The goal for this Open Source Solar Spectrum Analyzer was to collaboratively design the mechanical and electronic controls for a direct/diffuse data acquisition of wavelength and tilt angle dependent solar flux.
Thorough description, designs, hardware, etc: http://www.appropedia.org/Open_source_solar_spectrum_project
You'll need some libraries.
- AFMotor: http://www.ladyada.net/make/mshield/download.html
- Chronodot: http://planetstephanie.net/2011/04/09/chronodot-library-for-arduino/
They have also been included in this project for convenience.
For a refresher on how to install them, take a look at the Contributed Libraries section of http://arduino.cc/it/Reference/Libraries.
[this is an incomplete document]