Greenlee TG-600 Manuel D'instructions page 8

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Distance from Target
Infrared 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 background,
resulting in inaccurate readings.
1 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 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 ratio is stated in a
nominal value for the focus distance, which is 750 mm (30")
from the unit. Use the illustration shown here or the decal on
the side of the unit to determine the distance-to-spot-size ratio
for each application.
The nominal distance-to-spot-size (D:S) ratio at the focus
point is 12:1.
D:S = 12:1 at Focus Point
78 @ 1200
mm
44 @ 600
31 @ 300
3.1 @ 48
1.8 @ 24
1.2 @ 12
inch
Nominal Spot Diameter at 90% Energy
8