By Ron Briars, Product Manager, Ircon, Inc.
Reprinted with permission of Ircon, Inc.
E-Slope
Reflections
Small Targets
Thick Oxide
Physical Characteristics
Comparison with Less Sophisticated Two-Color Measurement Techniques
An infrared (IR) thermometer functions like a camera—if the lens is partially obstructed, the resulting image will be underexposed on the film. Similarly, partial obstructions in the Field-of-View (FOV) of an infrared thermometer will cause the thermometer to read incorrectly.
Incorrect temperature readings result when the measured object does not completely fill the FOV of the IR thermometer or when the IR energy emitted by the measured object is attenuated constantly or intermittently before it reaches the sensor. Examples include:
- Partial attenuation—a dirty lens or dirty IR window
- Intermittent attenuation—smoke, steam, or dust between the sensor and the measured object
- Partial obstruction—an induction heating coil partially blocking the view of the object
- Intermittent obstructions—clumps of material in a mixer or kiln falling through the FOV
- Low or changing emissivity of the measured object—changes in alloy or surface condition.
- Small objects (too small to fill the FOV of the IR thermometer) or moving objects (wire for example) that are difficult to keep in the FOV.
When an infrared thermometer "looks" at an object, it measures the intensity of the radiant energy from the object within its FOV. To obtain accurate readings from a "single-color*" IR thermometer, the following conditions must be met:
- the object must fill the FOV of the IR thermometer
- IR energy emitted by the object can not be attenuated by something (e.g. smoke, moisture, dust) between the object and the IR thermometer
*["Single-color" or single-wavelength IR thermometers are simple brightness thermometers that utilize a single waveband for measurement. The resulting temperature reading is based on the intensity of a single signal.]
Sometimes this is not possible. The object does not fill the FOV, an obstruction cannot be moved or the smoke or dust will always be present in the process area. In these cases the energy from the measured object is attenuated or partially blocked and the resulting temperature reading is incorrect—with a single-color thermometer.
A two-color** or "ratio" thermometer can usually solve these problems. While a single-color thermometer needs a clear unobstructed view of a target, some partially obstructed targets can be accurately measured with a two-color thermometer.
*["Two-color" or dual-wavelength IR thermometers utilize two separate wavebands for measurement. The resulting temperature reading is based on the ratio of the intensities of two signals that are attenuated equally by most ObstructionsÑhence, the ratio stays the same for a given temperature.]
A two-color thermometer consists of two single-color "brightness" thermometers in the same package. Using two detector layers, a two-color thermometer measures the target at two separate wavebands—simultaneously. The signals from the two detectors are then processed as a ratio. The calibration curve is based on the ratio of the two signals, which will be very accurate, as long as the partial obstruction or attenuation affects each of the wavelengths by an equal amount. If the signal loss in each waveband is 50%, the ratio is the same as for unattenuated signals and the two-color thermometer reports an accurate temperature!
Two-color thermometers are limited in regard to the amount of signal that can be lost. This is referred to as the "reduction ratio" and can vary from 5:1 (80% signal loss, i.e. 80% attenuation) to as high as 25:1 (96% attenuation). In other words, 96% of the signal could be lost and the two-color thermometer would still produce an accurate temperature reading.
If the two-color thermometer has a reduction ratio of 20:1, (maximum allowable signal loss is 95%) and the measured object has an emissivity of only 0.25 (75% of the signal is already lost) then only 20% more signal can be lost from all other factors combined. When the reduction ratio limit is reached, the instrument will indicate that the readings are invalid. An invalid reading means that the signal is so low that a repeatable result is not possible. Rather than indicate erroneous readings, the instrument provides an alarm of its inability to compute the temperature.
In addition, some applications require adjustments for "non-gray" behavior of the measured material. A good example is the measurement of molten metals where the emissivity of the material varies with wavelength. Even when a two-color thermometer is used to measure the temperature of some molten metals, the resulting temperature reading may be incorrect because the pre-programmed ratio or "slope" of the two signals is incorrect.
E-Slope To compensate for this type of error, most two-color thermometers have a user-adjustable "E-slope" feature that allows the user to set the correct emissivity slope for the material being measured. The E-slope control modifies the ratio of the two signals to correct for the unequal spectral emissivities of the target. When measuring a material or alloy type for the first time, the E-slope value is determined by adjusting the E-slope control so that the instrument reading matches the temperature reading from an accurate contact type device. The E-slope value is then known and used for that particular material. Once the E-slope is set, the problems of smoke, steam, dust, and so forth are handled by the instrument.
