Thermal Optics Explained

Q: What does NETD mean in thermal imaging?

NETD stands for Noise Equivalent Temperature Difference. It describes how well a thermal sensor or system can distinguish small temperature differences against its own image noise.

Q: What is pixel pitch in a thermal sensor?

Pixel pitch is the distance between the centres of neighbouring sensor pixels, measured in micrometres (µm). You will commonly see specifications such as 12 µm or 17 µm.

Q: 384 vs 640 thermal resolution: what is the difference?

Sensor resolution describes the number of thermal pixels available to capture an image.

Q: Does digital zoom improve thermal image detail?

Digital zoom enlarges a section of the existing image. It makes the subject appear bigger, but does not capture additional thermal detail.

Q: How do lens size and field of view affect thermal optics?

The lens and sensor work together to determine how much of the landscape you can see and how much detail is captured on a distant subject.

Q: What does 50 Hz mean on a thermal?

Frame rate, measured in hertz (Hz), describes how frequently the image updates. A 50 Hz device updates its image 50 times per second.

Q: Does thermal imaging work in fog and rain?

Thermal imaging works without visible light, but weather still affects performance.

Q: Which practical features matter when buying thermal?

Image quality is central, but everyday usability deserves equal attention.

Steelfoot

2 hours ago

6 min read

Choosing a thermal starts with understanding how you will use it. Whether you are observing wildlife, supporting deer management or checking your ground, the right combination of thermal sensitivity, sensor resolution and field of view makes a real difference.

This Steelfoot thermal buying guide explains NETD, pixel pitch, resolution and frame rate—and how to use those specifications when comparing devices.

What should you look for in a thermal?

Start with your usual viewing distances, surroundings and budget. A device suited to scanning woodland may offer a different balance of features from one used across open fields.

Your main use	Features to prioritise
Woodland and hedgerows	Wide field of view, manageable base magnification and straightforward focusing
Open fields and longer-distance observation	Enough sensor detail and suitable lens focal length to examine distant subjects
Conditions with little temperature contrast	Good thermal sensitivity, supported by capable optics and image processing
Long periods outdoors	Comfortable handling, manageable weight and practical battery options
General wildlife observation	A balance of image quality, field of view, portability and price

Compare the complete device. One impressive specification does not guarantee the best performance for your needs.

What does NETD mean in thermal imaging?

NETD stands for Noise Equivalent Temperature Difference. It describes how well a thermal sensor or system can distinguish small temperature differences against its own image noise.

NETD is measured in millikelvin (mK). A lower figure generally indicates greater thermal sensitivity when measured under comparable conditions.

For example, 30 mK is equivalent to a temperature difference of 0.03°C. A specification of NETD <30 mK describes performance under defined test conditions; it does not guarantee that every difference of that size will be clearly visible outdoors.

Greater sensitivity can help reveal subtle differences between an animal and its surroundings when thermal contrast is low.

When comparing NETD figures, check:

Whether the rating describes the sensor or the complete system.
Whether the measurements use comparable test conditions.
How the lens and image processing contribute to the final image.

NETD is also different from temperature measurement accuracy. A sensitive thermal monocular is designed to reveal temperature contrast, rather than necessarily provide precise temperature readings. FLIR’s specifications illustrate this distinction.

What is pixel pitch in a thermal sensor?

Pixel pitch is the distance between the centres of neighbouring sensor pixels, measured in micrometres (µm). You will commonly see specifications such as 12 µm or 17 µm.

Smaller pixel pitch can support a more compact sensor. With the same lens focal length, each pixel also covers a smaller angle of the scene.

However, 12 µm does not automatically mean a better image than 17 µm. A 640×512 sensor contains the same number of pixels at either pitch.

Pixel pitch needs to be considered alongside sensor resolution, lens design and thermal sensitivity. It should not decide the purchase on its own.

384 vs 640 thermal resolution: what is the difference?

Sensor resolution describes the number of thermal pixels available to capture an image.

Two common examples are:

Sensor resolution	Total thermal pixels
384×288	110,592
640×512	327,680

A 640×512 sensor contains almost three times as many pixels as a 384×288 sensor. With comparable optics and field of view, this allows finer detail to be captured.

A lower-resolution device can still be useful for locating heat sources and observing at suitable distances. Higher resolution gives you more information to work with when examining shapes or cropping the image.

