Could you explain the accumulated light logic a bit? Like what exactly are you looking for, the hours of sunlight till now? Or how many hours total during the day? Would cloud cover effect this drastically or would a simple cloud cover percentage adjustment work?
The reason I ask is this could likely be done by modifying the weather program and not needing an additional sensor at all. However, if it is for an indoor setup with artificial light obviously that idea would not necessarily work.
Another approach would be to have light accumulation act like a flow sensor of sorts and then the program could be adjusted based on that. What it comes down to though is exactly what you are trying to do.
So do this for me and I can probably bang out a quick modification to the firmware for you: Explain your setup, what you want to do, what you are measuring, and what you expect to happen.
For example, I have and indoor setup with artificial light that needs to be watered. I would like to water once the plants have been exposed to about 4 hours of light. I am measuring the time the lights are turned on. I want the water to only run every four hours of light exposure, even though it changes daily.
Or my outdoor greenhouse needs water whenever the sun has been shining on it for x number of hours. Clouds would change the measurement a little but estimating total exposure would be fine.