Measurement distance and measurement diameter
Thứ Sáu,
12/12/2025
You need to know the spot diameter to be measured according to the measurement distance between the object you want to measure and the radiation thermometer. The optical path diagram is used at this time.
Explaining the optical path diagram of our IN5+(Optics300) as an example. The optical path diagram shows the measurement distance and measurement spot diameter as shown in the figure.
When the measuring range is 300mm, the measuring spot diameter is 6mm, which can be seen at a glance. Here, the focal length is defined at the position where the measurement spot is narrowed down on the figure.
Measurement Distance and Spot Diameter
In this optical path diagram, the spot-diameter is illustrated until the measured distance is 1000mm. In fact, there are no restrictions on the measurement distance for the radiation thermometer. Measurements can be made even at distances higher than this. This section describes how to calculate the measured spot diameter at any measurement distance. (This is an approximate calculation. It should be used as a rough guide.) Calculated with IN5+. Scan the following numbers from the figure above: a :Focal length =300mm M: spot diameter =6mm D: aperture diameter =15mm (lens position: a=0)
1）For the measured spot-size M1 for distant a1 than the focal length,
M1 = a1 a ( M + D ) – D
2）For the measured spot-size M2 for distant a2 than the focal length,
M2 = a2 a ( M – D ) + D
Calculate with.
In this example, the measured spot diameter is obtained when the measured distance is 2000mm. When a1 = 2000, it is a1 > 300, so from M1 expression
M1 = 2000 300 x ( 6 + 15 ) – 15 = 125
Therefore, the spot-diameter is 125mm.
Securing the Field of View in the Optical Path
It is a matter of course to ensure a field of view in that range with respect to the optical path of the radiation thermometer. In general, in order to ensure accuracy and reproducibility, it is recommended to secure a field of view of approximately 1.5 times the optical path spot diameter.
Make sure that there is no obstruction to the field of view within this range.
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