Thermal imagers, also known as thermographic cameras or infrared cameras, are highly sophisticated sensing devices that provide a visual representation of the infrared
energy emitted by objects. Since more infrared energy is emitted from higher temperature objects, by sensing these wavelengths of light invisible to the human eye,
thermal imagers are able to see heat and how its distributed, making them an extremely valuable diagnostic tool.
Thermal imagers have benefitted tremendously from rapid advancements in technology. Once prohibitively expensive and short on options, thermal imagers have become an
essential piece of equipment for many technicians, quickly replacing other non-contact temperature measurement tools. Anywhere spot checking of temperature is necessary,
a thermal imager is the best option. Applications for thermal imagers include:
- Identify heat loss from buildings
- Inspection of electrical substation
- Locate radiant heating wires or pipes
- Locate potential areas for mold growth
- Flat-roof leak detection for buildings
- Detect thermal patterns on boiler tubes
- Mechanical bearing inspections
- Detect insulation leaks in refrigeration equipment
- Automotive applications
- Auditing of acoustic insulation for sound reduction
- Chemical imaging
- Nondestructive testing
- Research & development of new products
- Pollution effluent detection
About Heat and Light
Though thermal imagers are a temperature mapping instrument, they rely on infrared light as the basis of their measurements. Infrared light is one of many forms of electromagnetic energy.
A basic principle of electromagnetic radiation is that it travels in waves. The distance over which the wave’s shape repeats itself, the wavelength, determines the nature of the energy.
Wavelengths can vary from thousands of kilometers to a fraction of the size of an atom.
The wavelength of electromagnetic energy also correlates to the quantity of energy it contains. The longer the wavelength is, the lower energy it has. Electromagnetic energy with long
wavelengths, referred to as low-frequency, is used to transport television and radio signals. Short wavelengths, or high frequencies, include x-rays and gamma rays.
Visible light, electromagnetic radiation to which our eyes have adapted to see, is usually defined as having a wavelength between about 400 to 700 nanometers. Each color within the spectrum
of visible light is defined by its particular wavelength so that the shorter wavelengths (around 400 mn) are violet while the longer wavelengths (around 700 mn) are red.
Electromagnetic radiation with wavelengths just shorter than the human eye can see is ultraviolet light. If the wavelengths are just longer than the human eye can see, it’s infrared light.
It is this infrared light, extending from the nominal red edge of the visible spectrum at wavelengths from 700 nm to 1 mm, that is captured by thermal imagers.
Infrared radiation is often thought of as radiant heat though electromagnetic energy of any wavelength will heat surfaces that absorb them. In reality, all matter with a temperature greater
than absolute zero emits thermal radiation due to the vibration of its molecules at a given temperature. Most of this radiation is concentrated in the 8 to 25 µm (infrared) range. Generally
speaking, the higher an object's temperature, the more infrared radiation emitted. So while the popular association of infrared radiation with thermal radiation is only a coincidence, there is
a correlation between the two making infrared a useful proxy for temperature measurement.
Emissivity
One of the most important factors to consider when using a thermal imager is emissivity which refers to the ability of a material to emit thermal radiation. All materials absorb, reflect and
emit radiant energy. Certain materials, however, are better at doing this than other materials. Emissivity, therefore, is the ratio of the radiation emitted by a surface of a material to the
radiation emitted by a blackbody at the same temperature.
A blackbody is an idealized surface that is both a perfect absorber and emitter of light. All radiation absorbed by a blackbody will also be emitted by it. Blackbodies therefore have an emissivity
of 1.0. Though the true blackbody is purely theoretical, dark materials with a rough surface generally have a high emissivity. Asphalt for example has an emissivity of 0.90 meaning that it absorbs
and emits 90 percent of radiant thermal energy and reflects only 10 percent.
The bottom end of the emissivity scale would be the perfect reflector which reflects, rather than absorbs, all radiation. The emissivity of this theoretical surface would be 0.0. Bright, glossy
materials generally have a low emissivity. Aluminum foil, for example, has a thermal emissivity value of 0.03, meaning it absorbs and emits only 3 percent of radiant thermal energy while reflecting
97 percent.
Accurate temperature measurements of an object using a thermal image required the technician to account for the emissivity of the material of which the object is made. The emissivity can be taken
from a table and entered into the imager which will account for all factors before displaying an accurate temperature.
How Thermal Imagers Work
Thermal imagers operate in much the same manner as modern digital cameras. Generally compatible with infrared light with wavelengths ranging from 0.9–14 μm, imagers use a specialized lens to
focus the infrared light emitted by the objects within the field of view. The focused light is scanned by a phased array of infrared-detector elements which creates a detailed temperature
pattern called a thermogram. The thermogram created by the detector is translated into electrical pulses which are sent to a signal processing unit which translates them into data for the display.
Once the data is sent to the display, it appears as various shades or colors, the intensity of which is determined by the amount of infrared light captured by the detector. Through the different
combinations that came from the impulses made by different objects, an infrared thermal image is created.
Remember, as temperature strongly influences the amount of infrared radiation emitted by all objects, thermography allows people to see heat and how it is distributed. This requires no visible
light at all.
Images from an infrared camera tend to have a single color channel since they don’t use a sensor that distinguishes between the different infrared wavelengths. Color cameras would need to be much
more sophisticated and would suffer from the fact that color has less meaning outside of the spectrum of visible light as differing wavelengths do not map uniformly into the system of color vision
used by humans.
Color is often added to the monochromatic images produced by infrared cameras in order to better differentiate temperatures. When viewing these images it’s important to note that the color scheme
is a relative value that does not correspond to specific temperatures. For the most part the highest temperatures are colored white (without regard if the highest temperature is 100˚ or 10˚),
intermediate temperatures are reds and yellows, and cooler areas blues and greens. A scale is usually shown next to the image which relates the colors to temperature.
Specifications for Thermal Imagers
As with most instruments, thermal imagers are available in a range of styles with specifications suited to one’s budget and needs. Most imagers are handheld, making them ideal for spot checking
temperatures for maintenance, troubleshooting, or inspection. Some, however, are designed to be fix mounted as part of a process application.
Other important specifications of thermal imagers include:
Resolution: Resolution refers to the number of pixels in the sensor array and determines how sharp the image will be. Some lower-end imagers have sensor of just 80 x 60 (4800 pixels)
while high-end imagers boast large 640 x 480 (307,200 pixel) arrays.
Frame Rate: The frequency (rate) at which an imaging device produces unique consecutive images called frames.
Thermal Sensitivity: The ability of the imager to detect minor temperature differences. High-end imagers have a thermal sensitivity of better than 0.035°C, about four times
better than lower-end imagers.
Field of View: The field of view is the angle at which the detector is sensitive to electromagnetic energy. The field of view determines how much the imager can see.
Digital Image Overlay: Some imager include a digital camera which creates a photograph over which the thermal image is displayed. This results in an image in which the details
are much easier to see making the image more useful to most users.
Spectral Band: The wavelengths of infrared light captured by thermal imagers. This is generally around 0.9–14 μm though it can vary among imagers.
Additionally, thermal imagers often include a number of measurement modes, temperature ranges, color palates, focus features, proprietary measurement functions, communications options, data logging
and annotation features, lens options, ingress protection, and others.
If you have any questions regarding thermal imagers please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