Similar errors will occur if an improper window material is used. Pyrex windows, for example, are slightly colored and transmit differently in the two spectral regions. Once again, the E-slope control can be used to correct this problem.
Two-color thermometers may solve many measurement problems, but there are still some application conditions to be considered. These include reflections, small targets, scale and physical characteristics of the target.
- Reflections
Two-color infrared thermometers do not solve the problems caused by reflected energy. For example, steel in a hot gas-fired oven may be at 900°C (1650°F) while the oven walls are at 1100°C (2000°F). The hot steel is completely surrounded by a source hotter than the steel.
A two-color thermometer measures the composite radiant energy from the billet surface for each spectral channel, computes the ratio of these two signals, and displays the temperature equivalent of this ratio. Unfortunately, but as expected, the indicated temperature is neither that of the steel nor of the background. It is a composite of the energy from the billet and the reflected energy from the oven walls. How much was contributed by each source? Adjusting the E-slope will not correct this condition. The only way to correct reflection problems is to remove or block the source of the reflection.
- Small Targets
With a two-color IR thermometer, the measured object does not have to fill the entire FOV of the thermometer to obtain accurate temperature readings. Hot wires, hot rods, and molten glass streams are usually very narrow and do not fill the FOV. However, a very important factor when measuring small targets is; what is the temperature of the background or the objects that fill the remainder of the FOV? If the background is another hot object (similar or higher in temperature than the measured object) the ratio algorithm will be incorrect and an incorrect reading will result. If the background is much cooler than the measured object, the energy contributed by the background is negligible and will be processed as a signal that is attenuated equally in both channels. The resulting temperature reading from a two-color ratio thermometer will be correct.
- Thick Oxide
Two-color thermometers are often used in steel mills and foundries. A common misconception is that a two-color thermometer can "see through" the scale. Unfortunately, this is not true. If the scale fills the entire FOV, the instrument will simply measure the temperature of the scale (not the metal) and honestly report the results. The scale is usually cooler than the metal so the reading may seem inaccurate. However, an accurate temperature reading of the metal can be obtained, even if only a small glimpse of the bare hot metal can be "seen" by a two-color thermometer. Again, the small area of bare metal does not have to fill the FOV to obtain an accurate reading, if the reduction ratio for the instrument is not exceeded.
- Physical Characteristics
A two-color thermometer is a sophisticated sensor and signal processing system. Its unique combination of high performance electro-optical components allows it to accurately measure object temperatures when the energy emitted by the object is partially blocked or attenuated.
Two-color thermometers measure the energy emitted by an object in two adjacent IR spectral bands—simultaneously. They typically employ a layered "sandwich" detector with two IR filters to obtain the two distinct signals. This requires an optical system capable of focusing IR energy from two different wavelengths onto the same focal plane. When two different wavelengths pass through a normal lens, they bend differently. The longer wavelengths do not bend as much as the shorter wavelengths. Consequently, with a normal lens the two different wavelengths will not focus together—one will always be out of focus!
Two-color thermometers employ achromatic lenses, which solve this problem, because they bend each wavelength the same amount. That means that an achromatic lens can focus the image of the target from the two different wavelengths onto the same focal (detector) plane.
Through-the-Lens (TTL) sighting is a very important operating feature for two-color thermometers. TTL sighting allows the operator to view the measured object and determine:
- if the measured object fills enough of the FOV (as defined by the circular reticle in the viewfinder)
- if the background or objects in the background could be hot enough to cause an inaccurate reading
Comparison with Less Sophisticated Two-Color Measurement Techniques Sandwich detector systems provide faster and more accurate temperature measurement in comparison to instruments that use the filter wheel technique
- Sandwich detector systems measure the two spectral signals simultaneously and compute the ratio of the signals instantaneously, immune to inaccuracy due to phase shift.
- With filter wheel schemes, the two spectral signals are measured sequentially and then ratioed! The spectral signals were measured at different times and the ratio computation was displaced in time, resulting in slower operation and potential error.
- Sandwich detector systems produce superior temperature measurements under dynamic conditions, such as measuring the temperature of a moving object that is not always the same percentage of the FOV, because both detectors view and measure the same spot on the target at the same instant.
