LED based light detector circuit

Discussion in 'The Projects Forum' started by ssutton, Jan 15, 2013.

  1. ssutton

    Thread Starter New Member

    Sep 12, 2011
    Hi All,
    I have an application where I need to sense the changing opacity of a substance in an environment (chamber) where the temperature is steadily increased. I have set up a high temperature fiber optic glass cable set configured as a thru beam with the substance in the sensor gap. The question that I have is this....if I use an infrared type photo transistor emitter/collector to shoot thru the fiber optic cable will my results skew just because of the increased infrared light with the increasing temperature regardless of the opacity? If this is the case, what would be the recommended wavelength of LED emitter/receiver pair that would have immunity to heat?

  2. wayneh


    Sep 9, 2010
    You might consider a dual beam, with one passing thru water or some other material that will not change with temperature.

    Depending on the wavelength, you need to be sure everything in the light path passes that wavelength, or at least has minimal absorption.
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    I guess the first question is what wavelength of light are you testing for blockage of?

    You need to be sure your fiber is multi-mode so more than one frequency/path can exist in it at the same time.

    Another method would be to use a quartz window at the bottom of the test fixture, with the light source at the top, test material in the middle, quartz window at the bottom, below the quartz window you could place all your sensors for various wavelengths from UV to NIR without subjecting the sensors to the temperatures the test material will be undergoing.

    This point is moot if you have already spoken to a fiber company such as Corning and they have supplied you with a recommended product. However, if you are using fiber from some something like a home stereo link cable, then your results will not be linear.