How Absorption and Emission Heat Shape Indoor Temperatures
- SFI Team
- Jul 1
- 5 min read
Thermal energy indoors arises from two interlinked physical processes: emission, where objects release radiation based on their temperature, and absorption, where external radiation is taken in and converted to heat. Both mechanisms determine how environments warm or cool in response to sunlight.
These principles are particularly relevant in sunlit spaces like offices near windows. Sunlight entering through glass carries visible and infrared energy that structures and occupants absorb, heating surfaces. At the same time, objects emit infrared radiation themselves, attempting to cool the space—but at a much lower rate.
Managing indoor temperature involves understanding which wavelengths contribute most to heating and how surfaces interact with them. Techniques such as coatings and tinting films selectively block or reflect portions of the solar spectrum, maximizing comfort while preserving daylight. Below, we explore the physics and strategies behind thermal control.
Understanding the Electromagnetic Spectrum
The electromagnetic spectrum spans from extremely short-wave gamma rays to long-wave radio waves. When it comes to thermal energy and indoor heating, the most relevant parts are ultraviolet (UV), visible light, and infrared (IR) radiation.
Ultraviolet (UV): 100–400 nm
UVC: 100–280 nm — fully absorbed by the atmosphere
UVB: 280–315 nm — mostly absorbed by the ozone layer
UVA: 315–400 nm — the only UV type that significantly reaches Earth and contributes to heat (~5–10%)
Visible Light: 400–700 nm
Contributes ~40–45% of solar heating when absorbed
Infrared (IR): 700 nm to 1,000,000 nm (1 mm)
Near-IR (NIR): 700–2,500 nm — carries ~45–50% of solar heat and is the primary driver of solar heating indoors
Mid-IR (MIR): 2500–25,000 nm — contributes minimally to solar heating but important in emitted heat from objects
Far-IR (FIR): 25,000–1,000,000 nm — mostly associated with thermal emission from room-temperature surfaces
While UV photons are high in energy, they make up only a small portion of total solar power and are mostly blocked by the atmosphere or glass. By contrast, infrared radiation — particularly in the near-IR band — carries the majority of thermal energy and is the main contributor to indoor heat gain through windows.
Infrared also dominates nearly 99% of the thermal radiation spectrum, especially for emission from room-temperature objects. UV and visible light occupy a narrow slice of the spectrum but are still relevant for heating when absorbed. Visual diagrams showing the full spectrum and zoomed-in IR ranges help make this heat distribution clear.
How Radiation Interacts with Matter
Not all radiation produces heat. For heating to occur, radiation must be absorbed, not reflected or transmitted. Infrared is efficient at heating because photon energies in this band resonate with molecular vibration modes. In contrast, radio waves or certain artificial lights pass through materials without heating significantly.
Photon energy matters. Higher-energy photons like UV can cause chemical or biological damage but may not penetrate deeply or efficiently turn into heat. Visible light may be partly absorbed or reflect depending on surface characteristics. Heat results when absorption increases molecular motion.
Material properties influence interaction with radiation. Dark materials absorb across visible and IR wavelengths, while glossy or light-colored surfaces reflect more. Glass often transmits visible light but can block or absorb IR. Sunlight—strong in both visible and IR—warms more effectively than typical artificial lighting, which may lack strong IR content.
Absorption and Emission Heat
Emission: How Objects Radiate Heat
Objects emit infrared radiation proportional to their temperature. Room‑temperature surfaces (~20–30 °C) radiate predominantly around 10,000 nm, deep in the mid-IR band. Temperature, emission wavelength, and surface emissivity govern how much energy an object emits.
Parameters that influence thermal emission include surface temperature, wavelength, and emissivity. Together they define the radiated intensity without needing detailed equations here. For those interested, Planck’s Law (describing spectral emission distribution) is well explained Planck’s Law on Wikipedia.
Emission indoors from walls, furniture, and humans contributes to cooling but is generally much weaker than solar absorption—so it plays only a secondary role in everyday indoor heat balance.
