Mid-IR Borescope: Thermal Imaging of Hard-to-Access Area Application Note

Mid-IR borescope is an imaging instrument with customizable requirements for clear inspection of hard-to-access areas, such as boiler tubes, furnace walls, etc. The special environment must be considered in optical and mechanical design, especially the effect of high temperature on the lens. In some cases, customers also require light paths at certain angles in order to view the side wall.

Operation Principle

The overall design concept is shown in Figure 1. It consists of a thermal imaging lens, a Mid-IR camera, and a mechanical jig and apparatus for mounting and alignment. Optional water cooling, air purge unit, and protection window can provide optimum protection of the IR camera under rough environmental conditions.

Mid-IR Borescope 1
Figure 1. The layout of the Mid-IR borescope

The key specifications of the Mid-IR borescope are listed below. Compared with similar products in the market, we offer a larger field-of-view (FOV) and better pixel resolution.

MaterialSingle-crystal germanium
and silicon
Wavelength3.5 – 4 μm
Focal Length12 mm
Image Diagonal20 mm
Circular FOV79.6°
Detector Pixel Number640×480
Pixel Size25 μm
Table 1. Key specifications of Mid-IR borescope

A thermal Mid-IR imaging lens is a type of imaging lens that can withstand a temperature of up to 70°C (high thermal resistance) over long period of time. The exterior closure is designed to have high corrosion resistance. The layout is shown in Figure 2.

Mid-IR Borescope 3
Figure 2. The layout of the thermal imaging lens

For example, if the Mid-IR camera was chosen to have a pixel size of 25µm, it has a calculated resolution of 20lp/mm. The MTF is evaluated based on this resolution when designing. The results are shown in Figure 3.

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Figure 3. MTF curve with 20lp/mm spatial resolution


The Mid-IR borescope is suitable for use in hot or harsh environments and is typically placed beside a blast furnace or equipment with high operation temperature for real-time monitoring and control. It is designed to provide excellent monitoring of the image formed by objects at high temperatures. Figure 4. below shows the images formed by objects at high temperatures.

Mid-IR Borescope 7
Figure 4. Image result of thermal IR lens

The system could be used in but not limited to the following application scenarios:

  • Mobile or semi-permanent through-flame imaging inside power boilers, furnaces, and incinerators
  • Pinpoint problems before they cause outages
  • Inspect build-up of ash/slag on boiler tubes
  • Diagnose burner flame conditions
  • Measure temperature across the entire image
  • Record and analyze data to optimize combustion processes

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