>
>The purpose of this detector is to control (via a PIC of course)
>electrically actuated sun screens in front of a window of my house.
>

It may actually be easier than you think. Put the sensor "inside"
the house behind the movable sun screens - possibly in a box with
a diffuse translucent cover, pointed toward the window.

Feed sensor to a threshold comparator, and you'll never get more
light than you want. Change to a heat sensor, and you'll not
get more heat than you want.

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