Measuring darkness as a event trigger?

My HA system is built around the OmniPro II. I am interested in measuring the degree of darkness outside as a trigger for selected actions. As an example, I can turn on landscape lights based on time of day, sunrise or other calculated events. However, I would like to turn on my landscape lights earlier than normal if heavy clouds makes it darker earlier than normal.
I am thinking that I may be able to use a light sensor that is available for landscape lights as a way to determine darker conditions. However, I think they are an open/closed circuit based on factor settings and I don't know if I can adjust those to fit my desires. Any other product on the market that I could read the degree of darkness and make decisions based on the reading?

You should be able to do with a CdS photocell. Check out the following links for some more info:

AC power on/off sensor, A sensor module with variable resistance
How to monitor the status of a device using its status light

I had a few of these directly connected to the Elk M1 inputs, and used rules to check the value every x seconds and trigger tasks in certain conditions.