Emissivity Definition and 57 Threads

So basically, I stumbled across this concept of radiative cooling. There are a couple of YouTubers who posted videos on this topic. What I understood, was that in passive daylight radiative cooling, one applies some kind of coating or film with high sunlight reflectance and high longwave...

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...

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...

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...

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...

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...

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...

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...

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...

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?

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...

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...

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...

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...

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...

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...

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...

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?

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...

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...

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)...

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!

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...

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

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...

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

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...

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...

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...

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...

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...

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...

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...

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...

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.

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...

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

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...

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...

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...

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...

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...

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...

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

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...

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...

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...

Homework Statement Hello, I'm a little confused. I did an experiment today with a Leslie Cube. One of the faces was a black paint, and another a white paint. I know that white reflects well and has a low emissivity, at least in the visual wavelengths. In the Leslie's cube, was hot water, so...

Homework Statement A tungsten filament in a lamp is heated to a temperature of 2.30 x 10^3 K by an electric current. The tungsten has an emissivity of 0.31. What is the surface area of the filament if the lamp delivers 35.0 W of power? Homework Equations Stefan's Law (a greek letter...

Hi first time posting so apologies if this is the wrong section to post this in! I'm having a bit of trouble defining the difference between reflection and emission on the atomic level. As far as I can see both just essentially involve excitation of an electron by a photon, followed by...