Absorption: The Dominant Mechanism for Heat Gain
The primary source of indoor heat is the absorption of sunlight by surfaces. Solar radiation at Earth's surface (standard AM1.5 spectrum) consists of approximately 5–10% UV, 45% visible light, and 45% near-infrared (NIR). NIR ranging from 700–2500 nm is the most significant contributor to heat gain.
Absorption depends on the spectral irradiance of sunlight and material absorptivity. Though the full mathematical formulation uses integrals combining irradiance and absorptivity, key parameters include wavelength, incoming radiation intensity, and surface absorptivity.
Many surfaces—especially human skin—absorb across these bands differently. Brown skin, for instance, may absorb ~85% of UV, ~75% of visible light, and ~90% of NIR. Interior surfaces such as desks or floors behave similarly. When sunlight enters through glass, visible and IR light pass through, hit surfaces, and are absorbed—raising temperature quickly, especially where direct sunlight strikes.
Blocking Heat: Optimal Strategies for Radiation Control
Imagine having a filter that blocks any 500 nm-wide band of wavelengths—where would it block to maximize heat reduction while preserving brightness?
Option A (allow darkness): Block 400–900 nm, i.e. visible light. This removes heat but makes the room extremely dark—undesirable for most settings.
Option B (preserve brightness): Block 800–1300 nm (near-IR). This cuts substantial heat while allowing visible light to pass.
Blocking near-IR removes a heat-rich band without compromising daylight. UV (below 400 nm) is typically blocked by glass or coatings but contributes little to thermal load. Consistently, UV is filtered more for its damaging effects than its heat significance.
Mirroring, or reflective surfaces, offers another powerful strategy. Reflective coatings bounce IR and visible light away before entry, reducing heat gain without affecting indoor brightness. Combined approaches—reflection, absorption, UV blocking, and selective transmission—enable effective heat management without sacrificing comfort.
Window Film and Tinting Techniques
Modern window films and coatings deploy layered strategies that work together to reject solar heat:
Tinting absorbs visible and some IR light, reducing glare and warmth.
Mirroring (reflective coatings) bounces off solar radiation—especially IR—back outdoors.
UV filters cut nearly all UV (<400 nm), safeguarding against fading and damage.
IR filters target the near-IR band (700–2500 nm), blocking the most heat-intensive portion of sunlight.
By combining these techniques, films and glass systems often achieve >99% UV rejection, high IR reduction, and controlled visible light transmission. Factors like angle of sunlight, glass thickness, and multi-pane constructions further amplify heat-blocking effectiveness. Reflective low-emissivity (low-E) coatings are especially effective for IR rejection, maintaining clarity while reflecting heat.
Summary
Thermal energy indoors stems from both emission and absorption of radiation. Emission is the infrared radiation objects naturally emit at room temperature, peaking around 10,000 nm, but remains comparatively weak.
The dominant source of heat indoors is absorption of solar light, especially in the near-infrared band (700–2500 nm). To minimize heat gain while maintaining daylight, blocking or reflecting near-IR is key. Effective strategies use combinations of absorption, reflective mirroring, and UV/IR filtration in coatings and films to optimize comfort and energy efficiency.
FAQ
What is the difference between emission and absorption in heat transfer?
Emission is energy released by warm objects; absorption is energy gained when external radiation is captured. Emission helps cool; absorption drives warming.
Why do only some wavelengths cause heat?
Wavelengths that are absorbed convert photon energy into thermal motion. Wavelengths that are reflected or transmitted bypass surfaces without heating.
Why is near‑IR more significant than UV indoors?
Near‑IR makes up about 45% of solar energy and enters through glass easily. UV contributes little to thermal load and is mostly blocked by glass.
How does material color affect heat absorption?
Dark materials absorb more across visible and IR bands, converting more energy into heat. Light or reflective surfaces minimize absorption.
Where should solar filter focus for cooling without darkening the room?
Blocking near‑IR (≈800–1300 nm) reduces heating while allowing visible brightness (~400–700 nm).
What role does mirroring play in solar heat control?
Mirroring reflects solar radiation before it’s absorbed, effectively reducing heat gain while maintaining interior light.
Why does emission from room-temperature surfaces produce so little heat?
Because their temperature is moderate, emitted infrared energy is weak compared to incoming solar radiation, making absorption the dominant factor indoors.
