Greenlee TG-1000 Manuel D'instructions page 8

Distance from Target
Thermometers 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 thermometer will pick up the temperature of the back-
ground, 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.
Distance to Spot Size
The distance to spot size is a ratio that provides the appropriate distance to the target.
The nominal distance-to-spot-size (D:S) ratio for TG-1000 is 12:1.
8
Distance (D) to Spot size (S)
D:S=12:1
Nominal Spot Diameter at 90% Energy
1 2