Greenlee THH-500 Instruction Manual page 9

Distance from Target
Temperature sensors measure the surface temperature of an object by sensing its emitted,
reflected and transmitted energy. The sensor collects this energy from the area being
measured (spot size). The spot size varies as the distance to the target changes. The farther
the distance from the from the area being targeted, the larger the spot size.
When determining the appropriate distance from the object, the spot size must be somewhat
smaller than the target area. When accuracy is critical, the spot size should be half as large as
the target. If the spot size is larger than the target size, the thermometer will pick up the
temperature of the background, resulting in inaccurate readings.
Object 1 is the correct distance from the unit. Note the target is slightly larger than the spot
size being measured.
Object 2 is too far away. The thermometer also measures the background because the target is
smaller than the instrument's spot size. This can result in an inaccurate reading.
Note: The distance-to-spot size ratio (D:S) is outlined in the diagram on the battery
compartment cover.
1
25 mm @
150 mm
D 6
=
S 1
1" @
6"
50 mm @ 100 mm @
300 mm 600 mm
S
2" @ 4" @
12" 24"
D
THH-500
2
9