Pyrometer- Handbook - Contika

7. Spot Size and Measuring Distance
The optics of a pyrometer transmit the image of a section
of the target area of the measured surface to the detector.
This section is called the spot size 8). By using differently
shaped apertures in the pyrometer the spot size may be
round or rectangular. The laws of optics mean that the
image enlarges as the distance from the lens increases.
This is common knowledge in photography. It is possible
to measure small objects with pyrometers which are
designed for use over short distances. The larger the
distance between pyrometer and object, the larger the spot
size diameter.
Pyrometers are available with two types of optics:
1) Fixed Optics
2) Optics with variable focus
With fixed optics the minimum diameter of the spot size
requires a fixed distance for measuring; the nominal
measuring distance. A sharp image on the detector is the
result. A different optical variant, with different measuring
distances and spot sizes, enables the operator to use the
instrument correctly for various applications.
These optics allow the pyrometer to be focused on the
target from various distances. This kind of equipment is
preferred for portable pyrometers. The diameter of the spot
size can be calculated by using the distance to target ratio,
for example, 100:1. The resulting value expresses the
distance to diameter ratio. But there are also tables and
spot size diagrams which can be used to determine the spot
size diameter.
8) or spot size diameter
Pyrometer Handbook
fixed optics
optics with
variable focus
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