Photo Credit: pulsarwildlife.co.uk
Watching the Shadows
What NETD sensitivity really does for farm security after dark — and why the lowest number on the box won’t guard your fence line.
Summery
Thermal imaging is one of the most effective ways to spot people, animals and vehicles around a farm after dark. A big part of why some devices perform better than others comes down to a spec called NETD — a measure of how small a temperature difference the sensor can actually pick up. Lower NETD generally means better sensitivity, particularly on humid nights, in light mist, or when there’s a lot of vegetation in the way. But NETD is one piece of a bigger puzzle. Resolution, lens choice, field of view and image processing all matter just as much, and I’ve seen farmers spend extra on a low NETD number and end up with a device that’s wrong for their property. The right device is the one built for your terrain and your distances, not the one with the best spec sheet.
Here’s the detail
Why Thermal Matters More on a Farm Than in Town
Protecting a house in a suburb and protecting a farm are two completely different problems. A farm might have several kilometres of fence line, dark gravel roads, open camps, livestock scattered across paddocks, and next to no artificial light once the sun’s down. A standard security camera does a decent job watching a gate or a shed. It’s next to useless for the two hundred hectares behind it. Thermal doesn’t care about light. It reads heat instead, which means you can pick up a person, an animal or a vehicle in total darkness — conditions where the naked eye sees nothing at all. It also holds up better than ordinary cameras in light mist, smoke and dust, though none of that is unlimited — thick enough fog will beat any thermal device eventually.
Where this gets used on a working farm:
- Checking fence lines for cuts or gaps
- Scanning open camps and grazing land
- Keeping an eye on livestock at night
- Picking up movement near sheds, stores or the homestead
- Following activity through thinner bush or overgrown sections
- Sweeping large areas from a bakkie during a patrol
What is NETD?
NETD stands for Noise Equivalent Temperature Difference. Forget the name — what it tells you is simple: how small a temperature difference the sensor can detect before it’s just noise.
It’s measured in millikelvin (mK):
What is NETD?
- 40 mK = 0.040°C
- 25 mK = 0.025°C
- 15 mK = 0.015°C
Lower number, smaller temperature difference the sensor can pull apart. That’s really all it is.
Why That Matters in the Field
On a cold winter night, a person walking across a frosty camp stands out easily — their body heat is miles apart from the freezing ground around them. Almost any thermal device will catch that. Now take a warm, humid evening after a hot Karoo day. The soil’s still holding heat, the rocks are warm, the grass hasn’t cooled off yet. Suddenly that person isn’t nearly as different from their surroundings, and a weaker sensor starts showing you one flat grey blur instead of a shape. It’s worth knowing where the “standard” range figures on a spec sheet come from. The detection, recognition and identification distances quoted for a thermal imager are usually worked out assuming a healthy temperature gap — conventionally around 2°C — between the target and its background. The moment real conditions squeeze that gap smaller, those tidy numbers start to slip. That’s exactly where a lower-NETD unit earns its price. It doesn’t magically extend your range — it just gives you a usable picture when the temperature contrast has gone soft.
Thermal Crossover — the Problem Nobody Warns You About
On a cold winter night, a person walking across a frosty camp stands out easily — their body heat is miles apart from the freezing ground around them. Almost any thermal device will catch that. Now take a warm, humid evening after a hot Karoo day. The soil’s still holding heat, the rocks are warm, the grass hasn’t cooled off yet. Suddenly that person isn’t nearly as different from their surroundings, and a weaker sensor starts showing you one flat grey blur instead of a shape. It’s worth knowing where the “standard” range figures on a spec sheet come from. The detection, recognition and identification distances quoted for a thermal imager are usually worked out assuming a healthy temperature gap — conventionally around 2°C — between the target and its background. The moment real conditions squeeze that gap smaller, those tidy numbers start to slip. That’s exactly where a lower-NETD unit earns its price. It doesn’t magically extend your range — it just gives you a usable picture when the temperature contrast has gone soft.
“Trouble doesn’t wait for a cold, clear night with perfect contrast.”
