What is Emissivity: Definition and 61 Discussions

The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that may include both visible radiation (light) and infrared radiation, which is not visible to human eyes. The thermal radiation from very hot objects (see photograph) is easily visible to the eye. Quantitatively, emissivity is the ratio of the thermal radiation from a surface to the radiation from an ideal black surface at the same temperature as given by the Stefan–Boltzmann law. The ratio varies from 0 to 1. The surface of a perfect black body (with an emissivity of 1) emits thermal radiation at the rate of approximately 448 watts per square metre at room temperature (25 °C, 298.15 K); all real objects have emissivities less than 1.0, and emit radiation at correspondingly lower rates.Emissivities are important in several contexts:

Insulated windows – Warm surfaces are usually cooled directly by air, but they also cool themselves by emitting thermal radiation. This second cooling mechanism is important for simple glass windows, which have emissivities close to the maximum possible value of 1.0. "Low-E windows" with transparent low emissivity coatings emit less thermal radiation than ordinary windows. In winter, these coatings can halve the rate at which a window loses heat compared to an uncoated glass window.
Solar heat collectors – Similarly, solar heat collectors lose heat by emitting thermal radiation. Advanced solar collectors incorporate selective surfaces that have very low emissivities. These collectors waste very little of the solar energy through emission of thermal radiation.
Thermal shielding – For the protection of structures from high surface temperatures, such as reusable spacecraft or hypersonic aircraft, high emissivity coatings (HECs), with emissivity values near 0.9, are applied on the surface of insulating ceramics. This facilitates radiative cooling and protection of the underlying structure and is an alternative to ablative coatings, used in single-use reentry capsules.
Planetary temperatures – The planets are solar thermal collectors on a large scale. The temperature of a planet's surface is determined by the balance between the heat absorbed by the planet from sunlight, heat emitted from its core, and thermal radiation emitted back into space. Emissivity of a planet is determined by the nature of its surface and atmosphere.
Temperature measurements – Pyrometers and infrared cameras are instruments used to measure the temperature of an object by using its thermal radiation; no actual contact with the object is needed. The calibration of these instruments involves the emissivity of the surface that's being measured.

View More On Wikipedia.org
  1. gmason85

    I How do emissivity and absorptivity affect thermal imaging results?"

    Hi all, I’ve run into a number of paints that are sold as “thermal” paints, or insulation paints. I know it’s mostly bs, but want a clarification on how emissivity and infrared absorption relate. The salesperson says the paint surface has an emissivity of 0.91, and reflects 99.5% of infrared...
  2. F

    I Do you think emissivity of air makes sense?

    Hello guys :)In the frame of finding a physical model for the temperature of Earth's surface, talking about the very "idealized" two-layers model of atmosphere, I ask you now the question to the other physicists or engineers: does it make sens to associate an emissivity to a layer of air (+ some...
  3. H

    Misc. What emissivity should I use for LED strips?

    Hi! I want to measure the heat of LEDs on my DIY LED panel with my infrared thermometer. That means that wrong emissivity could be like +/- 10 C in some cases. Should I take emissivity of white plastic which is 0.84 or are there any better options? Also does IR thermometer care if light from...
  4. J

    Radiation - finding emissivity of a sphere

    So using the above equation, e=dQ/dt / (A*5.67E-8*303.8^4) The surface area of a sphere is 4(pi)r^2 and I get 136.8478 m^2. dQ/dt would be the net radiation (I think? Its in the correct units), 1074W. Plugging everything in I get 0.01625, but the answer is 0.0524. Now as I was writing this I...
  5. jdawg

    Total Emissivity as a Function of Temperature (Ceramics)

    Hello, I’m trying to better my understanding of how the total emissivity changes with temperature for ceramic materials. Currently it is my understanding that non-metals typically have a high emissivity. A sanded surface will result in a higher emissivity, and that spectral emissivity varies...
  6. jaumzaum

    I Why is the emissivity of metals low?

