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Guide

How to Understand NETD and Thermal Sensitivity

NETD is the specification that decides whether a camera shows the faint patterns building inspection depends on. This guide explains how to read the figure, what conditions it was measured under, what values suit which jobs, and how to test a camera's sensitivity in practice.

By the Akermin Editorial Team · 3 min read

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NETD (noise equivalent temperature difference) is the smallest temperature difference a camera can distinguish from its own noise, stated in millikelvin under defined conditions (usually a 30 °C target, f/1 lens, and the camera’s normal frame rate). To use it, read the figure with its conditions, compare it with the contrast your work produces (building defects 1 to 3 °C, HVAC patterns 0.5 to 5 °C, electrical faults 10 °C and up), and prefer a margin of at least ten to one. Under 70 mK suits building work; under 50 mK is better; 100 mK and above is for large contrasts only.

Reading the specification

  • The number: lower is better. 30 mK means 0.03 °C.
  • The conditions: “@30 °C” and sometimes “f/1.0” and a frame rate. Sensitivity worsens on colder targets and with slower lenses.
  • “Typical” versus “maximum”: typical is the average unit; maximum is the guaranteed worst case.
  • Processing: some makers quote NETD after noise reduction; the image looks cleaner but fine detail is smoothed. See what NETD is.

Matching NETD to the job

Task Typical contrast Comfortable NETD
Insulation gaps, thermal bridges (good conditions) 1.5 to 4 °C Under 70 mK
Same, marginal conditions (small inside-outside difference) 0.5 to 1.5 °C Under 50 mK
Moisture by evaporative cooling 1 to 3 °C Under 70 mK
Radiant floor loops, duct runs 1 to 5 °C Under 70 mK
HVAC coil unevenness 0.5 to 3 °C Under 50 mK
Electrical connections 5 to 50 °C Under 100 mK is fine
Motors, bearings, compressors 5 to 40 °C Under 100 mK is fine

How NETD shows on screen

With a narrow span (say 5 °C across the palette), a camera’s noise becomes visible as speckle or shimmer on uniform surfaces. A 40 mK camera shows a smooth wall; a 100 mK camera shows grain. Widen the span and the grain disappears, along with the faint pattern you were looking for. Cameras with low NETD tolerate narrower spans, which is exactly what marginal-condition surveys need. See what makes a good thermal image.

A side-by-side test

  1. Point both cameras at the same plain interior wall from the same distance, in the same conditions.
  2. Set both to the same palette and the same manual span (for example 18 to 23 °C).
  3. Compare the grain. Then place a hand on the wall for ten seconds and remove it; the fading handprint’s last visible moment is a rough sensitivity comparison.

Interactions

  • Resolution: more pixels of the same detector technology often means smaller pixels and worse NETD at a given price. See how to compare thermal camera resolution.
  • Frame rate: 9 Hz cameras can integrate longer and may quote better NETD than 30 Hz versions of the same core.
  • Lens: a slower lens (higher f-number) passes less radiation and worsens NETD.
  • Conditions: no NETD helps if the inside-outside difference is too small; see why thermal images change with weather.

Cameras with stated NETD are listed in best thermal cameras for home inspection; the selection process is in how to choose a thermal camera for home inspection.

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

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

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

Flir C5 Compact 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 NETD is good enough for home inspection?

Under 70 mK works; under 50 mK is comfortable. Insulation defects and damp often present as 1 to 2 °C differences, well above either figure, but on marginal days the margin shrinks.

Does a lower NETD make images sharper?

Cleaner rather than sharper. Low NETD reduces speckle so subtle gradients are visible; sharpness comes from resolution.

Can I test NETD myself?

Roughly. Image a uniform wall with a narrow span; the amount of speckle indicates noise. Compare two cameras side by side on the same wall with the same span.

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.