Need to Measure Light Freq. using photodiode and have it analysed by FFT, details below.

[chrisd]

Archimedes wrote:

> Since your instrument converts a frequency to a DC signal you should
> be all right sampling at 100 Hz or so. Keep in mind that at that
> sampling frequency your ciliary beat cannot exceed 50 Hz. For a
> better represntation of your signal I would sample 5 to 10 times the
> highest frequency in the ciliary beat.

Since you measure 80 Hz pulses, sampling at 100 Hz seems rather low to
me. Something like at least 300 Hz would be more appropriate, I would
say.
Chris

[BruceAmmons]

Julesy,

This is one way to measure cilia beat frequency (CBF). You will be limited by the update rate of the screen, which is 30 frames per second. This means you can only measure frequencies up to 15 Hz, due to Nyquist. If you sample at 60 Hz, you will have more data than you can use. You can just run the data through the "Single Tone" function to find out what the peak frequency is.

If you want to do a better job of measuring the CBF, you should look at the SAVA system my company developed. It is capable of measuring frequencies up to 60 Hz, and you can record samples and take multiple measurements. You can even analyze every point on the screen quickly and easily.

Feel free to contact me if you would like to discuss the SAVA system or other
CBF measurement methods.

Bruce Ammons
Ammons Engineering
bruce@ammonsengineering.com

[mir]

This is not quiet the case. To measure the light we can not deal with it as a electromagnetic waveform. It can't be treated as a simple signal either. The measurement is rather on light effect not on the light by itself. Thereofore you need a device,photodiode or perhaps a CCD, to convert the light energy to voltage and then acquire the voltage by an electronic board. In this case you don't need to have a sampling frequency of twice the light's frequency. But that would be great to deal with the light as a simple electromagnetic wave. We could save a lot of time bypassing FFT algorithms.