mvdsteen Says:

    Richard, it seems to me the absorption effect of O2, see also this image from Wiki.

  6. Anne Leugers, PhD Says:

    Richard- Thank you! Nice work! I’m looking at suppression of melatonin by light and have been told by 1 researcher that we don’t need to worry about moonlight because it contains little to no radiation in the 460nm-480nm range. Your nice ground-based data refutes that.

  7. Anne Leugers Says:

    mvdsteen- just a thought.. did you calibrate your integrating sphere first with white light standard and color standard to ensure wavelength and amplitude accuracy? Thanks!

  8. mvdsteen Says:

    Anne, I do agree there is energy in the wavelength range between 460-480 nm. However that amount of energy is very little. If I were to measure it in illuminance, then hat would be probably less than 1 lux. I am not sure whether such small amounts of energy would do anything.

    About the calibration: the spectrometer I used is calibrated by the supplier every year. He does the calibration on wavelength and also on absolute energy level.

  9. Prahallad I Says:

    Thanks so much Marcel.
    This is actually the only spectral data for moonlight that I could find.
    I am trying to compare the spectral differences between sunlight and moonlight. Accounting for the flux differences and instrumentation differences would you have the spectrum information for sunlight around the same day that you collected this moonlight data?
    Much appreciated!

  10. mvdsteen Says:

    Dear Prahallad. I also could not find any moon spectral data on the internet which is why I measured it. Now I had to measure in luminance mode, since in illuminance mode the signal would be too small. I measured 177 cd/m^2 in luminance mode, while the sensitive field of view for the spectrometer was 1.8 degrees (angular diameter). This corresponds to only 7.8E-4 sr. Assuming to measure illuminance, and also assuming only the moon to be a bright object, I would only have measured 0.14 lux (meaning the 177 cd/m^2 = 177 lm/sr/m^2 and as I have measured this over only 7.8E-4 sr, I multiply the 177 x 7.8E-4 and get 0.14 lm/m^2 is 0.14 lux). This is very small and would require a 60 seconds measurement time and 60 seconds dark measurement time. The resulting spectrum would be slightly higher than noise level of the meter.
    Now for the sun, I did not measure the sun with the specbos 1211. First, there is a lot of that spectrum to be found on the internet. Second, I cannot measure the sun with the specbos 1211 in luminance mode, since that would directly yield an overexposure. And even in the illuminance mode I would not be able to measure the sun with my specbos 1211 since it can only measure up to 10000 lux and the sun will go much higher than that.
    So I am sorry, I do not have the sun’s spectrum for you.

  11. Owen Says:

    While I agree there isn’t much benefit to obtaining yet another spectrum of the sun (plenty are out there), spectral features are distinguishable looking at the light of (brightly) illuminated surfaces. Obviously this should have some impact,varying based on setup, but as a first approximation you can get reasonable results in large bands without having to look at the sun directly with equipment that can’t manage.

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