    Hello! I am having trouble to understand why the emissivity of polished metals is much lower than if they are not polished. Consider, for example, non-polished aluminium at 300K, which is said to have an emissivity of 0.77. We put it floating in vacuum. There is an energy source near it, and in...
  7. O

    Emissivity of a very high temperature object

    I am looking for data on the total hemispherical emissivitie from a Platinum surface at high temperature 1400 - 2000 K. In this temperature range the Platnium surface will color from red, orange to white. I learned that, practically, the bove mentioned emissivity will approach to one in this...
  8. fluidistic

    A Getting the emissivity from scratch

    I have checked around 10 Solid State Physics and Condensed Matter textbooks, including the classic "Ashcroft and Mermin" and I've noticed that the "emissivity" of a solid is a totally neglected subject. This leaves me entirely knowledgless about how to compute the emissivity from scratch. I do...
  9. B

    I Why don't humans glow in the dark?

    Lets assume: emissivity of a human=0.91 T=310K Surface area body: 1.60 m^2 If we use the Stefan-Boltzmann Law we can find a value for the rate of emission of light by a human. Rate emission=762 J/s Given this rate of emission, why don't humans glow in the dark?
  10. M

    A How to measure emissivity of hot (~1000 centigrade) objects

    I want to measure the emissivity (or more correctly the absorbtivity) in the wavelength range 1 - 10 micrometer of very hot (~1000 degrees Celcius) ceramic materials. At room temperature this is typically done using an FTIR spectrometer with a gold coated integrating sphere. But for very hot...
  11. C

    How to calculate transmissivity of a set of reflective foils

    Suppose I have an infinitely thin foil that absorbs 1% of incoming radiation. It therefore emits 0.5% of the incoming radiation in both directions perpendicular to the plane of the foil. That is, transmission is 0.5%. How to calculate transmission for a series of such foils? There will be a lot...
  12. kal

    I Trouble understanding the idea of a cavity radiator being a Black Body

    I have been trying to understand the role of a cavity as a black body radiator in the derivation of planks black body radiation law but it has left me with 5 main questions: 1. If an object is a perfect absorber it must also be a perfect emitter, meaning that (allowing for a cavity not being a...
  13. P

    How much energy does a bar of iron radiate after losing 8.0 × 10^6J?

    A bar of iron is 0.5m long, 0.2m wide and 0.1m high (which means its volume is 1.0 × 10−2m3 and its surface area is 3.4 × 10−1m2 ). Iron has a density of 7900 kg m3 , a heat capacity of 400 J kg◦C , and a coefficient of linear expansion of 1.2 × 10−5 . The bar of iron is initially at 600K...
  14. Carlos de Meo

    Heat exchange, emissivity and reflectance

    Hi Guys I´m studying the heat exchange problem in furnaces and, to begin with, i started with Incropera´s book. One thing is actually driving me crazy On the last part of this exercise´s solution (part 3), the physical principle involved is not very clear to me. To calculate the absorptivity of...
  15. Carlos de Meo

    Thermal radiation, emissivity and Kirchhoff

    Hi Guys Studying thermal radiation here and I am kinda stuck at one point Kirchhoff law states that at thermodynamical equilibrium, the amount of energy absorbed must be reemited. But before the system reaches equilibrium, is there a model to predict how much energy of these photons absorbed...
  16. Carlos de Meo

    A Why does a higher absorption coefficient result in a smaller emissivity value?

    Hi Guys Im studying some thermal properties for my masters and while reading an article a weird thing happened: As far as i know, a good emitter must be a good absorber (since emission happens "after" absorption") Combining Fresnel equation for a medium that absorbs radiation and Kirchoff law...
  17. NatFex

    B Difference between albedo and emissivity?

    This has been asked already, but it was either answered incorrectly or I'm just not understanding it right. I was under the impression that albedo is the ratio of radiation reflected off a body compared to the total incident on that body. Hence black bodies have an albedo of 0 and more...
  18. J

    Emissivity of Flexinol Wire: 70°C & 90°C

    I searched all over the internet, but I couldn't find it. Does anyone know the emissivity of 70 degrees C and the emissivity of 90 degrees C Flexinol wire?
  19. T

    Thermal Management Question in Spacecraft

    Hello guys, Below I am presenting a question related to thermal management in spacecraft . Perhaps many of you might find it interesting! And this could be a real-life problem for thermal engineering in a spacecraft . So, the question is following: An instrument dissipates 10W and, in order...
  20. GM Jackson

    How does a black body emit as much energy as it absorbs?

