Video and PDF files describing how to draw a simple pyrometer optical diagram (Ircon term: "cone of vision")
The first tool to use should always be the Spot Size Calculator. From time to time it may be useful to make drawing.
It can also be useful for understand how to specify a certain distance and spot size and the consideration around that (90% spot and 100% spot), especially when using sighting tubes or view through apertures or windows into a process.
Also useful for actual and theoretical devices not in the spot size calculator.
Special Note:
Spot Size Calculator 2.9.0.2 allows unique focus distances for standard units - especially useful for working out non-focussed spot sizes for special builds of current sensors