Thermographic Measurement Techniques - Measuring Emissivity

Introduction
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

Emissivity
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.

Measuring Thin Film Plastics

A common application that is well suited to IR filters is that of measuring thin film plastics. Since the process of making thin plastic film itself is highly temperature critical, it is imperative to evaluate both the temperature and the uniformity of the plastic as it exits the extruder or web process. The product is typically moving at high speeds which precludes the use of contact temperature methods.

Most plastic films have spectral characteristics similar to polyethylene (depicted in the figure below) which is transmissive in both the short wave and long wave IR regions. Measuring thin film plastics can be challenging since without using a filter, you see “through” the plastic and measure the objects behind the plastic, rather than the plastic itself.

Spectral transmission of Polyethylene film with spectral response of plastics filter

Black Ice Thermal Images

Please see thermal images and associated visual images of black ice below:

Infrared Wildlife and Black Ice Detector - Looking for Feedback

by Rosaele Tremblay

Hello, I am a high school student writing a paper for my science project and I would like any feedback from the InfraMation readers (scientists or thermographers to see if I am on track with this idea or if anyone has suggestions as to how we can make this work. Thank you for any input.

You can provide comments and suggestions for Rosaele by leaving a comment on this post - Editor

Introduction
The electromagnetic spectrum includes gamma rays, X-rays, ultraviolet, visible, infrared, microwaves, and radio waves and each of them has a different wavelength and frequency. Infrared radiation is between visible light and the microwave portions of the electromagnetic spectrum and it is not visible to the human eye. Some animals do exist which see in infrared such as a few different snakes. Three categories exist in infrared: near, mid and far-infrared. Near-infrared is the closest to visible light and far-infrared is closer to the microwave portions. Infrared radiations are all around us every day coming from sunlight, a fire, radiator, a warm sidewalk and the TV remote. Everything on earth gives off heat when molecules begin to move and the higher the temperature of an object, the more the atoms and molecules will be moving which will produce a greater amount of infrared radiation. Objects with a temperature above absolute 0 radiate in infrared including the objects we perceive to be cold or freezing such as ice cubes or objects which are hot but do not visibly appear to be hot emit heat.

These shots of a coffee mug are in three different palettes to show that we assign the colors to gray steps. Human eyes see ten gray steps so to see the colors in definition we can assign 10 colors to them like in these shots. In these, white is hot and black is cool, but we can also invert these so that white is cool and black is hot, this is up to the thermographer.

Analyzing Building Images Acquired at Different Times

A camera user writes:

"I find that images taken of the same structure, not much separated in time, sometimes look very different.  This poses a problem for me as I’m trying to compare the heat images of different houses (to detect homes that need weatherization).  Presently I’m not confident that images of two homes reflect actual differences in the structures, or are caused by minor environmental changes or even by artifacts in the photography.  Here is an example.

Attached are two nighttime images of the front of my house, which faces east.  IR0310 was taken at 10:40 PM, IR0408 at 11:11 PM. (The clock on the camera is two hours fast.)  It was a cold night with little temperature change over the half hour between pictures.  The house thermostat was constant. 

I've set the palette and temperature range to give me good differentiation of houses along the street.  I took the first image of my house as I began imaging houses on my street, and I took the second image when I finished the street scan. 

I am surprised, first, that my house looks so different in the two images. And second, that the outside looks warmer in the later image.  If anything, I'd have expected the outside to have cooled.

Glad for any interpretation of this.  Needless to say, with this kind of variation on a single house, it is hard to get good images for comparing houses. "

Industry Outlook: Re-Envisioning The MRO Model

Written by Andy Teich, President, Commercial Systems Division, FLIR

Despite high unemployment numbers, our industrial base is dealing with a substantial shortage of skilled technicians. In particular, the skills gap has made it difficult to fill an increasing number of MRO roles, including non-destructive testing.

Many factors are at play, not the least of which is a rolling exodus of well-trained baby boomers heading for retirement. Expect skilled technician shortages in the millions, just in the U.S. Several strategies can address this dilemma.

First: training, training, training. It needs to be a key strategy for companies—one that’s protected and defended, even in adverse times. Training improves productivity, quality and job satisfaction. Continuing education keeps skill levels matched to advancing technology and cuts turnover and downtime. Job training and apprenticeship tax credits for companies offering training will go a long way in sustaining this priority.

Fresh thinking about MRO training will drive real progress on this front. Companies can increase access to training by de-emphasizing four-year degree requirements and increasing the role of tiered training certifications and two-year programs...

Read the entire article

Stainless steel reflecting sky temp

This is extracted from a recent message board post:

"I'm curious as to how to adjust for stainless steel reflecting sky temp. The apparent reflected temp of the ground and surroundings is approx 50F. The bottom half of the drum is reflecting this (SP4). The top half is reflecting the sky (SP5). 

In my software, if I set the emissivity to 1, the sky temp in the picture shows -34F.
SP4 and SP5 *should* be the same temp, within a few degrees, based on the operation of the equipment. If I enter 50F as RAT for SP4, I have to enter -95F as RAT for SP5 to get the two temperatures to match.

Why is it -95F and not -34F for SP5? I want to make sure I'm understanding and looking at the image correctly. "

Click image to enlarge

 

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