Single-Color vs Two-Color Pyrometers

Infrared pyrometers come in two main types: single-color and two-color pyrometers. The two-color variety is also known as ratio or dual-wavelength pyrometers. Both of these devices are designed to detect infrared radiation emitted by hot objects and convert that into a temperature reading, but they work on slightly different principles. A single-color pyrometer focuses on measuring thermal radiation at a specific wavelength or a narrow band. It essentially gauges how much infrared light is coming from the target at that wavelength and infers the temperature based on that intensity. However, with a single-wavelength measurement, the pyrometer can’t differentiate between temperature and emissivity. Emissivity refers to how effectively an object radiates heat compared to an ideal blackbody. If the sensor picks up a weaker signal, it is because the object is cooler, or because it is a poor emitter. To get an accurate temperature reading, the user needs to provide the single-color pyrometer with the emissivity of the object or calibrate it accordingly. In practice, this means the user inputs an emissivity setting based on the known emissivity of the material or measurements taken. While needing to know emissivity can be seen as a limitation of single-color pyrometry, it’s not as daunting as it seems. Many materials have established or consistent emissivity values under certain conditions. If the industrial process is stable, a single-color pyrometer can be incredibly accurate and reliable. When the emissivity is known and the measurement path is clear, a single-wavelength pyrometer can perform just as well as any other option. These devices are straightforward, cost-effective, durable, and widely utilized across various industries for these very reasons.

On the other hand, two-color pyrometers measure infrared radiation at two different wavelengths at the same time. The electronics calculate the ratio of the two measured intensities and use that ratio to determine the temperature. The concept is quite clever: by comparing two wavelengths, the measurement can theoretically cancel out the emissivity factor.

Each detector picks up a specific intensity of radiation, which depends on the temperature of the object and its emissivity at that wavelength. A two-color pyrometer works by comparing the ratio of these two signals. Since thermal radiation from a hot object rises with temperature, this signal ratio is primarily linked to the object’s temperature. If the emissivity at both wavelengths is the same, it cancels out in the ratio, allowing the result to reflect only the temperature. The device then uses this ratio to determine the temperature, often giving what’s known as a “ratio temperature.” Essentially, the two-color pyrometer is tackling two equations, one for each wavelength, to find two unknowns simultaneously. This method gives the impression that it can adjust for unknown emissivity, which it can in some situations, but only under specific conditions.

If the emissivity is identical at both wavelengths, it will drop out of the ratio calculation, enabling the pyrometer to report the true temperature without needing to know the emissivity. In simpler terms, a two-color system aims to determine temperature without requiring the input an emissivity value, which is a significant advantage. Manufacturers often market these devices as “emissivity independent” or capable of measuring “true temperature” directly. It’s crucial to understand that two-color pyrometers typically assume or require that the emissivity at the two wavelengths remains consistent relative to each other. In practice, many ratio pyrometers come with a setting called “slope” or emissivity ratio, where users can enter the expected ratio of emissivities. If this setting is at 1.0 (a common default), this assumes the object behaves like a “gray body”, which means the emissivity is the same at both wavelengths. If the emissivities differ, adjustments to the slope factor are required.

However, the main point to remember is that two-color pyrometry doesn’t eliminate emissivity from the equation. But the key takeaway is: two-color pyrometry doesn’t eliminate emissivity from the problem; it only reduces the problem to assuming a constant ratio of emissivities. The method hinges on that ratio staying fixed.

The Myth of Emissivity Independence (and Why It’s a Myth)

It’s worth directly addressing the common stereotype: “Two-color pyrometers are always better because they handle emissivity automatically.” This is a misconception.

A lot of real materials have an emissivity that changes with wavelength. This means they aren’t perfect gray bodies; their surface characteristics make them emit infrared radiation more effectively at certain wavelengths than at others. If the emissivities are mistakenly assumed to be the same, the two-color pyrometer could give an incorrect temperature reading.

Even if the right emissivity ratio (slope) is considered at the start of a measurement, changes in the material’s surface condition—such as oxidation, finish, or temperature fluctuations—can alter the spectral emissivity, causing that ratio to drift. Consequently, the two-color reading will also shift.

So, both single-color and two-color pyrometers are influenced by emissivity: single-color by the absolute emissivity value, and two-color by how stable the emissivity ratio is.

The system accuracy of each channel must be considered alongside slope changes. Since the ratio temperature depends on the individual’s temperature, the ratio temperature equation is calculated by assuming a deviated temperature reading from a single channel and evaluating it. Single-color uncertainties propagate through the nearly linear ratio pyrometer’s emissivity ratio sensitivity equation, which can lead to a higher deviation in the ratio temperature compared to the same device operating in single-color mode. Two-colour pyrometers may amplify minor channel mismatches and small errors in the assumed emissivity ratio, a consequence of the effective wavelength. Selecting closely spaced wavelengths can stabilize the emissivity ratio close to the assumption that they equal one, but this also raises the effective wavelength, increasing the temperature’s sensitivity to slope and inter-channel differences. As rule of thumb: two-colour temperature can shift markedly: for approximately every 1 °C difference between channels, the ratio result can move by up to ~10 °C.

Figure 1 displays a ±10% change in the emissivity ratio causes the two-color reading to deviate from the proper temperature, particularly at the lower end of the temperature range. In addition system accuracies are considered. At higher temperatures, the effect of the emissivity difference is less pronounced because the absolute signal levels are significantly higher, and the ratio is somewhat less sensitive in that regime.

