A well designed infrared pyrometer is inherently a very accurate measurement device, but because it measures from a distance, the focus and field of view (FOV) play an important part in the quality of the measurement, unlike other temperature sensors. The FOV is an invisible cone that extends from the lens out to infinity.
The FOV ratio is the diameter of the cone (Spot size) at a given distance from the lens. It is expressed in inches as a ratio e.g. 20:1, 60:1 etc. So if a 60:1 FOV pyrometer is focused at a distance of 120”, it will gather energy from a 2” dia. area on the object being measured. It will see only a small percentage of energy outside that circular FOV area based on the quality of its optics. The equation to determine spot size is (Focus Distance)/FOV = Spot Size.
To obtain an accurate temperature measurement, the FOV must be filled, smaller than the target at whatever distance the pyrometer is from the object being measured. The exception to this rule is the Two Color or Ratio Pyrometer which can measure accurately when only part of the FOV is filled. Listed below is an explanation of operation for one and two color pyrometry.
1. Depend on the emissivity of the target:
2. Measure the average temperature within the field of view:
1. Independent of emissivity of the target - (If emissivity changes proportionally, for each single color wavelength )
2. Measure the weighted peak temperature within the field of view.
3. Limited low temperature measurements to about 300° C
4. Higher cost solution compared to Single-Color.