1. Focal Length: The Key Parameter that Determines "How Far You Can See"
In infrared thermal imaging, focal length directly determines the maximum distance at which the system can produce clear images. Similar in principle to visible light lenses, special attention should be paid to the following:
Long focal lengths (e.g., 75mm, 100mm): Concentrate the thermal radiation energy from distant targets onto the detector, providing clearer thermal images. Suitable for long-distance temperature measurement or observation.
Short focal lengths (e.g., 6mm, 13mm): Offer a wide field of view, suitable for scanning large areas for heat sources, but with limited detail resolution at long distances.
Special Consideration for Thermal Imaging: Since thermal imaging relies on temperature difference detection, focal length selection must also account for the thermal contrast of the target. Low-contrast scenarios (e.g., a person in a forest environment) require longer focal lengths for effective identification.
2. Field of View: Determining "How Wide You Can See"
Horizontal Field of View (HFOV): The most commonly used metric, indicating the horizontal coverage range.
Vertical Field of View (VFOV): The vertical coverage range.
Diagonal Field of View (DFOV): The maximum imaging range of the lens.
Calculation Formula:
The Horizontal Field of View (HFOV) is calculated as: HFOV equals 2 times the arctangent of (Detector Horizontal Size divided by (2 times the Focal Length)).
3. Practical Correspondence Between Focal Length and Field of View
Focal Length: 6–9mm
Typical Field of View: 45°×34° or wider
Applicable Distance: 10–50 meters
Typical Applications: Building heat loss detection, electrical cabinet inspection
Focal Length: 13–19mm
Typical Field of View: Approximately 25°×19°
Applicable Distance: 50–150 meters
Typical Applications: Photovoltaic panel inspection, perimeter protection
Focal Length: 25–50mm
Typical Field of View: 15°×11° or narrower
Applicable Distance: 150–500 meters
Typical Applications: Power line inspection, forest fire prevention
Focal Length: 75mm+
Typical Field of View: 6°×4.5° or narrower
Applicable Distance: 500 meters or more
Typical Applications: High-voltage equipment temperature measurement, long-distance monitoring
4. How to Calculate the Required Focal Length Based on Target Size and Distance
Use the target coverage pixel formula:
Minimum Required Pixels equals Target Size divided by (Detection Distance multiplied by the Instantaneous Field of View (IFOV)).
Rule of Thumb:
Target Recognition: At least 3×3 pixels.
Temperature Measurement Analysis: At least 10×10 pixels.
Precise Diagnosis: Recommended 20×20 pixels or more.
Example: Inspecting a 0.5-meter transformer bushing at 100 meters, requiring at least 10×10 pixels:
Using a 25mm lens (IFOV approximately 1.7 mrad):
Pixel coverage = 0.5 / (100 * 0.0017) ≈ 2.94 pixels (insufficient).
Using a 50mm lens (IFOV approximately 0.85 mrad):
Pixel coverage = 0.5 / (100 * 0.00085) ≈ 5.88 pixels.