Weather: What Thermal Can and Can’t Do
Thermal owns the night. No argument there. It also beats visible-light cameras in light mist, smoke and dust. What it won’t do is punch through everything. Fog, heavy rain, thick smoke and dust absorb and scatter infrared energy before it ever reaches the sensor. The thicker it gets, the weaker your signal.
A better NETD figure buys you a bit more range and clarity as conditions worsen — but there’s a limit. Dense Cape winter fog rolling off the mountains will beat a R100,000 unit just as easily as a cheap one. Nobody selling you thermal should tell you otherwise.
Can It See Through Bush?
No. Not through anything solid — tree trunks, walls, rock, a closed door, or vegetation that’s genuinely dense and unbroken. What it can do is pick up whatever’s exposed through the gaps: a face between branches, a leg through long grass, movement through a thin hedge, an animal half-hidden behind a bush. A more sensitive sensor makes those small slivers of exposed heat easier to catch. But if the vegetation is genuinely thick, all you’re seeing is the temperature of the leaves in front of your target. This is why knowing your own property — the blind spots, the thick sections, the likely approach routes — still matters more than any spec on a box.
Detection Is Not Identification
This is the point I wish more buyers understood before they spend the money.
Spotting something on a thermal screen and knowing what it actually is are two very different things. The industry usually describes this with three practical stages — Detection, Recognition and Identification, often shortened to DRI. They come from the Johnson Criteria, the performance model developed by John
Johnson in 1958 that’s still the standard yardstick for rating thermal imagers:
Detection — something’s there, distinct from the background, but that’s all you know.
Recognition — you can generally tell what kind of thing it is: person, animal, vehicle.
Identification — you can confirm the specifics: who it is, what type of vehicle, what they’re carrying.
Worth a note for accuracy: Johnson’s original model actually defined a fourth level, Orientation — being able to tell which way a target is facing, or roughly what shape it is — sitting between Detection and Recognition. Most practical discussion drops it and works with the three DRI stages above, which is why you’ll rarely see it quoted on a spec sheet.
Thermal is superb at the first stage. It’ll tell you something’s moving where it shouldn’t be, often well before you’d have noticed with the naked eye. Recognition gets harder with distance, and identification is genuinely difficult on thermal alone — a shape carrying something dark could be a fence dropper, or a stick. You often can’t tell from the thermal image alone.
There’s one more reason identification is hard, and it’s worth knowing: thermal doesn’t read temperature directly, it reads emitted infrared, and different surfaces give off that infrared differently — a property called emissivity. A shiny or wet surface (bare metal, glass, a wet roof, still water) can read cooler than it actually is, while a dull surface shows its heat plainly. It’s another reason the picture isn’t always the clean “hot thing on cold background” the brochures suggest.
That’s why thermal should never be your only tool. Once it flags something, you still need a way to confirm it — a spotlight, a visible camera, binoculars, a second vantage point, or someone moving in to check.
Why NETD Isn’t the Whole Story
NETD gets pushed hard in marketing because it’s one clean number that’s easy to put on a spec sheet and easy to
compare. That’s exactly why buyers over-index on it.
The image you actually get depends on several things working together.
Sensor resolution — how many thermal pixels build the image. Common formats run 256×192, 320×240, 384×288
and 640×512, with 1280×1024 sensors available at the top end — though those are expensive and still fairly rare in
the handheld monoculars most farmers actually buy. More pixels means more detail, and it also means digital zoom
holds up longer before the image turns to mush. I’d rather sell someone a higher-resolution sensor with a slightly
worse NETD than the reverse — the difference shows up the moment they try to zoom in on something at distance.
Lens size — longer lens, more base magnification, narrower field of view. Shorter lens, wider view, less
magnification. A long lens sounds impressive until you realise you can barely find anything with it because your field
of view has shrunk to almost nothing.
Field of view — how much ground you cover in one look. Wide is better for patrols and scanning open camps.
Narrow suits watching one fixed point, like a gate or a known crossing.
Base magnification vs digital zoom — base magnification is real. Digital zoom just stretches existing pixels and
makes the image blockier the further you push it. Don’t let a big zoom number on the box impress you; it’s not doing
what you think it’s doing.
