Re: how do I measure phototropism ?

Yes, you have proposed a perfectly correct technique.  The angle, measured 
in degrees, between the shoot's original orientation and the post-
treatment orientation is a measure of the phototropism.  I might suggest 
securing the base of the shoot to a vertical stick as a reference point.  
And you are correct that different wavelengths of visible light will 
produce different phototropic effects.  It's been a long time since I've 
done this experiment, but you must correct for the different amounts of 
energy carried by different wavelengths.  Using the same length exposures 
for different wavelengths does not expose the shoot to the same amount of 
energy.  However, it was over 25 years ago and I've forgotten how to 
calculate and adjust the exposure time for different wavelengths.