Photodiodes have a propotional photocurrent to incoming light and
and can be used as light indicatior or more versatile intruments
like pyranomters.

In most cases photodiodes are shunted with resistor to show tha actual 
power and not the open voltage.

It is possibe to add photodiodes for ligt sensing at both analog inputs
ADC1, ADC2. 

This note describe the modificatins needed for each port.

ADC2.
-----
R54 was removed and replaced were ohm resistor. Or connect the two
pads with tin or small wire. This removes the voltage divider.
The 100k is left as shunt resistor.  One connect the photo diode
to the ADC2 connector. Cathode to GND. It's also possible to solder
and SMD type 0805 or 1206 directly to the connector. Compact and
wires. Due to rel. high shunt resistor this work as light detector.
Ie. TEMD70001 0805 Farnell 181-4881.

http://www.herjulf.se/products/WSN/sensors/pictures/smd-photodiode-edited.jpg

ADC1
----
R64 removed and pads short circiuted. Here R65 was replaced with 
825 ohm. The photodiode is TO-5 Hamamatsu S1223 Farnell 149-5587

http://www.herjulf.se/products/WSN/sensors/pictures/pyranometer-edited.jpg


Other:
Remove calibration and turn range into 1.6V. In firmware:

cali a1|a2 none