D:S ratio: D / S = 90
The number “90” means that at a distance D equal to 1 meter the unit will measure a circle of diameter S equal to 1.1cm (1meter divided by 90); at distance D equal to 2 meters it will measure a circle with diameter S equal to 2.2cm and so on in a linear progression.
Optical Diagram
The optical diagram shows the graphical representation of the optical ratio. The optical path of the infrared radiation is a cone that has its vertex at 61cm (24”) away from the front of the unit, and progresses at the same ratio towards infinity. The circle diameter can be calculated by dividing the distance by 90 at any point in the centerline of the cone. Below it is presented the diameters S for three different distances: 61cm (minimum distance), 2m and 5m. There is no maximum distance. Although, in practical terms, accurately measuring temperatures of targets further than 5m requires fairly big objects.
Figure 29. Illustration of Distance to Target / Spot Ratio
The explanation of optical resolution is key for the understanding of the next concept, vital for accurate temperature measurement in quantitative inspections.
Field-of-View
Make sure that the target is larger than the spot size that the unit is measuring. The smaller the target, the closer you should be to it.
July 1, 2003 | - 47 - | Version 1.0 |
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