May, 2011
Feature
Inspection News and Views from the American Society of Home Inspectors



Emissivity: Another Take on Infrared

STEVE NATIONS

Key Concepts

  • March infrared camera article warrants additional viewpoint

  • The three dimensionless values (emissivity, reflectivity, transmissivity) add up to 1 for all objects.

  • It's only in the emitted infrared energy that we can find an accurate temperature of a surface. So, infrared cameras allow the user to set the emissivity value, which the camera uses to determine the temperature of the surface.

  • For the majority of home inspection applications, finding an accurate temperature is not nearly as important as finding the temperature differences between objects and between areas of a surface.

  • To get an IR image suitable for home inspection work, often the most important factor is the level and span of the image.

  • If it is important to find an accurate temperature then more issues come into play, such as reflected apparent temperature.

  • For all objects, but especially for objects with high reflectivity, the temperatures of the other surfaces in the room will affect the infrared reading.

The article in the March ASHI Reporter on the use of infrared cameras to help reduce energy loss merits some background information, as well as another viewpoint regarding the important parameters.

Emissivity is a measure of how much infrared radiation a body emits compared to a perfect "black body."

Reflectivity is a measure of how well a body reflects back (rather than absorbs) infrared radiation that falls on it.

Transmissivity, in the case of translucent objects, is a measure of how well infrared radiation passes through an object. Together, these three dimensionless values (emissivity, reflectivity, transmissivity) add up to 1 for all objects.

That is, some of the infrared radiation that comes from an object is emitted from that object, some is reflected by that object and some is transmitted right through that object. The emissivity of painted drywall is about 0.95, depending on the color. For a metal vent connector, it's around 0.4, but may be significantly less if it's highly polished.

It's only in the emitted infrared energy that we can find an accurate temperature of a surface. So, infrared cameras allow the user to set the emissivity value, which the camera uses to determine the temperature of the surface.

In the article, Mr. Buchstaller notes he sets the reflected temperature parameter to get a temperature measurement that is as true as possible. But for the majority of home inspection applications, finding an accurate temperature is not nearly as important as finding the temperature differences between objects and between areas of a surface. It is these temperature differences that alert us to potential problems that need further investigation, such as the potential for water damage or the lack of insulation. When looking for moisture problems, for example, we're typically looking for signs of evaporative cooling. As water evaporates, it takes heat with it, leaving the affected area cooler than the surrounding area. Whether the wall is 74 F and the water spot is 71 F, or the wall is 71 F and the water spot is 68 F really doesn't matter.

To get an IR image suitable for home inspection work, often the most important factor is the level and span of the image. The level is the center point of the displayed temperature scale, and the span is the interval of the temperature scale. Together, these parameters determine the high and low temperatures shown in the image, with all temperatures above or below these values being shown by single-saturation colors.

A proper level-and-span setting creates an image that shows a good contrast, making detection of possible problem areas easier. A span that is too large tends to wash out the image, which doesn't provide good image contrast for detecting abnormalities. A small span tends to create large areas of saturation in the image, which requires the level to be adjusted frequently.

If it is important to find an accurate temperature, then more issues come into play, such as reflected apparent temperature. For all objects, but especially for objects with high reflectivity, the temperatures of the other surfaces in the room will affect the infrared reading.

For example, if there is a hot boiler in the room with your infrared camera, this boiler is emitting a great deal of infrared energy. Subsequently, a great deal of this infrared energy is being reflected by the surfaces in the room back to the camera, and this will make all the surfaces appear to the infrared camera to be hotter than they are, more so for those surfaces with high reflectivity.

To compensate for this reflected energy, the reflected apparent temperature parameter can be adjusted, allowing for a more accurate temperature reading.


Image-1-e_95-big.jpg
Image 1, above – An infrared image from a recent inspection, showing a cool spot on the ceiling below a bathroom. This is a qualitative image, alerting the inspector to a problem. The exact temperatures of the ceiling or the spot are not important. Follow-up with a moisture meter confirmed that the area was wet.



Image-2-e_45-rat_58-big.jpg
Image 2, above – The same image as Image 1, with the emissivity changed from 0.95 to 0.45 and the reflected apparent temperature from 68 F to 58 F. The image has changed imperceptibly, but the spot temperatures have jumped, and the level and span have increased. This image still cries out for further investigation, even though the temperatures are wildly inaccurate.



Image-3-Span-60_80-big.jpg
Image 3, above – The same infrared image with the same settings as Image 1, but with a larger span. While still visible, the wet area is much less distinct and harder to notice with the span set to 20 F.