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Physics instructors at all levels introduce infrared as an invisible, heat-related type of light. False-color infrared imaging in particular is a popular demonstration of the unseen world.1,2 Heat transfer by infrared light is less visual and arguably less popular for demonstrations and experiments in instructional laboratories. However, familiarity with the principles is important for practical topics such as radiant heating in buildings.3 This familiarity is crucial to understanding the connection between the atmosphere and surface temperatures on Earth. In this article, we describe a moderate-cost apparatus for radiant heat transfer experiments in the instructional physics laboratory. The apparatus includes an under-desk radiant heating panel and an infrared thermometer. The main innovation is the addition of an aluminized Mylar flap on the front of the heating panel. The flap blocks radiant (infrared) heat transfer between the front of the panel and the room. Associated experiments using the flap can then distinguish radiant heat transfer from convective heat transfer by air currents. We present energy balance diagrams that show the heat transfer processes for the bare panel and the panel with the flap attached. These diagrams support the presentation of similar energy balance diagrams that illustrate heat transfer processes for Earth’s surface and explain the atmosphere’s effect to increase Earth’s surface temperature.
Apparatus
The components are illustrated in Fig. 1 and listed below. Except for the aluminized Mylar flap, they are available from several manufacturers. The total cost of the components for each setup is about $150 at present.
Commercial electrical radiant heating panel. Ours was 0.30 × 0.48 m (12 × 19 in.) and 200 W. For safety, an internal thermostat limited the maximum temperature to 70 °C for our panel.
Handheld infrared thermometer.
AC watt meter—connected directly to an electrical outlet.
Inline dimmer switch to control the electrical power into the panel.
“Mirror flap” cut from a roll of aluminized Mylar or equivalent. In use, strip magnets flatten the flap against the panel. The shiny aluminized surface of the flap must face into the room.
Two radiant heating panels with aluminized Mylar flaps. The flap on the rear panel is wrapped around its back. The AC electrical power to the front panel first passes through a watt meter and then a dimmer switch (to adjust power). The panels are off and at room temperature (20 °C) in this photo.
Two radiant heating panels with aluminized Mylar flaps. The flap on the rear panel is wrapped around its back. The AC electrical power to the front panel first passes through a watt meter and then a dimmer switch (to adjust power). The panels are off and at room temperature (20 °C) in this photo.
The arrangement of these components is illustrated in Fig. 1. Two heating panels are shown, although only one is needed for the experiments. The frontmost panel shows the appearance when the mirror flap covers the front surface. The panel to the rear shows the appearance when the flap is folded over the back of the panel.
Infrared thermometers work by focusing infrared thermal radiation from a target onto a heat sensor. The sensor signal is converted to a temperature using “Stefan–Boltzmann law” calculations. These relate the intensity (W/m2) of infrared emission to the temperature of an object.
Aluminized Mylar film is readily available and is used for balloons, “space blankets,” and insulation products for buildings. The shiny surface is a good mirror for both visible and infrared light. It blocks infrared light from behind the mirror. Thus, the temperature measured by the thermometer when it points at the mirror belongs to the object whose image appears in the mirror.
Students should familiarize themselves with the infrared thermometer before starting the heat transfer experiments. The walls of a room will give a reading close to the air temperature in the room. The palm of one’s hand gives a higher temperature. Students should check that the aluminized Mylar is an infrared mirror. An example is illustrated in Fig. 1. The thermometer sends out a red laser beam that is reflected by the mirror and hits the palm of a hand. The thermometer reading in the photograph is 29.1 °C. It’s close to the temperature when the laser beam struck the palm directly, without first being reflected by the mirror.
Panel measurements
Heat transfer from the front surface of the radiant heating panel into the room occurs by two processes. One is radiant heating by the emission of infrared light. The second is warming of air currents (“convection”). When the Mylar flap is in place, radiant heating is blocked nearly completely. The electrical power into the panel then heats the room mainly by convection.
