THT70n
EN - 43
5.7 CHARACTERISTICS OF THE LENS
The instrument is provided with a 22mm focus lens. The theory of operation is reported
below, based on this 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 THT70n
FOV = 17° (Horizontal) x 12.7° (Vertical)
Focus lens = 22mm
Size of sensor's pxls = 17µm
IFOV (theoretical) = 17µm / 22mm = 0.77mrad
D:S (theoretical) = 1 / IFOV (theoretical) = 1298:1
Fig. 52: 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 THT70n:
D:S (real) ~ 433:1
