Infrared Training Center

Wednesday, February 29, 2012

About Emissivity Tables

Emissivity tables may or may not contain real useful information concerning the actual emissivities of the objects you wish to measure.

There can be many variations within and among different emissivity tables. Here are some factors of concern:

  • Total Normal (broadband, perpendicular) emissivity - This is the emissivity over a very wide waveband. It may or may not be close to the actual emissivity with respect to your infrared camera.
  • Midwave emissivity - Some tables are listed as shortwave (now called midwave) or at a specific narrow short waveband. Even if the emissivity is specified to be within the same spectral waveband as your infrared camera, it still may not be as accurate as you suspect. This is due to differences in camera detector responses.
  • Longwave emissivity - Longwave tables can also be somewhat unreliable for the same reasons described for the shortwave. Older cameras used different detectors that had different responses within the long wave band.
  • Narrow waveband - A narrow band can be just as unreliable as a wide band. Some materials can have significant changes in emissivity over small wavebands.
  • Temperature - Some tables take into account the temperature of the object when the emissivity was measured. If you consider the previously mentioned variables, this does not necessarily make the tables any more reliable.
  • Conditional (rough, smooth, corroded, rusty) - Conditional parameters seem to offer useful information concerning emissivities, but it is sometimes quite difficult to ascertain the condition of a metal surface by looking at it. If you use an emissivity table to determine the emissivity of copper, you may find values ranging from 0.05 to 0.86, depending on the surface. Copper that appears to be very tarnished can still have an extremely low emissivity.

So, what is one to do regarding emissivity tables? First, realize their limitations. They can offer a ball park estimate. If you really would like to use emissivity tables, the best thing to do is to create your own based on measurements taken with your camera.

Click this link for instructions on how to measure emissivity.

Thursday, February 23, 2012

Accuracy Specification of FLIR Cameras

How accurately can the camera measure an absolute temperature?

Most FLIR Thermography cameras have a specified accuracy of  ±2 ºC (±3.6 ºF) or ±2% (whichever is greater) of reading for a blackbody target (emissivity ~ 1).

For example, for objects that are 100 °C or lower, the temperature reading off a blackbody can be 98°C to 102°C and be within specification. Similarly for objects above 100°C, say 200 °C, the reading could vary between 196°C and 204°C.

Some science cameras such as the SC660 are specified ±1°C or ±1% of reading.

This means that any camera, at any environment condition (within specification), at any time will give a reading within the accuracy specification.

However, a particular camera, at the same environment condition, will have a statistical repeatability of measurement that is much better than this. Typically close to the NETD value. This is also applicable when you compare adjacent pixels, provided your target(s) are optically resolved. This means that much higher accuracies can be achieved by comparing values with a known reference source in the image scene.

Wednesday, February 15, 2012

IR Pictures Through a Grating or Mesh

Question from a customer: “Scanning through the steel grating I read a temperature 5 to 15 degrees Celsius lower than scanning without the grating. I know it is to do with the steel grating but I was wondering why.”

Great question. Let’s take a look at a typical situation with and without a grating, and then placing the grating at different distances to the camera.

The distance between the fuse and the camera remains constant, and the camera is always focused on the fuse. The only changes are the insertion of the grating, and the distance of the grating from the infrared camera.

Figure 1. Looking at a fuse directly with no grating. Max temperature is 51.4 C.

Figure 2. We have inserted the grating close to the fuse.
Max temperature dropped to 51 C, a small error.

Tuesday, February 14, 2012

Errors & Omissions Liability Insurance

Error and Omissions Liability Insurance is a topic that has come up repeatedly in our message boards through the years. I have consolidated the most relevant information from all or our posts here for convenience. Hope this is useful.

We found a reasonable policy through this agent:
Elsa Escobar
Commercial Lines Underwriter
Costanza Insurance Agency, Inc.  
800.300.9775 x12
It took her some time but she says it would be much easier now that she's researched it once. Send her an email and add "Thermography insurance referral" on the subject line.
We ended up purchasing a General Liability Policy with 3M coverage, and added a Marine Policy to cover the camera. (The Marine Policy) is also used for contractor's Tools, and anything movable. Your best bet appears to be with and Independent Insurance company who has access to a wide variety of companies to draw from.

Wednesday, February 8, 2012

Thermographic Measurement Techniques - Measuring Emissivity

An infrared camera measures and images the emitted infrared radiation from an object. The fact that radiation is a function of object surface temperature makes it possible for the camera to calculate and display this temperature.

However, the radiation measured by the camera does not only depend on the temperature of the object but is also a function of the emissivity. Radiation also originates from the surroundings and is reflected in the object. The radiation from the object and the reflected radiation will also be influenced by the absorption of the atmosphere.

To measure temperature accurately, it is therefore necessary to compensate for the effects of a number of different radiation sources. This is done on-line automatically by the camera. The following object parameters must, however, be supplied for the camera:

  • The emissivity of the object
  • The reflected apparent temperature
  • The distance between the object and the camera
  • The relative humidity
  • Temperature of the atmosphere

The most important object parameter to set correctly is the emissivity which, in short, is a measure of how much radiation is emitted from the object, compared to that from a perfect blackbody of the same temperature.

Normally, object materials and surface treatments exhibit emissivity ranging from approximately 0.1 to 0.95. A highly polished (mirror) surface falls below 0.1, while an oxidized or painted surface has a higher emissivity. Oil-based paint, regardless of color in the visible spectrum, has an emissivity over 0.9 in the infrared. Human skin exhibits an emissivity 0.97 to 0.98.

Non-oxidized metals represent an extreme case of perfect opacity and high reflexivity, which does not vary greatly with wavelength. Consequently, the emissivity of metals is low – only increasing with temperature. For non-metals, emissivity tends to be high, and decreases with temperature.

Infrared Thermography for Buildings

Learn how a Thermal Imaging Camera can benefit you for building work.

Infrared Thermography for Buildings

Measuring Body Temperature with an Infrared Camera

by Mikael Cronholm and Gary Orlove

From a biological standpoint, human beings are so called warm blooded animals. That means that we maintain a fairly constant body temperature, regardless of the surrounding temperature. The term body temperature (that we compare with when we decide whether a person has a fever or not) refers to the inside temperature, or core temperature of the body. The outside of the body is nearly always colder. It must be, because as we convert the energy from our food when we do work, we also produce heat. That heat has to go somewhere and if the outside and inside temperatures were the same, no heat transfer would be occurring.

None of us have a constant metabolism, or energy conversion, over time. It varies with our activity level. That means that the amount of heat we need to lose also changes with time. Our surrounding temperature also changes up and down, which means that sometimes we need to conserve heat and sometimes we need to increase the cooling by increased evaporation of liquid, we start to sweat more. Sweating is something we always do, just more or less depending on the situation.

To be able to use non-contact measurement for fever screening purposes, we need to find a point on the outside of the body that is close to the inside temperature – our “body temperature”. Because the outside is colder, and varies from place to place on the body, it is obviously the highest temperature on the outside of the body that is also the closest to the inside temperature. So we want to look for a warm spot on the outside.