Is there a straightforward formula that expresses the specified area of a surface receiving (varying strengths of) sunlight to be concentrated to a specified, much smaller area, please? For instance, were a twelve-inch receiving area concentrated to, say, half-an-inch, on an ambient, continuous 25C sunny day, what would be the temperature rise in the target one-inch area?

Variables: the amount of concentration (surface area to target area ratio) and the "sunniness" of the time of day, spread from dawn til dusk.

Thanks in advance, and apologies too - I know I ramble!