Check sensor resolution separately from display resolution. A high-resolution screen cannot recover thermal detail that the sensor did not capture.

Does digital zoom improve thermal image detail?

Digital zoom enlarges a section of the existing image. It makes the subject appear bigger, but does not capture additional thermal detail.

For example, a straightforward 2× digital zoom on a 640×512 sensor uses a central area of approximately 320×256 sensor pixels, enlarged to fill the display.

Image processing may improve its appearance, but the underlying information is still limited by what the sensor recorded.

When comparing thermal monoculars, look at image quality at base magnification and at the zoom levels you expect to use. Maximum advertised magnification is only useful if the image retains enough detail for your purpose.

How do lens size and field of view affect thermal optics?

The lens and sensor work together to determine how much of the landscape you can see and how much detail is captured on a distant subject.

For the same sensor:

A shorter focal length generally gives a wider field of view.
A longer focal length generally gives a narrower field of view, placing more pixels across a distant subject.

A wide view helps when scanning woodland, following hedgerows or locating animals nearby. A narrower view can help when examining subjects across open ground, but shows less of the surrounding area.

Compare the manufacturer’s quoted field of view, usually expressed in degrees or metres at 100 metres. This is more informative than comparing lens size alone.

What does 50 Hz mean on a thermal?

Frame rate, measured in hertz (Hz), describes how frequently the image updates. A 50 Hz device updates its image 50 times per second.

Higher frame rates generally make movement appear smoother when scanning or observing moving wildlife. Lower rates can make the image appear more fragmented during movement.

However, frame rate is not the same as latency. Sensor response and image processing also affect how quickly the displayed image follows the scene.

Thermal detection range vs identification range

A quoted detection range describes how far away a device may register an object under specified conditions. It does not necessarily mean you can confidently establish what that object is.

Term	What it means
Detection	Noticing that something is present
Recognition	Distinguishing a broad type of object
Identification	Seeing enough detail to establish what you are observing

These distances vary with the subject’s size, weather, thermal contrast and the criteria used by the manufacturer.

Choose around the detail you need at your normal viewing distances. A distant heat signature and a clearly identifiable animal are different levels of information.

Does thermal imaging work in fog and rain?

Thermal imaging works without visible light, but weather still affects performance.

Rain, fog and atmospheric moisture can reduce image contrast and useful viewing distance. Better sensitivity can help reveal subtle differences, but it cannot remove these limitations.

Thermal imaging also does not see through solid cover. You may detect an animal through gaps in vegetation, while branches and dense foliage obscure parts of the view.

Where possible, compare devices outdoors in conditions similar to those in which you expect to use them.

Which practical features matter when buying thermal?

Image quality is central, but everyday usability deserves equal attention.

Before choosing a thermal monocular, consider:

Battery life: Does the stated runtime suit your outings, and under what conditions was it measured?
Battery options: Can you replace the battery or use an external power source?
Weight and handling: Is it comfortable to carry and hold for extended periods?
Controls: Can you focus and operate it easily while wearing gloves?
Weather resistance: Is its protection rating suitable for your intended use?
Support: What warranty, servicing and repair arrangements are available?
Recording: Do you need photographs and video, or will you mainly use it for live observation?

A device that is easy to carry, focus and operate is more likely to be useful every time you take it out.

Choosing the right thermal for your budget

Decide what you need to see, at what distance and in which conditions. Then compare devices that meet those needs.

For woodland observation, a comfortable device with a useful wide field of view may offer more value than additional magnification. Across open ground, greater sensor detail and an appropriate lens may deserve more of your budget.

Where possible, look through shortlisted devices side by side. Compare the same scene at similar magnification, including background detail and subjects with less obvious temperature contrast.

The best thermal monocular for you is the one that balances image quality, sensitivity, field of view and usability at a price you are comfortable with.

Find your thermal optics with Steelfoot

At Steelfoot, we help you understand the specifications and choose equipment around your intended use.

Tell us your budget, typical viewing distances and the ground you cover. We’ll help you compare the options and identify the features that matter most.

Explore our thermal optics range or contact Steelfoot for help choosing your thermal.

Tell us what you’re trying to achieve. We'll help identify the route to get there.

Steelfoot, Precision, Done Properly.