What NETD actually measures

NETD stands for Noise Equivalent Temperature Difference. It is a signal-to-noise measure, expressed in millikelvin, that describes how small a temperature difference the imaging system can separate from its own noise.

Lower is better. A 25mK specification represents 0.025°C of temperature difference; 40mK represents 0.040°C. That does not mean the camera will measure absolute temperature to ±0.025°C or ±0.040°C. It means the imaging chain can resolve smaller thermal variations under the stated test conditions.

That distinction matters because thermal sensitivity, temperature accuracy and detector resolution answer three different questions:

25mK vs 40mK: the difference in numbers

The gap is 15mK. Put another way, 25mK is 37.5% lower than 40mK. But image quality is not a percentage score, so it would be wrong to say a 25mK camera is “37.5% better.” The visible result depends on the scene, the lens, detector behavior, image processing and display settings.

The practical question is not whether 25 is lower than 40. It is whether the target contains enough subtle temperature structure for that lower noise threshold to matter.

Can you actually see the difference?

In low-contrast scenes, 25mK can preserve smoother tonal transitions and cleaner edges before aggressive sharpening is applied. The result is not necessarily a dramatic “before and after.” It is often a quieter background, better separation between neighboring surfaces and fewer details buried in temporal noise.

Lower NETD matters most when the scene is difficult—not when the target is already glowing against the background.

A practical Boundvis 640×512 comparison

Boundvis lists two compact 640×512 modules: the B640A1 25mK thermal camera module and the B640A2 40mK thermal camera module. Both use a 9mm focal length and share a published 46.2° × 37.7° field of view. Their system specifications differ in more than NETD, which makes this a useful product-selection comparison—but not a controlled laboratory A/B test.

The compact 640×512 B640A1 is the more natural fit when weak thermal contrast is the main concern and a compact CVBS module meets the architecture. The 60Hz B640A2 configuration is worth considering when the design values motion rendering or its interface configuration more than the last step in thermal sensitivity.

What else controls the image?

Lens F-number

The lens determines how much infrared energy reaches the detector. NETD values should be compared at the same F-number. The B640A1 specification states its ≤25mK condition at f/1.0, 50Hz and 300K—details that should remain attached to the headline number.

Image processing

Automatic gain control, temporal noise reduction, edge enhancement and local contrast can change the apparent sharpness of a thermal image. Strong processing can make a noisy image look punchy in a screenshot while removing weak, useful temperature structure.

NUC and calibration state

Non-uniformity correction and shutter control suppress fixed-pattern variation across the detector. A module that is due for NUC may look streaky or uneven, regardless of the NETD value printed on its datasheet.

Focus, range and atmosphere

Poor focus can erase the benefit of a sensitive detector. At longer distances, humidity, fog, rain and atmospheric absorption reduce target contrast before the signal reaches the lens.

Where 25mK earns its place

• Building inspectionUseful when the task involves subtle surface patterns associated with insulation gaps, thermal bridges or moisture.
• ElectronicsHelpful for early-stage heating where neighboring components differ by only a small amount.
• Mechanical maintenanceMore valuable when looking for gradual temperature changes rather than obvious hotspots.
• Long-range observationCan preserve more target detail when the object approaches the thermal background, although the lens and target pixel coverage still set the range.
• Research and developmentBetter suited to work where weak thermal gradients must remain visible for later analysis.

When 40mK is already enough

There is no benefit in buying sensitivity that the application cannot use. A 40mK module can be entirely appropriate when:

• The target is normally much warmer or colder than its surroundings.
• The task is detection or navigation rather than subtle thermal analysis.
• 60Hz motion rendering is more important than the lowest available NETD.
• The project requires a particular PAL or UVC thermal camera interface.
• Size, power, cost and supply availability dominate the design decision.

For many surveillance, robotic and general monitoring systems, 40mK still provides strong usable contrast. A complete module that fits the signal path is usually a better engineering choice than a lower-NETD core that forces extra conversion hardware.

How to compare NETD figures fairly

Before placing two datasheets side by side, check the conditions behind the number:

1. F-number: compare f/1.0 with f/1.0, not with a slower optic.
2. Scene or detector temperature: 300K is roughly 27°C; sensitivity can be quoted at different temperatures.
3. Frame rate and integration time: longer integration can improve sensitivity but may affect motion performance.
4. Typical or guaranteed: a typical value and a maximum value are not equivalent promises.
5. Processing state: confirm whether the figure describes the detector, the camera core or the final processed output.
6. Measurement method: ask whether both products were tested using the same standard and setup.

If those conditions are missing, the NETD figure is useful as a clue—not as a complete decision.

Which one should an OEM choose?

Start with the scene, not the number. A complete thermal camera module selection process should define the smallest thermal difference that matters, the target size, working distance, field of view, output interface and frame-rate requirement. Then use NETD to refine the shortlist.

The decision is about the scene

25mK is not a universal “better image” switch. Its advantage appears when the scene contains weak thermal contrast and the rest of the optical chain is good enough to preserve it. In easy scenes, 40mK may deliver everything the application needs while offering a frame-rate or interface configuration that fits the system better.

For OEM selection, compare complete modules: NETD, resolution, lens, FOV, frame rate, interface, calibration behavior, power and mechanical envelope. The best specification is the one that survives contact with the actual product design.

Related Boundvis engineering notes

• PAL vs UVC for Infrared Camera Modules Choose between low-latency analog video and plug-and-play digital output.
• Multi-Interface Thermal Camera Modules See how interface choice, power input, NUC and mechanical fit affect OEM integration.
• How to Choose an Infrared Module That Will Not Ruin Your PCB Review connector, signal-integrity and mechanical-integration risks before design freeze.