Understanding Infrared Thermography Reading 3

Questions & Answers
Subject: Answer this web queries from: http://www.thesnellgroup.com/community/ir-talk/f/9/p/1402/5433.aspx
wonder if anyone can help me here. I am studying for my employer's Level 2 certification exam and I am using
the ASNT supplement booklet to help. They ask a few question about IFOV and spot size calculation and I do
not quite understand how they get the answers. basically it is not the answer I want but how they got to the
answers.
Question #1: A camera has an IFOV of 1.9 mRad. What is it's theoretical minimum spot size at a distance of
100 cm? Answer is: 0.19 cm (What formula is used for this and are there any units conversion like mm to cm or
mRad to something else?)
Question #2: The IFOV measurement of a radiometric system is 1.2 mRad. What is the maximum size object
this system can accurately measure at a distance of 25 m? Answer is: 3 cm (now clearly there are unit
conversions going on here from meters to cm. So how is it done?)
Question #3: You are looking at an electrical connection 20 m in the air. What IFOV measurement is required to
accurately measure the temperature on the 2.54 cm (1 in.) head of a bolt? Answer is: 1.25 mRad (I know it's
just a matter of transposing the formula, but again there is units changes and I do not know the formula to apply)
Last question: Using an IR system with an IFOV measurement ratio of 180:1. What is the smallest size object
you can accurately measure at a distance of 3m (3.3 ft)? Answer is: 16.6 mm or (0.65 in).
NOW this one I kind of figured out using: 1/180 = 0.0055 & 3 m = 3000mm therefore 0.0055 x 3000 = 16.5
Let me know if you all know how to do these problems. I think all I need is the formula and an understanding
when and which units to convert.
Charlie Chong/ Fion Zhang