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A Does increasing photon flux density increase attenuation?

  1. Jan 7, 2017 #1
    follow up from responses in old thread. https://www.physicsforums.com/threa...icient-decrease-with-increased-energy.569981/

    I have seen in my experiment using a saline media ( some oxygen bubbles) that an increase in Incident PFD is showing an slight increase in the attenuation coefficient. In theory I should be getting the same coefficient. Can an increased number of photons cause increasing number of photon collisions? My data is reporting a linear increase in attenuation coefficient at increased incident PFD.
     
    Last edited: Jan 7, 2017
  2. jcsd
  3. Jan 7, 2017 #2

    mfb

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    Increasing number of collisions of what?

    Nonlinear effects are possible, but usually they require very high intensities (= powerful pulsed lasers). If you have a conventional light source or work with x-rays (you see how vague your description here is?), the nonlinearity is probably somewhere else.
     
  4. Jan 7, 2017 #3
    We are using Halogen lamps (400 to 700nm WL) in a Bioreactor where the beams were set up to illuminate the surface of the bioreactor. A saline medium was used inside the bioreactor with a bubbling effect. We employed the use of light sensors to measure both incident PFD (Sensors on Front Surface) and attenuated PFD (Sensors on Back surface). The attenuation coefficient was calculated and the plotted as a function of incident PFD. Using the data we plotted we found tendencies for increased PFD to correlate with an increased attenuation coefficient in the saline media. I don't know what is causing the light to show an increased attenuation coefficient at increased PFD. A total of 20 sensors are being used and the general profile of the bioreactor shows an elevated attenuation coefficient in the middle of the bioreactor where incident light is expected to be the highest. Is it possible that increasing the number of photons is causing photons to collide with each other and the collisions are pushing photons off the sensor path , thus increasing the attenuation coefficient ?
     
  5. Jan 7, 2017 #4

    mfb

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    Don't jump to conclusions. How well do you know the linearity of your light measurements?

    Could heat from the light influence the setup? Do you see some time-dependence of the measured light intensity if you switch on the light?
    If your light would be 30 orders of magnitude more intense, yes. In your setup: no.
     
  6. Jan 7, 2017 #5
    We have measurements from SIX (6) different PFD levels in each bioreactor (x4) for a total of 480 data points. The relationship with attenuation tends to show a linearity at each PFD level ( 20 points each bioreactor and 80 points altogether each level), it has me puzzled as I try to figure out what is causing the variation. The measurements are taking the sum of the PFD over a 4 minute period before reporting to our servers, it resets and starts again, over and over. As soon as the lamps are turned on, we wait, then the second measurement from the server is used. This is done each time we place the sensors. ( If that is what you meant)

    So could it be the method we used that is causing in the variation ?
     
    Last edited: Jan 7, 2017
  7. Jan 7, 2017 #6

    mfb

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    Heat can certainly lead to nonlinear effects. To study that it would be interesting to take measurements after different operation times.
    Does any part of the setup heat up notably? If yes, do you wait for the things to cool down before you take the measurement? If no, can you change the order in which you take the measurements for different intensity, and see if the order matters?

    The sensors could be nonlinear as well: If the sensor gets twice the signal, it should report twice the intensity. But did you check that it actually does that?

    What is PFD, by the way? Neither Wikipedia nor Google lead to plausible results.
     
  8. Jan 7, 2017 #7
    PDF - photon flux density
     
  9. Jan 7, 2017 #8
    Thank you ! I appreciate all your input I really needed another angle.

    This data was taken a year ago. but I can definitely go back and take some sample measurements to see if I can come to a conclusion. Ill update you whatever comes of it.
     
  10. Jan 10, 2017 #9
    I suggest you check the stability of your voltage supply and regulator to the lamps....Even though Halogens are suppose to be very stable, they are voltage sensitive. If you are using Tungsten halogen lamps the spectral output can be very sensitive to the energy supplied to (and the temperature of) the tungsten filament. And spectral variation will affect attenuation (even though intensity is not predicted to be a factor). Here's a brief description with graphs of spectral output vs. energy and temperature. In doing exacting experiments this necessitates a very stable voltage source...and this may be your problem, especially since you may be running multiple lamps from the same regulator.
    https://www.intl-lighttech.com/applications/light-sources/tungsten-halogen-lamps (See figure 2)
    ............
     
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