THT100 - THT200 - THT300 - THT400
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5.6 CHARACTERISTICS OF THE LENS
The instrument is provided with a 9mm focus lens (THT300), 7.5mm (THT200,THT100) or
13.5mm (THT400). The theory of operation is reported below, based on the 9mm lens.
Definitions
➢
FOV (Field Of View)
→
Visible range
→
represents the maximum angular opening
(expressed in degrees) the instrument is capable of managing at a certain distance.
➢
IFOV (Instantaneous Field Of View
)
→
represents the smallest detail within the FOV,
that the instrument can detect at a certain distance. It is expressed in radians by the
following formula:
IFOV (theoretical) = (size of sensor's pixels) / (focus length of the lens)
➢
D:S ratio
→
Ratio Distance / Cross-section of the target object
→
it provides
information on the maximum distance between the instrument and the target having a
certain cross-section (spot) in order to obtain correct temperature measurements. It is
expressed as:
D:S (theoretical) = 1 / IFOV (theoretical)
Case of instrument THT300
FOV = 41.5° (Horizontal) x 31.1° (Vertical)
Focus lens = 9mm
Size of sensor's pxls = 17µm
IFOV (theoretical) = 17µm / 9mm = 1.89mrad
D:S (theoretical) = 1 / IFOV (theoretical) = 529:1
Fig. 48: Representation of the instrument's field of view FOV
➢
Typically, the IFOV parameter (real) is always considered as approx. 3 times higher
than the theoretical one, therefore the D:S ratio (real) is approx. 3 times lower than the
theoretical one. This means that, for THT300:
D:S (real) ~ 176:1
