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Diurnal optical variations

Discussion in 'General Electronics Discussion' started by daddles, Jul 18, 2011.

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  1. daddles


    Jun 10, 2011
    In a recent thread, I posted a plot of the voltage across a photodiode that was pointing out of the window next to me in my office. The plot showed a negative slope that was likely due to the rotation of the earth -- i.e., it was about 3 hours from sunset and this is the normal variation in light which our eyes don't typically detect.

    To check this, I decided to take the data for a 24 hour period. The experimental setup is a cheap Radio Shack photodiode whose axis is tilted about 45 degrees above the horizontal so that the diode is pointing towards the sky. The true bearing of the diode's axis is about 45 degrees true north because of the known orientation of my house. The location is the northwest US at a latitude of around 40 degrees north. A voltage reading of the diode is taken about every second and logged to a file.

    I thought it would be interesting to ask folks to predict what these plotted data will look like at the end of a day or two of taking data (the plot will be identical to the plot given in the above thread except that the time period will be longer). The person whose prediction is closest will get 10 absolutely worthless Daddles points and an autographed pictures of Daddles, our duck who is imprinted on humans (and even occasionally comes into our house wearing a diaper). If there is enough interest, I'll also post a story about Daddles and why she can recognize a man with a shovel at 50 m away and come running... :p
  2. daddles


    Jun 10, 2011
    I will post the first guess. I would have predicted an approximately sinusoidally-shaped curve with perhaps flattening around the peaks, as the orientation is roughly north-facing, so the peak a few hours around local noon should be pretty flat. At night, it gets pretty dark and stays dark.

    One thing I forgot to mention is that it's likely that the early morning causes the diode to see some of the sunrise in the morning, but since sunrise is around 3 hours before I normally get up, I haven't checked this.
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I predict that you'll find that 5PM was not too far distant (I'm guessing just after) to time at which the light intensity was falling the fastest.
  4. daddles


    Jun 10, 2011
    I've attached the plot; the data collection continued until 4 am this morning when for some reason there was a permissions error on the file I was logging to and the data collection stopped (I suspect it was my disk backup program temporarily accessing the data file and the logging script not being able to open the file).

    The starting time was 6:12 pm; local sunset is 8:20 pm and sunrise is 5:20 am. The peak at 13.1 hours is when the sun peeked over the local houses/trees. The noisy oscillations confused me a bit until I realized they were caused by the sun filtering through a climbing rose bush next to my window. The peak at 28.1 hours may have been caused by a car driving into my neighbor's yard. The smaller peaks slightly later might have been airplanes flying into town, as planes with their landing lights coming from the north often pass over our house. But I don't know what that small "mesa" near 30 hours is.

    This was a good time of the year to do this experiment, as there have been no clouds in the sky, which is a bit unusual here as there are often cumulus and summer thunderstorms.

    Attached Files:

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