Brenton WardSeptember 3, 2018 at 11:29 pm #402
Infrared Measurement is an ideal and simple solution for many demanding temperature measurement applications. The most difficult challenge people face is choosing the appropriate equipment and knowing how to use it properly.
Aside from the thermographer, equipment is the greatest limiting factor in performing an infrared inspection. Using incorrect instrumentation can significantly impact the accuracy of your infrared data.
When choosing equipment it is important to seek information and advice from qualified and certified professionals. Do not simply rely on what others tell you, as there is a great deal of misinformation when it comes to the use and application of thermal imaging equipment. Few people have formal training and the simplistic nature of the operation of a cameras leads to much misinterpretation. It is always recommend to consult a professional thermographer and not rely on the recommendations of sales people. Most sales personnel are not formally trained in thermography. To avoid increased liability, most manufacturers will not want want their staff providing professional advice on performing thermography, and as such sales personnel are only trained on how to sell thermal imaging systems, not how to apply them.
Always consult a professional thermographer!
There are multiple attributes that you will need to understand and consider before selecting equipment for your intended application.
– Resolution and Spot Measurement Size
– Thermal Sensitivity
– Temperature Range and
– Special requirements such as
o Frame Rates
o Camera Capture Speed
o Real Time Video
o Indoor/Outdoor Use
o Screen Size
o Image Display
o Memory Storage
Price should be considered last, as a decision based on price alone may result in the purchase of equipment that is completely ineffective for your application
You can find out more on selecting the right equipment by reading the other articles in this thread. Each article focuses on a particular attribute in detail.
There are no industry standards that govern the design or manufacture of thermal imagers, and prospective buyers are totally reliant on the information contained within sales brochures to determine if a piece of equipment is suitable. Manufacturers will suggest applications the equipment is suitable for, but they cannot guarantee that you will operate the camera under the conditions necessary to gather accurate data. It’s fair to say that many sales brochures are designed to appeal to the widest audience possible (to maximize sales), and often this information can be unreliable when it comes to choosing the correct equipment for your application.
Resolution and Spot Size
What resolution should I buy? This is the first big question that most people must answer when deciding on which thermal imager to buy.
The size and number of picture elements (pixels) produced by a thermal imager is the primary performance specification and is generally termed “detector resolution”. There are both qualitative and quantitative aspects to this performance characteristic and it is important to understand both.
There are a wide variety of detector resolutions and while every manufacturer may have a unique offering, there are some standardised types. I would categorise them as follows:
Low Resolution –
80×80 (6,400 pixels)
120×120 (14,400 pixels)
160×120 (19,200 pixels)
Example of a 160×120 resolution image
In general, low resolution cameras are mostly used “in house” by plant personnel and site operators. The image quality is usually insufficient for professional presentation or for accurate temperature measurement on small targets (ie electrical components etc) because the spot measurement size is usually insufficient. I personally would only use these resolutions for qualitative thermography.
Medium Resolution –
320×240 (76,800 pixels)
384×288 (110,592 pixels)
Example of a 320×240 image
Medium resolution thermal imagers would represent the bulk of the “contractor” market and professional thermographer (at this time of writing). These have an excellent qualitative image, and provide much higher levels of temperature accuracy with a spot measurement size that is usually half that (ie twice as good) as the lower resolution models.
High Resolution –
640×480 (307,200 pixels) Example of a 640×480 image
The high resolution models are used in all manner of high end applications. They are particularly effective in research applications, providing more than 300,000 points of measurement. They are also favoured by high end contractors who can provide visually stunning images for their customers.
Temperatures are again much more accurate with a much smaller spot measurement size. For commercial scale building diagnostics, the highest resolution models mean much less work. 1 standard image is equivalent to 4 medium resolution images or 16 low resolution images. This saves a lot of time on documentation.
Detector Resolution is the principal determinant of spot size, as the greater the number of measurement points usually equates to a higher density of measurement points over a given area. Of course it is only part of the equation, as the lens angle and manufacturer’s image processing programming will determine the actual spot size over a given area
When purchasing a Thermal Imager one of the most critical specifications to be considered is resolution and spot measurement size. Spot size is a specification that pertains primarily to quantitative thermography. If you intend to present accurate temperature measurement in your reporting and analysis, your camera’s resolution and resulting spot size is of ultimate importance.
Spot size will vary between different models of imagers and it cannot be changed. Although spot size is primarily a quantitative issue, it is important to understand its importance prior to purchasing an imager or prior to capturing data that may be quantitatively analyzed in the future.
When attempting to measure temperature, spot size determines a radiometer’s ability to accurately measure temperature. Spot measurement size is defined as the area from which temperature data are derived. This is a combination of an imagers detector resolution, detector pitch size, lens angle and proprietary measurement algorithms. Spot size is NOT determined by display crosshairs, laser dots, etc.
The following recommendations serve as a general guide only. Please speak to one of our qualified thermographers before deciding what is appropriate for your application.
• The smaller your target the more resolution you will need.
• The greater the distance from your target, the more resolution you will need.
• Accurate temperature measurement requires the correct spot size for the target and for measurement accuracy; the spot size must be smaller than target area.
• It should be noted that it is impossible to compensate for incorrect spot size.
• Spot size must be appropriate at the time of measurement and/or at the time of image capture.
• When spot size is larger than the target, the imager will measure the target and the area beyond averaging the temperatures of both together.
• In some cases, this can produce temperature values that may be inaccurate by hundreds of degrees.
Thermal Sensitivity is an often overlooked objective specification. The number is small, and seemingly insignificant, but has major implications to image quality and measurement capabilities.
