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I have to study the diffraction curve of a laser passed through a slit. To measure the intensity i am using a photodiode and a voltmeter; I move the photodiode and measure the intensity every $200\mu$m.

My question is about the uncertains assigned to the mesaurements, how can i assign it ?, i don’t want to use the multimeter resolution because is too small and the sensibile surface area of the photodiode is $200\mu$m wide, so it returns a mean value of the intensity and not the value at that point.

Thanks for the help!

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Your plot of photodiode output will be the diffraction pattern of the slit convolved with the area of the photodiode. (Spatial convolution)

If you increase the distance between the photodiode and the output aperture, the spatial filtering effect of the area of the will be reduced. - this reduces the output (inverse square law) and will reduce the SNR - eventually the limiting the resolution of the diffraction pattern.

Uncertainty is related to filtering effect of the spatial size of the photodiode (you can deconvolve to remove this effect) and the noise on the number of measurements (and your model) of the diffraction pattern.


You want to be measuring the photodiode (PD) current (linear with light) rather than the voltage across the PD (not linear) and you want to arrange that the maximum current drawn from your PD (if it is unbiased) is less than about 100 uA. (value depends on actual PD used)

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  • $\begingroup$ This is Not what i was looking for, i need to quantify the mesaurement’s uncertains Not reducing it $\endgroup$ – Sergio Piccione Apr 22 '18 at 0:15
  • $\begingroup$ @SergioPiccione Start with the basics as described above and setup the experiment logically and correctly then the description of what the errors are will be clearer. $\endgroup$ – MJC Apr 25 '18 at 13:45

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