Fizz57
Thanks for that clarification, which changes some long held misconceptions. I thought the inverse square law would operate locally, as well as 'globally'. In other words, attenuation in air will also increase according to the ISL and is much easier to quantify horizontally. Vertically it is more complicated because of the change in density. Of course, the difference between 60k and 40k is miniscule compared to the distance from Earth to the Sun, but given that a finite amount of radiation arrives at the limits of atmosphere, it will still decay from that level according to the ISL, as well as be the action of the atmosphere?
As for my TLD, my eyes are well and truly opened. I simply hadn't realised it was so inaccurate when measuring 'background' effectively. I know the old film badges had to be specially calibrated for places like Aberdeen, with its high concentration of granite buildings.
Thanks again for the information.
Roger.