I have also had a interest in this topic for use in a greenhouse environmental monitoring (and one day control) project that I'm working on in my spare time.
I thought I'd add the little insight that I have acquired. The light spectrum between 400-700nm is what is referred to as Photosynthetic Active Radiation (PAR) or Photosynthetic Photon Flux (PPF). It is, as you might suspect, the wavelength of light that plants require for photosynthesis.
The key with measuring this narrow band of wavelength is having the proper photodiode and even more importantly the proper filter on the sensor head. There are a handful of vendors of PAR/PPF sensors out there, **broken link removed**is one **broken link removed** is another. Depending on your point of view, the price really isn't all that bad (at least for Apogee) considering the accuracy of the measurement that you get in return. Of course, if you wanted a dozen of these, then you'd really start to feel the pain.
Several years back the B.C. Ministry of Forest required a quantity of these sensors for their research. They decided to roll their own to keep their budget reasonable and were generous enough to
publish the results. They experimented with various photodiodes and were able to obtain decent results in the process.
I hope to use this document as a guide in building my own sensors at some point down the road when I have the time, meanwhile I purchased one from Apogee to get me up and running and to use as a "standard" for comparison purposes when I do get around to taking a shot at building these from scratch.
At any rate, best of luck in your endeavors.