Image processing — the software behind the sensor (noise reduction, edge enhancement, contrast handling) can
make two devices with near-identical specs look completely different in the field. This is the part you genuinely can’t
judge from a datasheet. You have to see it working.
Why NETD Isn’t the Whole Story
This is a practical guide, not an industry standard — treat it as a starting point, not gospel.
~ 40 mK Fine for good conditions with strong contrast: open fields, shorter distances, basic observation.
≤ 25 mK Noticeably better when target and background are close in temperature. Useful in humid conditions and areas with more vegetation.
≤ 20 mK Strong sensitivity, holds detail well in low-contrast situations. Common now in decent mid-to-upper-range monoculars.
≤ 15 mK Excellent detail in tough conditions, but only if the rest of the device — sensor, lens, processing — is pulling its weight too. A sub-15 mK unit paired with a poor lens is still a poor device.
Don’t assume you need the lowest number on the shelf. Plenty of farmers would get more real-world use out of a
mid-range NETD with the correct lens and field of view than a top-spec sensor that doesn’t suit their terrain.
Reading Spec Sheets Properly
Manufacturers aren’t consistent about how they quote NETD, so before you compare two units, check:
- Whether the figure is for the sensor alone or the whole imaging system
- Whether it says “typical,” “less than” or “maximum” — these aren’t the same thing
- What lens aperture was used for the test
- Whether every model in that product range actually shares the same sensor (often they don’t)
- That the number you’re reading applies to the exact model you’re buying, not just the range
Never assume the whole range shares one spec sheet. Pull the datasheet for the exact model.
Working Out What You Need
Before you buy anything, work out the job the device has to do.
How far do you realistically need to see? Is the terrain open or thick with bush? Foot patrol or from a vehicle? Do you need a wide scanning view or a fixed narrow watch on one spot? Is early detection the priority, or do you need to identify what you’re looking at? What’s the weather like most nights where you are? How long does the battery need to last, and do you need recording or a rangefinder built in?
And get real numbers, not guesses. Pace out or estimate the distance to your main gate, your nearest fence line, across your biggest camp, to the livestock camps, and along the routes someone would actually use to get onto your property. Those numbers decide your lens and magnification — not a “long-range” label on a box.
Thermal Is One Layer, Not the Whole System
A thermal monocular gives you early warning. It was never meant to be your entire security plan.
A setup that actually works usually layers several things together: thermal observation, visible-light cameras, decent perimeter lighting, motion sensors, an alarm system, working radios, patrols, and — this is the part people skip — an actual plan for what happens once something’s detected. Thermal finds the problem. Everything else is what lets you confirm it and respond without walking into something blind.
Get Advice That’s Just About Your Property
Every thermal device looks roughly the same on a spec sheet until you’re standing in your own camp at 2am trying to use it.
Survival Africa supplies selected HIKMICRO and Pulsar thermal monoculars and binoculars for farm security, wildlife work and general outdoor use. When you get in touch, it helps to have:
- Your rough viewing distances
- A sense of the terrain — open, bushy, hilly
- What you actually need to watch
- Foot or vehicle use
- A realistic budget
Give us that and we can point you at something built for your farm, rather than whatever happens to have the flashiest number on the box.
Bottom Line
NETD matters because farm conditions are rarely the clean, cold, high-contrast nights that make thermal look effortless in the brochure photos.
A lower NETD figure genuinely helps when your target and the background are close in temperature. But it’s one spec among several, and chasing the lowest number on its own is how people end up with the wrong device for their property.
Get the sensitivity, the resolution, the lens and the field of view matched to your actual farm, and you’ll get far more out of it than someone who bought on NETD alone.
Talk to a Survival Africa Specialist to discuss your needs before you buy
Contact Johan Dempers on 082 394 3742.
Every farm and ranch block is different — terrain, target species, budget, and whether you need detection only or detection plus identification all change the recommendation. If you’d like guidance on what fits your situation, get in touch with Survival Africa.
Survival Africa supplies thermal imaging and night vision optics to farmers, game ranchers, and wildlife enthusiasts across South Africa.