can anybody suggest me an alternative for Honeywell SAF25 which is used for measuring the solar irradiance?
There are lots of low cost light sensors on the market. Photoresistors cost about 5 cents each.
You will have to calibrate them yourself if you need some specific spectral sensitivity.
What type of interface do you need?
What accuracy do you need?
What spectral sensitivity do you need?
What levels of light do you need to measure?
What speed do you need?
