RayTek MI Miniature Infrared Sensor - Manual (Page 22)

Installation
14 MI
5 Installation
5.1 Positioning
Sensor location depends on the application. Before deciding on a
location, you need to be aware of the ambient temperature of the
location, the atmospheric quality of the location, and the possible
electromagnetic interference in that location, according to the sections
described above. If you plan to use air purging, you need to have an
air connection available. Wiring and conduit runs must be
considered, including computer wiring and connections, if used.
5.1.1 Distance to Object
The desired spot size on the target will determine the maximum
measurement distance. To avoid erroneous readings the target spot
size must completely fill the entire field of view of the sensor.
Consequently, the sensor must be positioned so the field of view is
the same as or smaller than the desired target size. For a list
indicating the available optics, see section 3.2
Optical Specifications
on page 6.
The actual spot size for any distance can be calculated by using the
following formula. Divide the distance D by your model’s D:S
number. For example, for a unit with D:S = 10:1, if the sensor is
400 mm (15.7 in.) from the target, divide 400 by 10 (15.7 by 10), which
gives you a target spot size of approximately 40 mm (1.57 in.).