Found this on the Opel owners club strangely
The solar sensor is a photoresistor that changes resistance when exposed to light and is connected to the input stage of the A/C amplifier. The round, tinted plastic cap that snaps on top of the sensor is a light filter that is there to calibrate the solar sensor to the auto A/C system, basically by controlling how much light intensity is required to produce the proper output voltage signal from the solar sensor during daylight. The auto A/C system uses the solar sensor to sense how much radiant heat from sunlight is coming through the windshield and heating up the interior of the car while sitting in a parking lot, driveway, etc.. You've all probably noticed that if you get into your car after it's been sitting for awhile in the sun and the interior is scorching hot that turning on the blower switch to "Auto" immediately causes the blower to run at full speed for a period of time regardless of the temp. control setting. This is done to help purge the interior of stagnant hot air and to help cool down the interior before the in-car and ambient temp. sensors take over and sample the interior and exterior air temp.. "I think" the solar sensor is responsible for this function of the auto A/C system