    Homework Statement A black body absorbs all incident electromagnetic radiation, including visible light which has wavelengths from 380nm to 750nm. IR radiation has wavelengths that are so long they are measured in microns. That suggests that visible light has a higher frequency than IR, and...
  21. L

    Emissivity & internal blackbody

    Please confirm my understanding and inquiry.. in material without transmission... 100% = Emissivity + Reflectivity... when using thermal camera (I own one).. we have to adjust the emissivity In two objects that is both 30 Celsius.. one a near black object with emissivity nearly 1 (black body)...
  22. A

    Paint high T, high emissivity for Black Body simulation

    Hi all! I would be interested in paints that show very high absorptivity/emissivity and that can withstand high temperatures (ideally above 1000°C) and used in vacuum. Do you have any commercial suggestions? Thank you!
  23. H

    Measuring the emissivity of a surface

    Hi Fellas, Need your help to solve a practical problem I've been facing. I run a company that makes products for foundries. Recently, we started work on developing a high temperature, high emissivity ceramic coating to paint the inside walls of a furnace. The motive behind the particular...
  24. P

    What is the emissivity of Nifethal?

    I've been searching and I haven't found it yet. I will really appreciate it if someone can help me. Thanks
  25. M

    Emissivity of Skin: Unity & Ice | Need Help & Guidance

    I have found from various sources that the emissivity of human skin, irrespective of pigment, is close to unity. I am also unsure as to why ice has such a high emissivity too- perhaps my understanding of emissivity is fundamentally flawed! Any help or guidance on this topic would be...
  26. S

    Can there be different IR emissivity & IR absorptivity

    At a particular wavelength (In IR range), & at particular temperature, can anybody or coating has different IR emissivity & IR absorptivity?
  27. R

    How Does Particle Size Influence Emissivity in Circumstellar Dust?

    In this 1984 paper on circumstellar dust, the author says, speaking of the emissivity/absorption of dust grains at visual and infrared wavelengths: My question is: where did the expression ##\lambda_p / 2\pi## come from, and what is the physical justification for it? I understand that small...
  28. F

    Temperature of filament using its area, power, and emissivity

    I have entered the emissivity in the calculation so that we can treat it as a blackbody, allowing us to use I_{tot} = \sigma T^4. My book tells me the correct answer is 2.06*10^4, which I'd normally put down to a misprint, but if I use my value, I get a value for the next part which is a factor...
  29. S

    Emissivity of Tungsten Carbide

    Hi, For the purpose of my FYP I am carrying out temperature analysis on a single point cutting tool while machining on a lathe. I am using a handheld infrared camera to determine this. In order to carry this out accurately the emissivity of the material must be inputted to the device as a...
  30. D

    How to find emissivity of tungsten from ln graph?

    Homework Statement I'm doing a lab where I'm trying to find the emissivity of a tungsten filament. I measured voltage and current of a bulb with a tungsten filament and found the power. Then I plotted Power vs Temperature^4 and used the slope along with surface area and Boltzman's constant...
  31. R

    Reflectivity, emissivity, and absorptivity of opaque materials

    Homework Statement what is the relation between reflectivity, emissivity, and absorptivity for an opaque material? if an opaque material's reflectivity is 0.075 for solar radiation in the visible range and 0.95 for radiation of a heated surface of 80 degrees Celsius in the far infrared...
  32. A

    Temperature, brightness temperature, emissivity

    Hello, I have two different materials, wood and iron in two different environments. I have both the wood and iron temperature, respectively Tw, and Ti. I also know their emissivities, Ew, and Ei. What I want to know is the temperature of the iron if it was wood (i.e. if its material was wood...
  33. B

    Blackbody emissivity different to thermometers

    I am doing some research on thermometers and have a blackbody to test some infrared ones. Unfortunately the blackbody has an emissivity of 0.97 +/- 0.02 and the thermometer has an emissivity of approximately 0.96? How do I correct for the differences? I have had a quick look online and other...
  34. A

    Emissivity and IR Thermometers

    Hi I am trying to get my head around Emissivity and was wondering if anyone could help. There is a "blackbody" with emissivity of 0.97. We want to use it to see whether some IR thermometers are giving suitable readings. Unfortunately the emissivity of the thermometer is unknown and...
  35. C

    Emissivity, radiation and heat transfer

    Homework Statement A space station in outer space (far from the sun) has a total surface area of 580 m^2 with emissivity of .62. The temperature of the outside surface is 156 K. the walls are .25 m thick with an average thermal conductivity of .038 Wm^-1K^-1. Find the temperature of the inner...
  36. C

