Greenlee TPIR-J Instruction Manual page 8

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 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 sensor will pick up the temperature
of the background, resulting in inaccurate readings.
TPIR Target Spot Sizes
7.5 mm @
25 mm
0.3" @ 1"
Note: The distance-to-spot size ratio (D:S) is outlined in the table on the side of the unit.
8
1
21 mm @
76 mm
14 mm @
50 mm
0.8" @ 3"
0.6" @ 2"
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
2
smaller than the instrument's
spot size. This can result in an
inaccurate reading.