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Sensitivity of photosensors

  1. Dec 22, 2015 #1
    Hi there,

    My name is Anar Kazimov and I want to carry out an experiment related to the sensitivity of the photosensors(photodiode,photoresistors and etc.). To find their sensitivity I am going to change the distance, from the light source to the sensors, and compare the change in output voltage(see the circuit). But the problem is that I am a high school student and I don't know how to describe my experiment more scientifically. Can I say something like this: "The sensitivity of the photosensors to the change in lumen flux"? I am hesitating in this description because lumen flux doesn't change with the distance from the light source, then what quantity should I use to describe the sensitivity of photosensors?

    Kind regards.

  2. jcsd
  3. Dec 23, 2015 #2
  4. Dec 23, 2015 #3
    Hi Anar,
    That's a great experiment. LOTS to learn.
    Did you think this up on your own, discover it in a source or is it a school project?

    Generally, you'll need to think about both source and receiver [detector] characteristics. That includes not only the 'sensitivity' of photosensor but the associated amplifier as well.

    Different measures of illumination are listed here....
    I don't know which would be most suitable....and probably shouldn't tell you if I did know.

    After a quick scan, I noticed different terminology relative to the article in Post #2.

    For your experiment, be sure you give careful thought to "sensitivity"....define it precisely. See the following for some perspective.....For example, your light source might or might not register ANY output regardless of distance.
    The basics of what's happening is described here:


    "..... electrons are only dislodged by the impingement of photons when those photons reach or exceed a threshold frequency. Below that threshold, no electrons are emitted from the metal regardless of the light intensity or the length of time of exposure to the light. To make sense of the fact that light can eject electrons even if its intensity is low, Albert Einstein proposed that a beam of light is not a wave propagating through space, but rather a collection of discrete wave packets (photons), each with energy hf.

    check here for the type of photodetector you may be using:

    I remember a little bit about photo resistors and Wikipedia explains the fundamentals of what happens when light falls on them:


    " In intrinsic devices the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities, also called dopants, added whose ground state energy is closer to the conduction band; since the electrons do not have as far to jump, lower energy photons (that is, longer wavelengths and lower frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms (impurities), there will be extra electrons available for conduction."

    Good luck.
    Let us know how your experiment progresses.
  5. Dec 23, 2015 #4
    Thank you very much for your assistance, I was a little bit surprised to see such a broaden answer. It will take me some time to digest everything written above). As for my experiment, I had this light bulb moment when was thinking about research question for my extended essay(Every IB student should write it). The aim of my essay is to compare different parameters of semiconductor photosensors and sensitivity is among them. It is a very good advice to think about definition of sensitivity. I suppose, that "sensitivity of the photosensors to the change in the apparent brightness" will be quite suitable. But if you have some other ideas I will be very glad to read and analyze them.
  6. Dec 23, 2015 #5
    This subject is a good one; you could write a book on it...in fact, multiple books!! Fortunately, the basics have been worked out over the last 100 years or so. I remember doing experiments like this many years ago, always fun, sometimes frustrating, always a great learning experience.

    My suggestion would be to define any "parameter" you think you can measure as well as 'brightness'. Then you need to be sure you have measuring instruments to capture/observe what you want. For example, do you want to measure 'brightness' at your detectors? If so how. Exactly what will you measure as output?
    What might the problem be if you get no output from your detectors [assuming the electrical stuff is working]?
    The answer happens to be in the posted quote above: the light frequency [color] may be insufficient.

    See, for example the chart here:

    A 'red' light source may not be sufficiently energetic regardless of distance, violet is a lot more energetic.
    I wonder what happens if you were to use a much higher or lower energy 'source:
    Don't try gamma rays, they are lethal. I wonder is the radium from a 'glow in the dark' wristwatch would work??

    For example, if I asked you to determine the 'sensitivity' of a given transistor to different inputs, you would have to define exactly what you are measuring at both the input and output, and it would be good to figure out what type of transistor you were testing. For example, a common measure relating output/input would be the current amplification.
  7. Dec 23, 2015 #6
  8. Dec 23, 2015 #7
  9. Jan 1, 2016 #8
    A good experiment would be to map the data values of your photo resistors to the lux/lumen output of your light source. An easy way to do this would be to use a lux/lumen measuring app on your cell phone.

    You could then go on to talk about how light affects the voltage drop and current across your photo resistors. From this you could go on to talk about the different properties of the semiconductors used in the photo resistors and their exact coefficients.

    You can also simplify your circuit by using the internal pull-up resistors on the Arduino's analog pins and then connect your photo resistors directly from ground to analog pins A0, A1 and A2.

    Just use the code:
    pinMode(A0, INPUT_PULLUP);

    This will draw the voltage at the pin up to 5V using the internal resistor and eliminate the need for an additional resistor.
  10. Jan 1, 2016 #9
    This is amazing the circuit will be so basic now.!!! Yes I have already decided to use lux to measure illuminance. But thank you guys for all your assistance, you really helped me)
  11. Jan 1, 2016 #10
    Keep in mind lux is a measurement of light/area (and is equal to 1 lumen/ m^2), so you will need to use the exact same space (room) for you measurements. Lumens is a measure of overall light emitted by a light source. So in order to talk about the intensity of a light source you want to use lumens. To talk about the intensity of the observed light intensity in an area (room) use lux. Try not confuse the two or you will lose credibility.

    P.s. You will measure in lux, and then convert to lumens to talk about the intensity of the light source.
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