I would use the variance in the Brownian motion using equipartition theorem to calculate the trap stiffness, but I could not understand how I choose the points that is representative to be used for the calculation,
shall I use the just after calibration of position detector, or the straight-line on which beads are trapped by light before I start the real experiment.
As both values of variance differs and the difference is big enough to create different results (SD=2.1, 5.8 nm).
Does difference between these values mean something wrong in the our optical tweezer
When You are getting different values of standard deviation you can check few things-
1. If there is any power fluctuation in laser
2. Particles are of same size
3. If there is any drift in the medium (may be because of loading of sample or temperature difference)
Why do not use the power spectrum method (PSD) to get both stiffness (k) and sensitivity(S)?
The PSD is calculated by Fourier transforming the QPD signal in Volts and is fitted with a lorentzian function to determine two constants: the plateau and the corner frequency, which define S and k.
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