The following measurements distinguish the panel’s heating of a room by convection and by radiant heating. Without the flap in place, 40 W (0.27 kW/m2) of electrical power raised the panel’s center temperature to 45 °C in a room at 20 °C. With the flap in place, the radiant heating is much smaller. Students can be asked to predict whether the mirror flap will increase or decrease the panel temperature. The correct prediction is that the temperature will increase until convective heating carries off about 40 W of power. With the mirror flap in place, the center temperature of the panel increased to 60 °C after several minutes. This panel temperature was measured by lifting the flap and quickly remeasuring the temperature.
As we discuss subsequently, there is a rough analogy with the effect of Earth’s atmosphere on its surface temperature. The atmosphere absorbs most of the infrared emitted by Earth’s surface and then emits more than half of that back down to the surface.
When the flap is removed, the panel temperature begins to fall. Students should then increase the electrical power to maintain the temperature at 60 °C. After a few minutes, it took 64 W (0.43 kW/m2) to maintain the center temperature of the panel at 60 °C. About 40 W of heat was transported into the room by convection at the front of the panel as well as some heating by the back. The remaining 24 W is used for radiant heating of the room by the front of the panel. It is interesting that the radiant room heating of 24 W by the front of the panel is smaller than the additional 40 W.
These measurements are summarized in the “energy balance” diagrams of Fig. 2. Figure 2(a) shows the heat transfers when a mirror flap blocks radiative heat transfer between the panel and the room. The width of the “AC” bar is proportional to the electrical power per unit area (0.27 kW/m2) flowing into the panel. The convection bar has about the same width since radiative transfer is blocked. The bars labeled “Panel IR” and “Room IR” represent potential infrared heat transfers that are nearly completely blocked when the mirror flap is in place. The width of the “Panel IR” bar represents the intensity of infrared light from the panel that was blocked from entering the room. The width of the “Room IR” bar represents the intensity of infrared light from the room that was blocked from absorption in the panel. The widths of these bars come from Stefan–Boltzmann law calculations in the supplemental materials.5
Energy balance diagrams for the radiant panel at 60 °C with and without the aluminized Mylar flap covering the front of the panel. Potential power flows into the panel are the electrical (AC) power and room temperature thermal infrared radiation (Room IR). Power flows out of the panel are convective heating (air currents—Conv) and radiant heating (Panel IR). (a) Panel with the aluminized Mylar flap blocking infrared heat transfer between the panel and the room. (b) Panel with both convective and infrared heat transfer between the panel and the room.
Energy balance diagrams for the radiant panel at 60 °C with and without the aluminized Mylar flap covering the front of the panel. Potential power flows into the panel are the electrical (AC) power and room temperature thermal infrared radiation (Room IR). Power flows out of the panel are convective heating (air currents—Conv) and radiant heating (Panel IR). (a) Panel with the aluminized Mylar flap blocking infrared heat transfer between the panel and the room. (b) Panel with both convective and infrared heat transfer between the panel and the room.
The mirror flap is removed in Fig. 2(b), and the width of the “AC” bar increases by 0.16 kW/m2. The panel temperature doesn’t change, and neither do the widths of the other bars. Since the panel temperature isn’t changed, the convective heat transfer bar isn’t changed from Fig. 2(a). The difference in the widths of the “Panel IR” bar and “Room IR” bars indicates net radiative heating from the panel into the room, and agrees fairly well with the Stefan–Boltzmann law calculation.
Analogy with the atmosphere’s effect on Earth’s surface temperature
The energy balance diagrams for the radiant heating panel can be used to introduce the diagrams used to explain Earth’s surface temperature. For Earth, sunlight is the main source of energy that warms its surface above the temperature of space (3 K, or −270 °C). There is a rough analogy with the electrical power that warms the heating panel above room temperature. Averaging over the surface and the year, about 0.24 kW/m2 of power is absorbed by Earth from sunlight. The remainder of the incident sunlight is reflected back into space.
