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Thermal Sensitivity Explained: Why NETD May Matter More Than Resolution

NETD is thermal sensitivity: how faint a temperature difference the detector can register before it disappears into its own noise. For building work, where a missing bat of insulation may be only 1.5 °C cooler, it matters as much as resolution.

By the Akermin Editorial Team · 3 min read

NETD, noise equivalent temperature difference, is the smallest temperature difference a thermal camera can distinguish from the random noise of its own detector. It is quoted in millikelvin (mK): a camera with 40 mK NETD can reliably show a 0.04 °C difference between adjacent areas under the specified conditions, while one with 100 mK needs a 0.1 °C difference. Lower is better. For building surveys, where the interesting defects are often only a degree or two cooler than their surroundings, NETD is as important as pixel count.

Where the noise comes from

Each element of a microbolometer detector warms by a tiny fraction of a degree when infrared falls on it, and the electronics that read that change add random noise. NETD is the temperature difference at which the signal from a target equals that noise, giving a signal-to-noise ratio of one. It is measured under stated conditions, usually a 30 °C blackbody target, f/1.0 lens and a defined frame rate, so figures from different makers are only roughly comparable.

Typical values

Camera class Typical NETD
Entry-level phone attachments and pocket cameras 70 to 150 mK
Mid-range compact cameras (FLIR C-series, TOPDON, HIKMICRO) 40 to 70 mK
Professional handhelds (FLIR E-series, testo 8xx) 30 to 50 mK
High-end and cooled research cameras Under 20 mK

What it looks like in the image

A camera with poor NETD shows a grainy, shimmering image on uniform surfaces, and small temperature differences vanish into the grain. A camera with good NETD shows smooth gradients and subtle patterns: the studs behind drywall on a mild day, the outline of a warm pipe under a floor, the slightly cooler stripe where insulation has slumped. Manufacturers’ image processing can smooth noise and improve the apparent result, at some cost in fine detail.

NETD and resolution together

Resolution decides how small a defect can be seen; NETD decides how faint. Neither substitutes for the other. A 640×480 camera with 100 mK NETD gives a sharp image of large temperature contrasts and loses the subtle ones; a 160×120 camera with 40 mK gives a blocky image in which those subtle contrasts survive. For building work the faint defects are the common ones, so a modest resolution with good NETD often serves better than the reverse. See thermal camera resolution explained.

NETD and conditions

The camera’s sensitivity is fixed, but the temperature contrast in a building is not. Insulation defects show up when there is at least a 10 °C difference between inside and out; on a mild day the contrast may be under the camera’s NETD and nothing shows. Weather, sun and wind change what any camera can see; see why thermal images change with weather.

Reading specifications

  • Look for the figure with its conditions, such as “<40 mK @ 30 °C".
  • Be wary of a camera that does not state NETD at all.
  • Treat figures better than about 30 mK on inexpensive cameras with suspicion; they may be measured after heavy averaging.
  • Remember NETD is not accuracy; the two are separate lines on the sheet.

Cameras with stated NETD are listed in best thermal cameras for home inspection; for the detector physics see how a thermal camera sees heat.

Flir C5 Compact Thermal Imaging Camera, WiFi, 160x120 MSX, Ignite

Flir C5 Compact Thermal Imaging Camera, WiFi, 160x120 MSX, Ignite

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Flir E5 Pro Thermal Imaging Camera, WiFi, 160x120 MSX, Ignite Cloud

Flir E5 Pro Thermal Imaging Camera, WiFi, 160x120 MSX, Ignite

View on Amazon

Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.

Frequently Asked Questions

What is a good NETD value?

Under 50 mK is good for building inspection; 30 to 40 mK is typical of professional cameras; 70 to 100 mK is common in entry-level units and adequate for spotting large differences.

Is NETD the same as accuracy?

No. NETD is sensitivity to small differences within an image. Accuracy (typically ±2 °C or ±2%) is how close an absolute reading is to the true temperature.

Does a lower NETD make the image sharper?

It makes it cleaner: less speckle, and subtle patterns stand out. Sharpness (fine detail) comes from resolution.

Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.