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Labjack with Quantum Light Sensor

light sensor impedance

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5 replies to this topic

#1 alkaram

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Posted 03 June 2014 - 02:06 PM



We are trying to measure the light intensity with an Apogee Quantum light sensor (https://www.apogeein...Q-200manual.pdf).  Still very new to using labjack and data acquisition and we have a few questions as our data logging revealed a clearly sinusoidal pattern from multiple light sources.  


1) How do we know what range to select if logging on the UDlogger? Currently we have selected LB_rgBIP5V because the sensor is a 5V calibrated sensor. But honestly, I struggled finding documentation on how to determine which range to select so selecting this range was all but a guess.


2) Will will need to connect the ground for sensor to an earth ground due to this sinusoidal pattern? 



#2 LabJack Support

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Posted 03 June 2014 - 02:33 PM

1.  The U6 has 4 analog input ranges:  +/-10V, +/-1V, +/-0.1V, and +/-0.01V:





You generally want to use the smallest range possible such that your signal does not exceed the range.  If you are expecting a 0-5V signal, you will need to use the +/-10V range (LJ_rgBIP10V).



2.  I looked at your link.  You want to connect White to VS, Clear to GND, and Green to AINx.


One likely reason you would see a sine is if you are picking up light from AC-powered lights.  AC lights powered by 60 Hz generally produce light that varies at 120 Hz.

#3 alkaram

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Posted 03 June 2014 - 06:25 PM

Thank you for the help. I will set the range to +/- 10V from here on out.  


I had my light sensor connected in the way you described. However, based on logging data so far, I'm not sure whether or not AC current is the only reason for the sine pattern (though I'm not sure). Do the graphs in the Imgur link below suggest that the sine pattern is from AC powered lights? The pattern is clearest in the halogen type bulb but there is still a slight pattern when the light came from my cell phone. I'm not an expert on light generation but I was thinking that since the cell phone light was being powered by phone's battery there wouldn't be any major fluctuations.  However, we always had the background fluorescent lights on for each of these tests (as shown in the third graph). 


Logging data graphs for 3 different light sources: 


#4 alkaram

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Posted 04 June 2014 - 10:40 AM

Minor update:


I tested the light sensor outside in the sunlight and this wave pattern disappeared (http://imgur.com/bNVAgix). Definitely think the wave pattern is either a function of the light or electromagnetic background noise or perhaps both?  The manual states that "For best measurements, the shield  wire must be connected to an earth ground. This is particularly important when using the sensor with long lead lengths in electromagnetically noisy environments."


Any advice on using an earth ground or determining if this is necessary? 

#5 LabJack Support

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Posted 04 June 2014 - 11:45 AM

If you cover up the sensor so all light is blocked, does the sine go away?  That would tell you it is the light that is sinusoidal.  If it was EMI introduced into the wires that would not be expected to change based on light.


You can connect the cable shield to GND on the U6, but I doubt that connection is needed.

#6 LabJack Support

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Posted 04 June 2014 - 11:52 AM

The data you show in your images is likely aliased.


To see if the signal is varying at 120 Hz, you need to sample faster.  Run LJStreamUD, set scan rate to 4000, set Graph History to 400, and then click Start Stream.

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