Figure 3(a) is the diagram for a hypothetical bare Earth that has no atmosphere. The figure is based on a textbook calculation.6 The hypothetical surface temperature is calculated by using the Stefan–Boltzmann law and matching the emission of radiant heat to the absorbed solar power of 0.24 kW/m2. The calculation predicts a frigid average temperature of −18 °C. It’s much colder than the measured average of surface temperatures, which is around 15 °C. A no-atmosphere calculation does account fairly well for the surface temperature of the planet Mars, where atmosphere effects are minor.
Energy balance diagrams for 0.34 kW/m2 average solar illumination of Earth’s surface area (Solar). Of this, 0.10 kW/m2 is reflected (Refl). (a) Without an atmosphere, the solar power absorbed by Earth’s surface (0.24 kW/m2) is returned directly to outer space as thermal infrared light (IR) radiated by Earth’s surface. The calculated average temperature of Earth’s surface is −18 °C. (b) With the atmosphere, some sunlight is absorbed by the atmosphere before 0.16 kW/m2 reaches Earth’s surface. Thermal infrared radiation from Earth’s surface is nearly completely absorbed by the atmosphere. There is also some additional heat transfer from transport of water (H2O). The atmosphere radiates more than half of the power it absorbs back to Earth’s surface as infrared thermal radiation. The resulting average surface temperature with the atmosphere is 15 °C. It’s 33 °C warmer than without the atmosphere.
Energy balance diagrams for 0.34 kW/m2 average solar illumination of Earth’s surface area (Solar). Of this, 0.10 kW/m2 is reflected (Refl). (a) Without an atmosphere, the solar power absorbed by Earth’s surface (0.24 kW/m2) is returned directly to outer space as thermal infrared light (IR) radiated by Earth’s surface. The calculated average temperature of Earth’s surface is −18 °C. (b) With the atmosphere, some sunlight is absorbed by the atmosphere before 0.16 kW/m2 reaches Earth’s surface. Thermal infrared radiation from Earth’s surface is nearly completely absorbed by the atmosphere. There is also some additional heat transfer from transport of water (H2O). The atmosphere radiates more than half of the power it absorbs back to Earth’s surface as infrared thermal radiation. The resulting average surface temperature with the atmosphere is 15 °C. It’s 33 °C warmer than without the atmosphere.
Figure 3(b) includes the effects of Earth’s atmosphere. It is a simplified version of the widely reproduced diagram published by Trenberth et al.7 The same 0.10 kW/m2 of sunlight is reflected back into space. About 0.16 kW/m2 passes through the atmosphere and is absorbed by Earth’s surface. The remaining 0.08 kW/m2 is absorbed in the atmosphere.
The atmosphere increases Earth’s surface temperature through its interactions with infrared light. Almost all of the thermal energy emitted at the surface is absorbed by the atmosphere. In turn, the atmosphere radiates an infrared intensity of about 0.33 kW/m2 back down to the surface—which is larger than the 0.16 kW/m2 absorbed by the surface directly from sunlight. The atmosphere also broadcasts 0.24 kW/m2 of infrared back into space. This isn’t unexpected: it’s colder at high altitudes than at the surface. The net result is a calculated average surface temperature of about 15 °C that matches measurements. The calculation, and Earth’s surface temperature, are changed as the properties of the atmosphere change. The atmosphere does change from natural and human causes. Volcanic eruptions of particles and gases into the atmosphere have led to a transitory cooling of the Earth’s surface that lasted until the atmosphere recovered. Emissions of gases like methane and carbon dioxide, which increase the strength of the atmosphere’s absorption of infrared light, cause a warming trend. The warming will be reversed when the gas concentrations are lowered.
Acknowledgments
This work was supported by a contract from the New York Empire State Development Fund (C200183) to the Center of Excellence in Environmental and Energy Systems; the agency does not endorse or approve publication by its contractors. The author thanks Samuel Sampere for his advice and support on the radiant heating project, and the John Ben Snow Foundation for funding distribution of the apparatus to local high school teachers.
References
As a physics professor at Syracuse University, Eric Schiff has prioritized development of the undergraduate laboratory experience. His research group has focused on semiconductors and solar cell device physics. He has served as department chair, executive director of the Center of Excellence on Environmental & Energy Systems, and as a program director at ARPA-E.
