How do you measure radiant heat?

In the fields of thermodynamics and energy, radiant heat refers to energy transferred from an object or surface through electromagnetic waves, without relying on a medium such as conduction or convection. Whether in high-temperature industrial furnaces, solar energy assessment, or building thermal comfort studies, accurately measuring radiant heat is a critical issue. This article will provide a detailed explanation of the definition of radiant heat, measurement principles, main measuring instruments, and practical applications, helping to understand how to scientifically measure this important physical quantity.

What is Radiant Heat?

Radiant heat is energy emitted by an object through electromagnetic radiation, closely related to the surface temperature of the object. Any object with a temperature above absolute zero (-273.15°C) emits radiant energy, and the intensity of this radiation increases with temperature. Radiant heat is one of the three modes of heat transfer (conduction, convection, and radiation). It is especially significant in high-temperature environments and can be transmitted to another object’s surface without any medium.

Core Principles of Radiant Heat Measurement

Relationship between Radiant Heat and Temperature

The intensity of radiant heat is not only related to temperature but also depends on the emissivity of the object’s surface. According to thermodynamic theory, the radiant heat flux density of an ideal blackbody per unit area is proportional to the fourth power of its absolute temperature, as described by the Stefan-Boltzmann law. Therefore, measuring radiant heat often requires consideration of both temperature and emissivity.

How to Measure Radiant Heat

Unlike ordinary temperature measurement, radiant heat measurement focuses on the intensity of energy per unit area, typically expressed in watts per square meter (W/m²). Different applications require different measurement methods:

1. Radiant Heat Meters

Radiant heat meters are specialized sensors used to measure the energy of electromagnetic radiation. Their core principle is to convert absorbed radiant energy into measurable electrical signals or temperature changes, which are then used to output radiant heat intensity data. Typically, the absorbed radiation causes the sensor temperature to rise, altering resistance or voltage, which allows the calculation of radiant heat intensity.

These instruments are highly sensitive and can capture radiation across different wavelengths, making them suitable for research, industrial, and environmental monitoring applications.

Typical implementations include:

Black Globe Thermometers: Using a high-radiation-absorption surface (usually coated black) to approximate a blackbody, with an internal thermometer measuring the equilibrium temperature to estimate surrounding radiant heat intensity.

Thermocouple-Based Devices: Equipped with an array of heat-sensitive elements that respond to external radiation, generating electrical signals proportional to the incident energy.

2. Heat Flux Sensors (Gardon Gauges)

In fire testing and high-intensity radiation environments, heat flux sensors, such as Gardon gauges, are commonly used. These devices directly measure radiant heat flux density. They typically consist of a heat-sensitive element with a black coating and a sophisticated thermal management system. When radiant heat acts on the sensor, it generates an electrical signal proportional to the incident heat flux, which can be calibrated to obtain radiant heat intensity.

These instruments are standard tools for high-intensity radiant heat measurement in high-temperature testing and material heat resistance experiments.

3. Infrared Thermometers and Indirect Measurement

Although infrared thermometers primarily measure surface temperature rather than radiant heat flux directly, they are fundamental tools for obtaining radiant heat-related data. By focusing the target’s infrared radiation with a lens and converting it into an electrical signal, the device calculates and displays temperature. By properly setting the emissivity, it can be used to analyze radiant heat distribution.

Infrared thermometers are especially useful in high-temperature, hazardous, or difficult-to-access environments, providing an indirect method of measuring radiant heat.

4. Radiometers and Total Radiation Measurement

In scientific research, such as meteorology and solar resource assessment, total environmental radiation or solar radiation needs to be measured. Integrated sensors, such as heat radiation sensors or radiation recorders, can measure various types of radiation, including visible, ultraviolet, and infrared radiation. These devices convert received radiation into electrical signals through the thermoelectric effect, which are then calibrated to obtain radiant flux values.

Practical Considerations in Radiant Heat Measurement

1. Emissivity and Measurement Error

Most radiant measurement instruments rely on the emissivity of the target surface for accurate calculation. Different materials have varying emissivity—for example, blackbody surfaces are close to 1. while smooth metals are significantly lower. Setting or calibrating emissivity is necessary to avoid measurement errors.

2. Environmental Interference and Compensation

Factors such as reflected radiation, background temperature, and air convection can affect measurement accuracy. High-quality instruments often include environmental compensation algorithms or calibration mechanisms to minimize interference.

3. Instrument Selection and Application Scenarios

Different applications require different measurement devices:

Industrial high-temperature monitoring: Use heat flux sensors or high-precision infrared thermometers.

Building thermal comfort assessment: Use black globe thermometers to measure mean radiant temperature.

Solar energy and meteorological research: Use integrated radiometers and total radiation sensors.

Typical Applications of Radiant Heat Measurement

Material Fire Resistance Testing

Heat flux sensors are used to simulate heat exposure on materials, evaluating critical performance by measuring radiant heat intensity.

Industrial High-Temperature Automation

Non-contact radiant measurements monitor furnace temperatures in steel, glass, and other manufacturing industries, providing feedback for process control.

Thermal Comfort and Energy Efficiency in Buildings

Indoor comfort depends not only on air temperature but also on surrounding surface radiation. Black globe thermometers and similar instruments help accurately evaluate thermal comfort indices for building design and energy efficiency improvements.

Measuring radiant heat involves capturing, converting, and quantifying electromagnetic radiation energy. Conventional thermometers cannot directly reflect radiant heat intensity, which led to the development of specialized instruments such as radiant heat meters, heat flux sensors, infrared systems, and integrated radiometers. By converting radiant energy into temperature or electrical signals, these instruments provide reliable means for high-temperature measurement, material testing, and environmental thermal analysis.

Accurate and scientific measurement of radiant heat depends not only on the instruments but also on considering the target’s emissivity, environmental conditions, and measurement context, ensuring precise results in various practical applications.