Real-World Considerations: When Ratio Pyrometers Shine (and When They Don’t)

There are certain scenarios where ratio pyrometers offer clear practical advantages despite their emissivity sensitivity. These mostly relate to difficult measurement conditions, such as dirty optics, obstructions, or targeting issues, as well as some cases of unknown emissivity:

• •  Dirty, Dusty, or Obstructed Views: One big advantage of the two-color method is its ability to compensate for uniform signal losses in the optical path. If something partially blocks the infrared signal equally at both wavelengths, for example, a dirty viewing window, smoke, dust, or steam that dim both channels similarly, a ratio pyrometer will largely cancel out that attenuation. The displayed temperature remains correct because both detectors see the same percentage drop, so their ratio stays the same. Single-color pyrometers, by contrast, would simply see a weaker signal and would under-read the temperature in those cases. This makes ratio pyrometers well-suited for harsh environments like industrial furnaces, combustion chambers, or smokestack monitoring. Important: if the obstruction is not equal for both wavelengths – e.g. dust that absorbs short wavelengths more than long ones – then the ratio method will not perfectly cancel it and will introduce error, just as if emissivity changed. So the obstruction has to be “gray” for the compensation to work.

• • Varying Target Size or Partial Field of View: Two-color pyrometers are generally less sensitive to the target not filling the sensor’s field of view, as long as both wavelengths see the same portion of the target. For example, when measuring a small, moving piece of metal, and at times it only covers half of the measurement spot, a single-color pyrometer’s reading would drop. A ratio pyrometer, however, will maintain a consistent ratio as both channels receive half the signal, allowing it to still read the correct temperature even if the target only partially fills the view or moves around slightly. This makes ratio pyrometers useful for moving targets, small objects, or situations where alignment isn’t perfect.

• • Rapidly Changing or Unknown Emissivity: If a process has truly unpredictable emissivity changes that cannot be practically accounted for, a two-color pyrometer might at least reduce sensitivity to those changes compared to a single fixed-emissivity setting. A classic example is measuring molten metals or materials that change phase – their emissivity might be in flux. A ratio pyrometer assuming a constant emissivity ratio might not be 100% accurate if emissivity swings, but it could be closer to the mark than a single-color reading without frequent emissivity.

A two-color pyrometer delivers the best results when the target has equal emissivity at both wavelengths and any obstructions reduce both signals equally—such as with a “gray” target viewed through uniform smoke. Under these conditions, the ratio method can outperform single-color devices.

However, it’s not a universal solution. On non-gray surfaces, the slope behavior still needs to be known or adjusted for the emissivity difference. Likewise, if a filter or viewing medium affects one wavelength more than the other, the ratio reading will be distorted—just as in a single-color pyrometer, though for a different reason. The method remains bound by the physical properties of the target and measurement path.

Practical Trade-offs: Precision, Repeatability, and Simplicity

Given the above, how to decide between a single-color and a two-color pyrometer for a given job? It comes down to understanding the measurement conditions and priorities:

• • If the target’s emissivity is reasonably well-known, stable, or can be determined and the environment is clear, a single-color pyrometer is often the best choice. These devices are straightforward – set the emissivity and go.
  • If the minimum temperature or signal quality is important: Single Color Pyrometer have only one detector and one signal, so the noise can be lower and the start up temperature is lower as well. In fact, under identical conditions, a single-color pyrometer can be more accurate than a two-color because it isn’t performing a ratio calculation that could amplify errors.
  • If the target or environment is challenging – unknown or fluctuating emissivity, dirty optics, intermittent view – and a consistent emissivity setting is changing, then a two-colour pyrometer might be worth the extra complexity. They are especially useful in high-temperature applications, where targets are glowing hot, might be moving, and keeping optics clean is tough. Two-color pyrometers often have a higher low-end temperature limit. They come into their own when the conditions meet those ratio assumptions – for example, measuring through a flame or plume that uniformly obscures the view, or looking at a target that emits similarly in two bands. In those cases, a ratio pyrometer can maintain accuracy where a single-color would falter.
  • Maintenance and ease of use: Single-color pyrometers are usually simpler to set up, if the emissivity is known. Two-color pyrometers may require more care – the user might have to calibrate both channels, set the emissivity ratio (“slope”) correctly for the material, and verify that both optical paths are clean and aligned. If one detector gets out of alignment or dirtier than the other, the ratio will drift. Thus, from a user perspective, single-color units can be more forgiving in maintenance. Ratio units might come with more sophisticated optics and alignment tools due to the dual channel nature.

In short, a two-color pyrometer isn’t automatically the better choice. When emissivity is the only concern and can be controlled, a single-color unit often delivers equal or better results with lower noise and uncertainty. But in environments where emissivity or visibility can’t be kept stable, a two-color pyrometer can help reduce—though not eliminate—those problems. As a guideline: use a single-color pyrometer for steady processes with known emissivity and a clear view; choose a ratio pyrometer for dynamic conditions, changing emissivity, or dirty and obstructed measurement paths.

While ratio pyrometers are powerful tools, they are not universally superior. Any commercial two-color systems marketed for “true temperature” measurement will only live up to that claim if the emissivity ratio is well characterized and remains constant. In real-world conditions, if that isn’t true, they won’t yield more accurate results than a properly used single-color pyrometer – in fact they could be less accurate. The key is to match the tool to the application. Sometimes a ratio pyrometer offers no benefit and can even complicate things, compared to a simpler single-color unit.