THT80
EN - 39
5.5 CHARACTERISTICS OF THE LENS
The instrument is provided with a 2.3mm focus lens. The theory of operation is reported
below.
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 THT80
FOV = 50° (Horizontal) x 37° (Vertical)
Focus lens = 2.3mm
Size of sensor's pxls = 17µm
IFOV (theoretical) = 17µm / 2.3mm = 7.4mrad
D:S (theoretical) = 1 / IFOV (theoretical) = 135:1
Fig. 50: 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 THT80:
D:S (real) ~ 45:1
