If you’re selecting a pyrometer based only on temperature range, you could be solving the wrong problem. In practice, measurement errors are often caused by changing emissivity, dust, smoke, moving targets, or partially blocked sight paths rather than the instrument itself.
That’s why the debate between single-color and two-color pyrometers continues across industries such as steel, forging, foundries, heat treatment, and glass manufacturing. Both can deliver excellent accuracy, but only when matched to the right application. In this article, we’ll look at the key differences between the two technologies and explain how to choose the pyrometer that will perform reliably in your actual operating conditions.
What Is a Pyrometer?
A pyrometer is a temperature measurement device that measures the temperature of an object without being in contact with it. An infrared ray that is emitted by an object is able to pass through an optical system that includes a lens or fiber optic cable, and after passing through it comes into contact with the transducer, which makes it into a small electrical signal.
There is also electronic equipment within the pyrometer itself, which will amplify, filter, linearize, and correct for the signal, depending on parameters such as emissivity, wavelength, response time, and calibration data. The temperature value is transmitted to a display, PLC, DCS, recorder, or software through 4–20 mA, RS-485, Modbus, relay, or USB. Accuracy mainly depends on the correct emissivity setting, clean optics, proper alignment, suitable wavelength, and correct spot size.
Single Color Pyrometers
A single-color pyrometer measures infrared radiation in one wavelength band and converts it into temperature. It works best when the target surface is clean, emissivity is known, and the object fully fills the spot. Typical wavelengths are 1.0 µm, 1.6 µm, 2–2.6 µm, or 8–14 µm, depending on material. Accuracy is commonly around ±0.3% to ±1% of reading. It is used for billets, forging, induction heating, graphite, ceramics, plastics, rubber, paper, textile, hot rolling, cold rolling and painted surfaces.
Two Color Pyrometers
A two-color pyrometer, also called a ratio pyrometer, measures infrared radiation at two nearby wavelengths and calculates temperature from their ratio. This makes it more reliable when emissivity changes, scale forms, the target moves, or dust, smoke, steam, or vapor partly blocks the view. Typical wavelength pairs are around 0.7–1.15 µm (InGaAs pair) or 1.5–1.6 µm (Si/Si pair), with accuracy commonly around ±0.5% to ±1% of reading. It is used in foundries, molten metal streams, forging, reheating furnace heat treatment, cement, paper kiln pulp and harsh furnace applications.
Head-to-Head: When to Choose Each
| Field condition | Better choice |
| Clean surface + known emissivity | Single-color |
| Stable plastic/rubber/paper surface | Single-color long wavelength |
| Hot metal with clean surface | Single-color short wavelength |
| Cold metal with clean surface | Single-color short wavelength – 2.3 µm |
| Hot metal with scale | Two-color |
| Dust/smoke/steam partly between sensor and target | Two-color short wavelength |
| Smaller target | FOPL single/dual color |
| Molten metal stream | Special two-color model |
Use single-color for clean, stable, repeatable conditions.
Use two-color for dirty, moving, changing, or partially blocked conditions
Final Thoughts
Single color and two color pyrometer both deliver excellent accuracy (except aluminium), in the right conditions. Single color has a known emissivity and a clear sight line. Two color removes that dependency through ratio measurement, making it the practical choice wherever manufacturing reality involves scale, oxidation, steam, or variable surfaces.
Choose based on what your process actually looks like. Tempsens engineers work with steel mills, foundries, glass plants, and petrochemical facilities to match the right measurement technology to the right conditions, because the right pyrometer, properly configured, is one you never have to think about again. Both of these models are available in online and handheld models.
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People Also Ask
What is a two-colour pyrometer?
It measures infrared radiation at two wavelengths and calculates their ratio to determine temperature — making it largely independent of emissivity changes and optical obstructions.
How accurate are radiation pyrometers?
Single color achieves ±0.3% +1°C; two color achieves ±0.5% +1°C, both with 0.1% repeatability. Real-world accuracy depends on matching the pyrometer type to your actual application conditions.
What pyrometers were used to measure visible light?
Disappearing filament pyrometers (developed in 1917) operate in the visible spectrum (0.4–0.7 μm), requiring human observation to match filament brightness against the target. These are largely obsolete today, replaced by infrared systems.