Most uncooled cameras will have sensitivity of around 0.1°C. This is considered average. The better uncooled sensitivities are less than 0.08 and the worst are closer to 0.2°C. Thermal sensitivity has a large impact on image quality as it results in either a crisp (good) or noisy/grainy (bad) image.
From a measurement perspective, higher sensitivity means we will see more temperature variation within a narrow band of temperatures.
Those applications which benefit most benefit from a high thermal sensitivity include
• building sciences,
• veterinary/medical applications,
• professional thermographer’s who need quality definition in their imaging.
Compare the following: Both cameras have the same resolution of 320×240, meaning the image resolution size is the same, but have different thermal sensitivities, meaning their thermal resolution is different.
Thermal Sensitivity of 0.1°C or NETD of 100mk
Thermal Sensitivity of 0.03°C or NETD of 30mk
The temperature range and span of each range can vary considerably between models. While some imagers can measure up to 2000°C, most entry level models will only measure up to 250°C. Professional cameras will measure in the range of -20 to 600°C or higher. Many cameras designed for building sciences or veterinary work, may only measure as high as 100°C, therefore making them unsuitable for electrical and mechanical applications.
As prices progress, so does the temperature range. The first thing to be sure of is that your camera is capable of measuring the desired surface temperature of your target. Be aware that the “optional” temperature ranges quoted in brochure will only work if the camera was ordered with this function. This option is specially added to the camera during manufacture as it generally involves the addition of filters and special calibration process. It can be quite expensive (depending on the range required), so it pays to make sure a camera has a desired range before purchase.
There may be some performance trade-offs with a specific temperature range. Typically narrow ranges improve image quality, while broad ranges can diminish a camera’s sensitivity. Also, a camera’s range may be “split” over several ranges. For example a camera with -20 to 600°C capability, might have 3 ranges to achieve this. ie -20 to 120, 100 to 300, 250 to 600. Where the concern may arise is if you needed to view a target that varied in temperature between 150 and 400°C. With the above set ranges you would be unable to view the target seamlessly in 1 single range, instead having to use range 2 and 3. This would make analysis in a single image impossible. You cannot measure a temperature outside a given range.
Again it pays to speak to a qualified consultant.
There are many applications which will have special needs with respect to the operation of a camera and will need to be considered when purchasing a unit. The following are examples where the physical characteristics of a camera are important.
• Frame Rates
• Camera Capture Speed
• Real Time Video
Frame Rates –
Cameras capture images at a variety of speeds. Many entry level cameras are NOT real time and will blur when looking at moving targets, or when used in a scanning motion. Generally entry level cameras are less than 10Hz (10 frames per second), and must be used while stationary, on stationary objects.
There are many applications where a faster frame rate will be required, otherwise the image will be blurred and distorted. When monitoring rotating or moving equipment, or when the thermographer themselves is moving (walking or in a vehicle), you will need a faster frame rate. Higher speed frame rates are usually above 30hz, with the faster rates at 50 or 60Hz.
If you have an application which occurs in a very short time interval, ie less than a few seconds, then you will also need to consider video capture in real time. This is where a camera is able to gather several seconds or minutes of streaming video (usually at 30 frames per second), and can then break each frame down individually for analysis. This is mostly found in high end professional cameras.
With this kind of application you should definitely seek the advice of a seasoned professional who has performed this kind of measurement.
Indoor or Outdoor Use –
If you are going to use your camera outdoors then special attention should be given to the type of viewing options the camera provides. Most external LCD screen are adversely affected by Sunlight, so much so that they are virtually impossible to see in bright sunlight.
Many entry level cameras provide some kind of sunshield which slide over the screen to form a protective tunnel. This is a crude but relatively effective remedy for short term use.
Some models do offer a day bright screen, which is specially lit to provide better viewing in outdoor elements. These are also quite effective, but the best solution is an internal LCD or CRT viewfinder otherwise known as an eyewell.
This solution is found in most professional cameras which offer both an external :LCD screen for indoor use, and an internal viewfinder (eyewell) for use outdoors in sunlight. If you perform extended work outdoors, the latest internal viewfinders offer low radiation OLED (organic LED) viewfinders which make it much more comfortable for use over long periods of time.
Screen Size, Onscreen display and Image Display –
There is a great variety of screen sizes and display resolutions, and the appropriateness of each is completely up to the end user’s personal preference. Screen sizes are measured diagonally across the screen and will range from as small as 2 inches to 5 inches.
Subjective characteristics to keep in mind when looking for an appropriate screen size are:
• Can I see the screen properly?
• Am I able to read all the small numbers clearly? Can I see the difference between an 8 and a 0 ?
• Can I see sufficient detail in the image (ie for high sensitivity applications).
• What is it like outdoors?
Bear in mind that in general, the larger the screen the higher the power consumption. If long battery life is an important factor having a smaller screen, or perhaps energy saving (screen saver) functions will be of benefit.
Is there enough information on-screen? –
The amount of data represented on-screen is also an important consideration for many applications. It may be important to have specific data in front of the operator at all times. Information such as the emissivity, image range, temperature range or filter, hot seeking cursors or other temperature related information may need to be instantly recognized by the operator.
Why does it look better on the camera than on the PC? –
Always bear in mind the image display is usually NOT the same as the detector resolution. In most cases (especially when larger screens are in use), the image will be enhanced or digitally increased in size to be presented on the camera’s screen.
Many 160×120 native resolutions (detector) will be enhanced to 320×240 or 640×480 on the rear screen of the camera for display purposes. This only serves to aesthetically enlarge the image and has no effect on measurement capabilities.
Often this enhancement is not carried through to the software, whereby the image will open in its native format. It pays to always compare the on-screen performance to the downloaded image to avoid getting a false sense of image quality. How the image looks in the PC software will be the best representation of how it will look for the customer when you prepare the report!
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