If you've started looking at thermal optics, there's a good chance you've already come across terms like 384, 640, 12μm and NETD.
One of the biggest jumps you'll see — particularly in price — is between a 384-class thermal sensor and a 640-class sensor.
So what are you actually paying for?
And more importantly, do you actually need 640?
Let's break it down.
WHAT DOES 384 VS 640 ACTUALLY MEAN?
When people refer to a “384 thermal” or “640 thermal”, they're generally talking about the resolution of the thermal detector.
Two common resolutions are:
384 × 288 = 110,592 detector pixels
640 × 512 = 327,680 detector pixels
That means a 640 × 512 sensor contains nearly three times as many detector pixels as a 384 × 288 sensor.
Each detector pixel measures infrared energy from part of the scene.
More detector pixels give the thermal system more spatial information to work with, which can result in a more detailed image.
But there's an important catch:
Sensor resolution isn't the only thing determining how good a thermal looks.
Lens size, pixel pitch, thermal sensitivity, image processing, display resolution, magnification and environmental conditions all play a part.
WHAT DOES THE DIFFERENCE LOOK LIKE IN THE FIELD?
This is where the additional resolution starts to matter.
At relatively short distances, a good-quality 384 thermal can produce an excellent image.
You may easily detect an animal, understand its general shape and observe its movement.
As distance increases, however, the target occupies fewer pixels on the thermal detector.
That's where having more available detector resolution can become valuable.
A 640 sensor gives the system considerably more pixels across the scene, potentially allowing finer details to remain visible depending on the optic, lens and conditions.
This can make observing and interpreting distant heat signatures easier.
384 THERMAL — WHERE DOES IT MAKE SENSE?
Don't make the mistake of assuming 384 = bad.
Modern 384-class thermals can be extremely capable.
For many users, 384 offers an excellent balance between:
Image quality
Useful range
Size and weight
Battery consumption
Price
If most of your thermal use occurs at relatively modest distances, paying significantly more for 640 may not provide enough practical benefit to justify the additional cost.
A quality 384 unit can also outperform a poorly implemented higher-resolution thermal in some circumstances.
The number printed on the box doesn't tell the entire story.
WHY CHOOSE 640?
The biggest advantage is simply having more thermal information available to create the image.
A 640 × 512 detector contains approximately 327,680 detector pixels, compared with approximately 110,592 in a 384 × 288 detector.
That additional resolution can be particularly useful when:
- Observing smaller targets
- Viewing targets at greater distances
- Trying to distinguish finer details
- Using digital magnification
- Scanning large areas
- Operating in situations where image detail is particularly important
For someone who uses thermal frequently or wants higher-end image performance, the jump to 640 can be worthwhile.
WHAT ABOUT DIGITAL ZOOM?
This is another area where sensor resolution becomes important.
Digital magnification doesn't magically create additional thermal detail.
Instead, the system enlarges a smaller portion of the detector image.
As digital magnification increases, fewer original detector pixels are being used to represent the target.
That's why thermal images generally become increasingly pixelated as digital zoom increases.
Starting with a higher-resolution detector gives the system more original pixels to work with.
As a result, a 640-class thermal will generally have a significant advantage when digital magnification is required, although the final result still depends heavily on the complete optical and processing system.
DOES 640 MEAN MORE DETECTION RANGE?
Not automatically.
This is one of the biggest misconceptions surrounding thermal specifications.
A thermal's detection capability is influenced by numerous factors, including:
Detector resolution
Lens focal length
Pixel pitch
Target size
Base magnification
Atmospheric conditions
Thermal contrast
A 640 sensor doesn't automatically detect something twice as far away as a 384 sensor.
In fact, two thermals with different resolutions can have similar quoted detection distances depending on their lens and detector configuration.
Where the additional resolution can become particularly valuable is in providing more image information to interpret what you've detected.
DETECTION ISN'T IDENTIFICATION
Seeing a heat source and knowing exactly what you're looking at are two different things.
A manufacturer may quote a very long detection range, but that doesn't necessarily mean you'll have enough detail at that distance to confidently identify the object.
Think about thermal performance in three stages:
Detection — Something is there.
Recognition — You can determine the general type of object.
Identification — You have sufficient information to determine what the object actually is.
This distinction becomes increasingly important when comparing thermal optics based purely on advertised detection distances.
WHAT ABOUT NETD?
Resolution tells you how many detector pixels are available.
It doesn't tell you how sensitive those pixels are to very small differences in temperature.
That's where NETD, generally expressed in millikelvin (mK), becomes relevant.
A thermal with excellent detector resolution but mediocre thermal sensitivity won't necessarily provide the best image in every environment.
Likewise, a high-quality 384 detector with strong thermal sensitivity and good image processing can produce an extremely impressive image.
We'll break this specification down properly in another article:
What Does NETD (mK) Mean in Thermal Imaging?
WHAT ABOUT PIXEL PITCH?
You'll also commonly see specifications such as:
12μm
or
17μm
This refers to pixel pitch — essentially the spacing of detector elements on the thermal sensor.
Pixel pitch interacts with detector resolution and lens focal length to influence characteristics such as field of view and the angular size represented by each detector pixel.
It's another reason you shouldn't compare thermal optics using resolution alone.
We'll cover this separately in:
What Does 12μm Pixel Pitch Mean on a Thermal?
SO, IS 640 WORTH IT?
It depends entirely on what you expect from the optic.
384 MAY BE THE BETTER CHOICE IF:
You want strong thermal performance without moving into the price bracket of higher-resolution units.
Your typical observation distances aren't extreme.
You don't rely heavily on digital magnification.
You'd rather spend your budget on a higher-quality overall unit than chase sensor resolution alone.
640 MAY BE WORTH IT IF:
You regularly observe at greater distances.
Fine image detail is important to you.
You use digital magnification frequently.
You're buying a higher-end thermal and want greater available detector resolution.
You simply want the additional image performance that a good 640 system can provide.
DON'T BUY A THERMAL BASED ON ONE NUMBER
This is probably the most important takeaway.
384 vs 640 matters — but it isn't the entire thermal.
When comparing two units, look at the complete system:
Sensor resolution
NETD / thermal sensitivity
Pixel pitch
Objective lens
Base magnification
Field of view
Display
Image processing
Battery system
Ergonomics and controls
A well-designed thermal brings all of these together.
The best thermal isn't necessarily the one with the biggest number on the specification sheet.
It's the one whose specifications and design best suit how you're actually going to use it.
THERMAL EXPLAINED — BY APEX TACTICAL
Thermal technology can look complicated when every product page throws numbers like 640 × 512, 12μm and <20mK at you.
Thermal Explained breaks those specifications down into practical information so you can understand what you're looking at and make a more informed decision.