Understanding Emissivity

 

Cold Can Thermal Image

Can you explain the hot spot?

This can of beer is ice cold straight out of the fridge. When scanned with an infrared camera you would expect the entire image to be relatively even in temperature and to appear "cold" in relation to the background. Can you explain the "hot" spot in the center of the can. (Hint: it's not a fingerprint!)

The live image reveals truth.

The paint on the outside of the can has been scratched off in a small area. The bare aluminum has a different emissivity than the painted aluminum. The camera can only allow for one emissivity setting at one time so to the detector the bare aluminum "images" hotter than the rest of the can.

Examples like this one show how emissivity can cause false readings in the field.

 

Emissivity Definition

Emissivity is a measure of the thermal emittance of a surface. It is defined as the fraction of energy being emitted relative to that emitted by a thermally black surface, known as a black body. A black body is a material that is a perfect emitter of heat energy in that it emits all energy it absorbs and has an emissivity value of 1. In contrast, a material with an emissivity value of 0 would be considered a perfect thermal mirror and imaging this material would result in readings of reflected energy only and not the actual material. For example, if an object had the potential to emit 100 units of energy but only emits 90 units in the real world. That object would have an emissivity value of 0.90. In the real world there are no perfect "black bodies" and very few perfect infrared mirrors so most objects have an emissivity between 0 and 1. Emissivity is a variable that makes it very difficult to obtain exact temperature readings with an infrared camera or spot radiometer. This is due to the fact that it is highly impractical to measure the emissivity of every object in your field of view. For example, if you are scanning an electrical panel in a predictive maintenance application you will be imaging wires, fuses, nuts, bolts, and other materials all of which will have a different emissivity value. So how do we obtain accurate reliable information?

 

Managing Emissivity

As stated before, it is hard to determine the emissivity of all objects in your field of view. It is also difficult to determine the emissivity of a single known material. This is because emissivity is a measure of the "surface" emittance of an object. The surface of objects (especially metals) change with the passing of time. For example, if you look at the table below you will see that corroded copper (E 0.78) has a significantly different emissivity value than shiny copper (E0.02). This difference introduces a subjective judgement call on the part of the thermographer. How do you determine how corroded or shiny a piece of copper is? The answer is you can come close but you cannot be exact. Additionally, the numbers in the emissivity table are approximates or averages, in the real world values may be skewed. So how do we manage emissivity? It is possible to determine actual emissivity values for a material but it is not practical in the real world. The process requires a spectrometer (expensive and usually not available) or dismantling the object and testing each piece. Even if you determine an accurate E value, the next time you scan the item that value will have changed rendering your old value useless. Fortunately, In most infrared applications an exact temperature measurement is not necessary. For example, if a circuit has a fault limit of 150° F and your instrument measures 100° F and the E value skews the temperature reading by 5°F you are left with a +- 5°F variance which in this case is negligible. Additionally, most thermal infrared applications rely on temperature difference (Delta T ) rather than exact temperature readings. To use our previous example of the circuit we measured, there would most likely be more than one circuit next to each other. If you use the same E value for both circuits they will both be skewed the same amount. If the one circuit was reading 100°F (which we will assume is normal operating temperature) and the adjacent circuit reads 150°F we are left with a Delta T of 50°F which would indicate a problem and as you can see negates the emissivity problem. E values become even less of a problem when trending an area over time. If the same circuit with the reading of 100°F has a reading of 110°F the next time you scan it and a reading of 115°F the next time, with the same emissivity setting, we know a problem is developing regardless of the error introduced by emissivity.

Dealing with emissivity is not as hard as it would seem. The important things to remember are that exact temperature measurements are difficult to obtain (do not promise this to your clients unless you are sure you can back it up), temperature difference (Delta T) is more important than exact readings in most applications, and that trending an object can reveal problems regardless of E value error. In the real world you pick an emissivity value that approximates the scene you are imaging and then you record it and maintain that same setting every time you scan that object.

We hope this article answers your questions regarding emissivity. A full explanation of emissivity is beyond the scope of this web site. We have included a brief emissivity table below listing some of the more common materials you will encounter. If you have any questions regarding this topic contact Sierra Pacific Innovations at 702-369-3966 or email us.

This is not a comprehensive list and should be taken as a guide only.

Emissivity Table of Common Materials

Material

Temp °C/°F

E

Aluminum Foil 27/81 0.04
Aluminum Disc 27/81 0.18
Aluminum Household (Flat) 23/73 0.01
Aluminum (Polished Plate 98.3% Pure) 227/440 0.04
  577/1070 0.06
Aluminum (Rough Plate) 26/78 0.06
Aluminum (Oxidized @ 599°C) 199/390 0.11
  599/1110 0.19
Aluminum Surfaced Roofing 38/100 0.22
Aluminum Colorized Surfaces @ 599°C    
          Copper 199/390 0.18
          599/1110 0.19
           Steel 199/390 0.52
  599/1110 0.57
Asbestos Board 23/74 0.96
Asbestos Paper 38/100 0.93
  371/700 0.95
Asphalt (Paving) 4/39 0.97
Brass (Highly Polished):    
73.2% Cu- 26.7% Zn 247/476 0.03
Brass (Hard Rolled - Polished w/Lines) 21/70 0.04
          (Some What Attacked) 23/73 0.04
Brick (Red - Rough) 21/70 0.93
Brick (Silica - Unglazed Rough) 1000/1832 0.80
Carbon (T - Carbon 0.9% Ash) 127/260 0.81
Concrete - 0.94
Copper (Polished) 21-117/70-242 0.02
Copper (Scraped Shiny Not Mirrored) 22/72 0.07
Copper (Plate Heavily Oxidized) 25/77 0.78
Enamel (White Fused On Iron) 19/66 0.90
Formica 27/81 0.94
Frozen Soil - 0.93
Glass (Smooth) 22/72 0.94
Gold (Pure Highly Polished) 227/440 0.02
Granite (Polished) 21/70 0.85
Ice 0/32 0.97
Iron & Steel:    
          Iron Galvanized (Bright) - 0.13
          Iron Plate (Completely Rusted) 20/68 0.69
          Rolled Sheet Steel 21/71 0.66
          Oxidized Iron 100/212 0.74
          Wrought Iron 21/70 0.94
          Molten Iron 1299-1399/3270-2550 0.29
Lead (Pure 99.9% - Unoxidized) 127/260 0.06
Marble (Light Gray Polished) 22/72 0.93
Nickel Wire 187/368 0.10
Paper (Black Tar) - 0.93
Paper (White) - 0.95
Plaster (White) - 0.91
Plywood 19/66 0.96
Tin (Bright Tinned Iron Sheet) 25/77 0.04
Water - 0.95
Wood (Freshly Planned) - 0.90
Zinc Galvanized Sheet Iron (Bright) 28/82 0.23