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Introduction

Non-contact temperature measurement is based on an optical measuring technique. The extent to which the measurement result is influenced by pyrometer characteristics is often underestimated. When considering the measurement uncertainty of various instruments, one often merely compares this parameter as specified in the data sheets. Serious measurement errors, however, can also result from the use of inferior, poorly selected or incorrectly adjusted optics. This article aims to explain the principles and effects of imaging errors, as well as explain the specifications of the optical parameters of pyrometers. An easy way for the user himself to control the quality of the optics will be introduced.

Principles of Optical Imaging Errors
Spherical Aberration

The focus is only perfect for light beams which pass through the center of the lens. Light beams passing through the edges of the spherical lens will cause the image to be blurry. In optical systems consisting of several lenses, spherical aberration can be reduced by the clever arrangement of a combination of lenses.
(1) single lens made of crown glass
(2) Achromat cemented for the visible range	
(3) Achromat cemented for the infrared range
(1) single lens made of crown glass
(2) Achromat cemented for the visible range
(3) Achromat cemented for the infrared range

Chromatic Aberration

The focal length of an uncorrected lens depends on the light’s wave-length. Light, or rather radiated energy, of differing wavelengths will focus on slightly differing focal points. The inspected object will be imaged at various distances to the lens and the image will appear cir-cled by a rainbow-coloured halo. This chromatic aberration can be greatly reduced by the use of lenses which have been corrected for two wavelengths (achromat) or three wavelengths (apochromat). The lens materials are selected so that the lens aberrations mutually compen-sate each other.

Specification of Pyrometer Optics

In a data sheet, the pyrometer’s optics will usually be specified in one of two ways: either in terms of the target spot size at a certain distance, or in terms of the ratio of the measuring distance to the target spot diameter. The target spot size is stated with reference to a fixed percentage of the maximum amount of radiated energy which can be absorbed at that target spot. 100% refers to an infinitely large radiating body. The specified target spot size is typically based on either 90%, 95% or 98% of the maximum absorbed energy.
Target spot based on  90, 95 and 98 % of the maximum absorbable energy
Target spot based on 90, 95 and 98 % of the maximum absorbable energy
At a greater percentage of radiated energy, the correlating target spot size will increase as well. Therefore target spot sizes can only be truly compared to each other when they refer to the same percentage of absorbed energy. Unfortunately, not all pyrometer manufacturers state the percentage of radiated energy, or they declare a quite small percent-age in their data sheets in order to suggest a smaller target spot. Defining a target spot size in this way is deliberately misleading. Furthermore, the specifications of some manufacturers do not take tolerance values of the lens into consideration.

Effects of Optical Errors

Pyrometers can be distinguished between instruments which have focusable lenses and fixed focus optics. The only way to obtain a sharp image of the target spot is by altering the distance between the pyrometer and the measured object. When operating the instrument beyond the focus range the infrared radiation might not be evenly absorbed by the sensor.

The infrared energy which the sensor detects will not be uniform within the entire target spot area. Temperature changes in the centre of the spot will have a greater effect on the reading than temperature changes detected at the edges. Incorrect focusing especially of extremely small measurement objects which are barely larger than the pyrometer’s target spot can lead to significant measurement errors. Even when the pyrometer peers through sighting tubes, inspection glasses, kiln walls or other openings, a poorly adjusted optical system or improper focusing will often result in a constriction of the cone of vision, and give incorrect measurement readings. When inferior optics are used to measure objects distinctly larger than the pyrometer’s target spot, a change in the distance to or the size of the target spot will result in a change in the temperature reading. This size of source effect is to a greater or lesser extent a source of error for any pyrometer. This effect can stem from imaging errors of the optical system, stray light and reflec-tions, as well as diffraction due to the wave nature of light.

Measuring at shorter wavelengths reduces the size of source effect. The effect can also be minimized by an accurate correction of the optical imaging errors, the use of antireflection-coated optical surfaces, and avoiding stray light and reflections in the instrument. In practice, the pyrometer user can reduce errors to a minimum by accurately focusing at the proper distance.

Depending on the object’s tempera-ture, the infrared energy emitted from the object is primarily in the wavelength range from 0.6 – 20 µm, in other words usually beyond the wavelength of visible light. First of all, this means that the pyrometer’s optical system must be adjusted according to the pyrometer’s specific wavelength range.
Comparing point spread functions of focused and defocused optics
Comparing point spread functions of focused and defocused optics
If the user wishes to visually focus the instrument, then the optics must be arranged in such a way that optical imaging errors for both the visible and the infrared wavelengths are corrected in equal measure. Single lenses are only adjusted for one specific wavelength. Thus the focal points of the infrared radiation and the visible radiation will not coincide. When the pyrometer is focused using the sighting device, it might still not be in sharp focus as far as the infrared rays are concerned. Only a fairly complex arrangement of two or three lenses can accomplish the task and eliminate errors as far as possible. The PZ 40 AF 90 pyrometer, for example, has high-grade precision optics with broadband antireflection-coated lenses. This instrument makes it easy to accurately measure the exact temperature of filaments having a target spot diameter as small as 0.3 mm.

Examining the Imaging Quality

There is a simple way for the user to check the imaging capability of the pyrometer’s optical system. Aim the pyrometer at a defined source of radiation. The surface area of the radiating body should be noticeably larger than the pyrometer’s target spot. Now place an open iris diaphragm between the pyrometer and the radiating body, spaced at the pyrometer’s focal length (a).

Determine the temperature at an emissivity setting of ε = 1. The temperature reading itself is irrelevant, what matters is that it be considerably above room temperature. Then adjust the emissivity to, e.g. 0.95. Subsequently, reduce the aperture diameter of the iris diaphragm as far as needed in order to get the same previous temperature reading. Repeat this test with other comparable pyrometers, using the same previously chosen emissivity settings respectively. In this manner, the optical properties and imaging capabilities, including the effects of lens defects, can be evaluated and compared.

For pyrometers with a spot light sighting system or through-the-lens sighting, this test is also useful for determining if the focused target spot in the infrared range and the focused target spot in the visible range are identical, in other words whether or not the marked target spot size coincides, in size and position, with the spot actually measured by the pyrometer.
A setup for testing optical properties
A setup for testing optical properties

Summary

When choosing a pyrometer, one should not only consider the metrological parameters and features of an instrument. It is equally important to closely compare the quality characteristics and capabilities of the optical systems. Unfortunately, the information which can be obtained from the brochures of many manufacturers is often inadequate. To truly judge such pyrometers, detailed questions must be asked, such as precisely how the specifically stated target spot was obtained, and whether lens imperfections and alignment tolerances were taken into account in the claims made. A true comparison between pyrometers is only possible if their stated specifications regarding the optical systems have been defined or derived in identical fashion. To exclude all possibility of doubt, it is best to perform the test described above to double check the statements made in the brochure. After all, what good is a pyrometer that claims a measurement uncertainty of less than 1 %, however due to the use of poor quality lenses, produces considerable temperature reading errors?

Basics

Optical factors which influence non-contact temperature measurement

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