    High Emissivity Paint for Aluminium Surface

    Hey, I am making a furnace...which uses Aluminium material. Does anyone knows a paint which can be used for applying on Aluminium Surface and which works upto 500 degree Celsius and has high emissivity.
  37. S

    The relationship between absorbtion, emissivity and reflection

    As I understand it, the absorption coefficient of a surface of material is the same as its emissivity (due to Kirchoff's law of thermal radiation), and that the net of this and the tranmissibility is emissivity + reflectivity + transmissibility = 1 Is this a general statement about...
  38. M

    Connection between emissivity and albedo

    Hi, I am studying thermal radiation. Does anyone know what is a connection between emissivity and albedo. Regards, Mike
  39. lavalamp

    Measuring emissivity of material based on reflection

    Homework Statement I have a university project that involves comparing various paints with regards to how they affect the insulating properties of houses. As part of that I would like to measure the emissivity of the paint for a variety of wavelengths. Unfortunately I can not simply heat...
  40. B

    Question about black bodies, emissivity.

    Imagine a diamond sphere radius of 1 cm traveling in space far from any heat sources. Embedded in the center of the sphere is a magic heater that can produce 0.3 W for an indefinite amount of time. If the emissivity of the surface is zero, what temperature does the sphere reach? Why? The...
  41. S

    What causes emissivity to be less than 1?

    Everything I have read about emissivity simply says that in thermal equilibrium, emitted radiation has to be equal to absorbed radiation. That's a glib explanation. What about an object that is not in thermal equilibrium with its surroundings? What about an object that is in a vacuum that...
  42. J

    Heat Transfer Problem - Radiation missing Emissivity value

    Homework Statement Two students, A and B, are discussing a heat transfer problem regarding what the surface temperature of a cartridge electrical heater would be when exposed to a water flow (CASE 1) and an air flow (CASE 2). Case 1: hw = 5000 W / (m2 * K); Tsurr = 20ºC Case 2...
  43. R

    Emissivity & Thermal Transmission

    Stainless steel has emissivity of 0.12. It means it only absorbs 12% of radiation and reflect the rest of the 88% as well as tranmitting the 12%. Now if you heat the stainless steel to say 300 Celsius. It won't transmit the 300 Celsius but only a little of it. This is why thermal imagers can see...

    Gas Radiation - Emissivity Value

    I work in the field of high temperature equipment and I am using Hottel's curves from his classical book "Radiative Transfer" then I bought a much newer book by Michael F. Modest, "Radiative Heat Transfer" and this book uses curves by Leckner. I rewrote the FORTRAN code from Modest's book into a...
  45. S

    Spectral Directional Emissivity

    Hi all, I am trying to understand why emissivity changes with direction. Specifically within the thermal band [7-14micron] and while looking at non-metallic surfaces (like a flat water surface). What I'm thinking is that since vibrating atoms are basically accelerating charges and create...
  46. R

    Emissivity of radiation from a surface, Leslies cube

    Homework Statement Hello! I am going to do a demonstration involving the Leslie's cube to demonstrate the emissivity of a thermal radiation from different surfaces. I have been reading some about black body radiation which has emissivity = 1 but this is not the case. Leslie's cube has 4...
  47. M

    Emissivity Dimensional Analysis

    Homework Statement Does anyone know where can i find (book?) the dimensional analysis of emissivity? Homework Equations The Attempt at a Solution
  48. S

    Studying Emissivity Effects on a Copper Box Experiment"

    Homework Statement The experiment we did was pretty basic, it was a copper box with five different metal rods (unlagged and of equal lengths and diameter) welded onto the bottom, on the bottom of each were pins stuck with petroleum jelly. Hot water was then added to the box and the order at...
  49. I

    Plotting a Suitable Graph to find Emissivity of Tungsten Filament

    Plotting a suitable graph to find emissivity of tungsten It is given that: Q = pσ2πal(T^4 - t^4) Where Q is the Energy Loss Rate, p is Emissivity and T,t are the wire and room temperature. Other symbols are constant. I have data for Q and (T^4- t^4), which for simplification purposes...
  50. B

    Find High Emissivity Paint for Accurate Infrared Temperature Measurements

    I'm supposed to measure temperature with an infrared camera. But when used on materials such as gold and aluminum or iron I get false readings (the emissivity is low). To solve this problem I figured I would need a paint that gives the surface high emissivity (low-reflective). My question...