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LED - Light Intensity vs. Current Relationship

Discussion in 'LEDs and Optoelectronics' started by biomed_dude, Apr 3, 2014.

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  1. biomed_dude

    biomed_dude

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    Nov 25, 2013
    I am building a fiber optic blood oximeter which is founded on the basis of the different light absorption of oxyhemoglobin and deoxyhemoglobin. My red LED can withstand much more current than the IR LED. Should I drive them with the same current or can I use different current values (will this affect my output on the photodiode reception end?)?
     
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    The light output from an LED is reasonably linear with current. But the human eye does not see it that way but a sensor will. You need to choose the correct current for each LED for the purpose of which they are being used. You could drive the red LED at the same current as the IR LED but this might not work for your project. You need to choose the correct power devices so you achieve the expected results. Give us a bit more information like over what distance between sensor and photodiode and what medium is the light passing through. If you have the details of the devices to use, post them also
    Adam
     
  3. jpanhalt

    jpanhalt

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    Nov 12, 2013
    I am sure there are many articles about the best relative intensities. One piece of information you need is the detector sensitivity at the two (?) wavelengths you will be using.

    Here is what you are looking at:
    Capture.PNG

    Obviously, you don't want one curve grossly displaced from the other. So, as a first approximation, use the appropriate current for each diode to approximate the same detected intensities.

    Is this homework?

    John
     
  4. biomed_dude

    biomed_dude

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    Nov 25, 2013
  5. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    So is this light traveling down the fibre optic over that distance? Or are you just using the photodiode as is. The response of this device looks like it is worked out assuming you are using a fibre optic cable because it does not reference a surface area.Some IR photo diodes will use a relationship of mW/cm^2 to workout the current versus irradiance. That's fairly easy to work out but I am not sure with this because the units don't seem quite right. You may have to just try it and as long as you don't exceed the pulsed rating of either device you should be ok.
    Adam
     
  6. jpanhalt

    jpanhalt

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    Nov 12, 2013
    Please clarify, are you doing blood oximetry (e.g., with a sample of arterial blood) or pulse-oximetry?

    If the former, which constituents are you going to measure? Only oxygenated hemoglobin vs. hemoglobin (deoxyhemoglobin)? Do you have to worry about carboxyhemoglobin, methemoglobin, or fetal hemoglobin? I am assuming you are talking about human studies without any other abnormal hemoglobins.

    If you are doing pulse-oximetry (e.g., the finger type), the relative intensities of the two LED's doesn't matter much, so long as each produces a reasonable, in-range response with your detector. That also applies to blood oximetry, so long as there are only the two forms or normal hemoglobin present.

    If pulse-oximetry, why is the measurement distance so large?

    John
     
  7. biomed_dude

    biomed_dude

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    Nov 25, 2013
    I am designing a blood oximeter. Right now, the cables are simply testing in a beaker of sheep blood (that is kept refrigerated). I realize I will not get a fairly accurate oxygen saturation percentage, since I cannot actually test in vivo.

    I started the design by making sure an output could be seen in different solutions (starting with water, then adding corn starch and food coloring). So, since I am doing blood oximetry, does it matter that the LED intensities are not the same?
     
  8. jpanhalt

    jpanhalt

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    Nov 12, 2013
    That still doesn't answer why you need such a long light path. Three to 4 inches is enormous.

    It probably doesn't matter what the two LED intensities are, just so you have calibration standards, and the range of transmitted intensities is within the essentially linear range of your detector. I don't know much about other confounders, such as carboxyhemoglobin in sheep. Presumably, these are non-smoking sheep. :D

    As a small point, in vivo generally refers to testing in a live subject (examples: pulse oximetry, indwelling biosensors); in vitro generally applies to testing of a specimen that has been removed from the subject.

    John
     
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