1-Wire Solar Radiation Sensor

Background

According to Parallax's Earth Measurements page 84 There are four tools for measuring light intensity:

A Pyranometer measures light energy hitting a surfaces during a given period of time. Useful for solar energy systems. The measurement unit is watts per square meter.
A Photometer measures the light intensity our eyes see. The measurement unit is lux or footcandles.
A PAR (Photosynthetically Active Radiation) measures the light intensity as it effects the growth of plants. The measurement unit is micromoles of quanta per square meter per second.
A Spectrophotometer determines how much light energy is in each narrow band of wavelengths across the light spectrum.

DS2438 Smart Battery Monitor available from Digi-Key #DS2438Z-ND for < $4.
Two 1N5817 Schottky diodes available from Digi-Key #1N5817DICT-ND for < $1.
Photonic Detectors Inc. PDB-V113 Blue Enhanced Silicon Photodiode is available from Digi-Key #PDB-V113 for < $2.
100 Ohm 1/4W 5% Resistor Radioshack #271-1311 for < $1.
0.1 MFD 50V radial ceramic capacitor Digi-Key #399-2054-ND for < $1.

There are several different approximations that can be done. The current in photodiodes is equal to a constant times the light intensity.

Full summer sun on a clear data is approximately 1000 watts per square meter (75 watts per square foot).
Full summer sun light is approximately 110,000 Lux or a 100 watt lightbulb viewed from 1 meter is approximately 100 Lux.
Full summer sun light is approximately 2000 moles per square meter per second.

Eric Vickery at Hobby Boards now has a Solar Radiation Kit based on the DS2438 and the Clairex Photodiode.
Jim Jennings use to have several kits. His web pages have disappeared, but you can still see them using the Internet Archive ( Weather Station Module Related Information