engineering

Low emissivity

Low emissivity is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Low emissivity rather than just read about it. In short: Low emissivity (low e or low thermal emissivity) refers to a surface condition that emits low levels of radiant thermal (heat) energy. All materials absorb, reflect, and emit radiant energy according to Planck's law but here, the primary concern is a special wavelength interval of radiant energy, namely thermal radiation of materials.

Key takeaways

Low emissivity belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Low emissivity to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Low emissivity from memory before moving on to harder problems.

Reference excerpt

Low emissivity (low e or low thermal emissivity) refers to a surface condition that emits low levels of radiant thermal (heat) energy. All materials absorb, reflect, and emit radiant energy according to Planck's law but here, the primary concern is a special wavelength interval of radiant energy, namely thermal radiation of materials. In common use, especially building applications, the temperature range of approximately -40 to +80 degrees Celsius is the focus, but in aerospace and industrial process engineering, much broader ranges are of practical concern.

Definition Emissivity is the value given to materials based on the ratio of heat emitted compared to a perfect black body, on a scale from zero to one. A black body would have an emissivity of 1 and a perfect reflector would have a value of 0. Kirchhoff's law of thermal radiation states that absorption equals emissivity opaque (εopaque) for every specific wavelength/frequency (materials often have quite different emissivities at different wavelengths). Therefore, if the asphalt has an emissivity value of 0.90 at a specific wavelength (say wavelength of 10 micrometers, or room temperature thermal radiation), its thermal absorptance value would also be 0.90. This means that it absorbs and emits 90 percent of radiant thermal energy. As it is an opaque material, the remaining 10 percent must be reflected. Conversely, a low-e material such as aluminum foil has a thermal emissivity/absorptance value of 0.03 and as an opaque material, the thermal reflectance value must be 1.0 - 0.03 =0.97, meaning it reflects 97 percent of radiant thermal energy. Low-emissivity building materials include window glass manufactured with metal-oxide coatings as well as house wrap materials, reflective thermal insulations, and other forms of radiant thermal barriers. The thermal emissivity of various surfaces is listed in the following table.

Low-emissivity windows Window glass is by nature highly thermally emissive, as indicated in the table above. To improve thermal control (insulation and solar optical properties) thin-film coatings are applied to the raw soda–lime glass. There are two primary methods in use: pyrolytic chemical vapor deposition and magnetron sputtering. The first involves the deposition of fluorinated tin dioxide at high temperatures. Pyrolytic coatings are usually applied at the float glass plant when the glass is manufactured. The second involves depositing thin silver layers with antireflection layers. Magnetron sputtering uses large vacuum chambers with multiple deposition chambers depositing 5 to 10 or more layers in succession. Silver-based films are environmentally unstable and must be enclosed in insulated glazing or an Insulated Glass Unit (IGU) to maintain their properties over time. Specially designed coatings may be applied to one or more surfaces of insulated glass. One type of coating (low-e coatings) reduces the emission of radiant infrared energy, thus tending to keep the heat on the side of the glass where it originated while letting visible light pass. This results in glazing with better control of energy - heat originating from indoors in winter remains inside (the warm side), while heat during summer does not emit from the exterior, keeping it cooler inside. Glass can be made with differing thermal emissivities, but this is not used for windows. Certain properties such as the iron content may be controlled, changing the thermal emissivity properties of glass. This "naturally" low thermal emissivity is found in some formulations of borosilicate or Pyrex. Naturally, low-e glass does not have the property of reflecting near infrared (NIR)/thermal radiation; instead, this type of glass has higher NIR transmission, leading to undesirable heat loss (or gain) in a building with that type of window.

Criticism of low-E windows It has been suggested that the high reflectivity of low-E windows can contribute to a concentration of solar radiation which can potentially cause damage to their surroundings; damage to the sidings of homes and to automobiles has been reported not only in news stories, but may cause legal issues as well. Low-e windows may also block radio frequency signals. Buildings without distributed antenna systems may then suffer degraded cell phone reception.

Reflective thermal insulation

Reflective thermal insulation is typically fabricated from aluminum foil with a variety of core materials such as low-density polyethylene foam, polyethylene bubbles, fiberglass, or similar materials. Each core material presents its own set of benefits and drawbacks based on its ability to provide a thermal break, deaden sound, absorb moisture, and resist combustion during a fire. When aluminum foil is used as the facing material, reflective thermal insulation can stop 97% of radiant heat transfer. Recently, some reflective thermal insulation manufacturers have switched to a metalized polyethylene facing. The long-term efficiency and durability of such facings are still undetermined. Reflective thermal insulation can be installed in a variety of applications and locations including residential, agricultural, commercial, aerospace, and industrial structures. Some common installations include house wraps, duct wraps, pipe wraps, under radiant floors, inside wall cavities, roof systems, attic systems, aircraft fuselage systems, space probe systems, and crawl spaces. Reflective thermal insulation can be used as a stand-alone product in many applications but can also be used in combination systems with mass insulation where higher R-values are required.

Military applications Low emissivity coatings have found applications in stealth technology, reducing the thermal infrared emissions from military equipment in the short-wave, mid-wave and long-wave infrared portions of the electromagnetic spectrum.

Worked examples

Example 1 — a first encounter with Low emissivity

Start with the simplest possible case. Write down what Low emissivity claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Low emissivity before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Low emissivity ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Low emissivity

In research: Low emissivity appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Low emissivity in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Low emissivity is common in secondary-school and first-year university syllabi. It links to neighbouring topics Glass coating and surface modification, Thermal protection, Windows, so understanding it makes those chapters shorter.
In everyday life: Look for Low emissivity outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Low emissivity in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Low emissivity means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Low emissivity out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Low emissivity in simple terms?

Why does Low emissivity matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Low emissivity?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Low emissivity.