HT-A2 thermal imager with a resolution of 320 × 240 pixels

About five years ago, I published a review of a thermal imager, where I went into detail about what it is. Now it’s time to update that review. I looked at various models, but eventually decided on the HT-A2 from Hti. I ordered one, received it, used it for a while, and decided to put together a short review.


Before ordering, I spent a long time digging around on AliExpress and found the cheapest option. At the time, it cost $446.71, but since there was a sale at the end of March, I ended up paying about $425. The price is now $522, which surprised me when I saw it.
At other stores, almost every seller has it for $530. The only place where I found it cheaper was for $487, but I think that’s temporary.

I first started thinking about upgrading my thermal imager about a year ago. Based on my experience with the old one, I knew I wanted a thermal imager that:
1. Is a standalone device, not a smartphone attachment
2. Has both a regular camera and an IR camera (some models don’t have a regular camera).
3. Has an IR resolution of at least 220x160, preferably higher. The old one was 60x60.

There were quite a few options that met these criteria, such as a thermal imager from Uni-t, but I wanted something better. Of course, I could have tried to get a thermal imager with a 25 Hz frame rate, but that would be expensive and difficult, and, for the most part, pointless for everyday use. The same can’t be said for resolution.

In the end, I decided on a product from Hti. The company has four popular models that are essentially different only in resolution and body shape.
The HT-A1 and HT-A2 look more like smartphones. The only difference between them is the IR sensor resolution: 220x160 on the A1 and 320x240 on the A2, which is noticeably higher.
It later turned out that the models also differ in their field of view: 27° * 35° for the HT-A1 and a wider 56° * 42° for the HT-A2. The stated focal length is the same for both, at 4 mm.


There are also two pistol-grip models, the HT-18 and HT-19, with resolutions of 220x160 and 320x240, respectively.
All four thermal imagers are built on the same platform, use the same software, and differ in just two key ways: resolution and body shape. The HT-19 is also slightly more expensive than the HT-A2 (by just a few dollars).

I’ll get ahead of myself and say that, in my opinion, the “pistol” shape is more convenient.


The store page compares the two models, although I have a strong feeling that some “Photoshop” was involved here.


Anyway, I ordered it, received it, checked it out, and now it’s time to tell you about it.
The box is universal, for two models, and very compact. The HT-1x series comes in a box like the one my first thermal imager came in.
There’s foam inside, and they even made a cutout to make it easier to take the device out of the box.


The package includes:
1. HT-A2 thermal imager
2. USB-to-microUSB cable
3. Charger
4. Wrist strap
5. Manual
6. Some kind of brochure.


1. Brochure and manual
2. A very ordinary cable, three rubles a bundle.
3, 4. 5 V, 2 A power supply
5, 6. The strap is designed so you can detach the thermal imager from it fairly easily. I’ll get ahead of myself again: the body design turned out to make the strap essential, since the thermal imager keeps trying to slip out of your hands. That’s exactly why the “pistol” shape is better.


There’s nothing particularly interesting in the manual, apart from a table of specifications for both versions of the device.


Device specifications in Russian.
Model - HT-A2
Display: 3.2" TFT
IR camera resolution: 320*240
Visible-spectrum camera resolution: 640*480 (300000 pixels)
LCD resolution: 320*240
Field of view/focal length: 56 ° * 42 °/4.0 mm
Thermal sensitivity: 0.07 ℃
Temperature measurement range: From -20 ℃ to 300 ℃ (from -4 ℉ to 572 ℉)
Measurement accuracy: ± 2% / ± 2
Emissivity: Adjustable from 0.01 to 1.00
Thermal image frame rate: 9 Hz
IR wavelength: 8-14um
Focus mode: Fixed
Color palettes: Rainbow, iron oxide red, cool, black and white, white and black
Display: Thermal image blending in 25% increments with the visible image.
Storage: Built-in 3 GB (more than 20 thousand images)
Supported file formats: JPG

The thermal imager looks like some kind of overly thick smartphone. When I first picked it up, I immediately wondered why it was so big. For some reason, I’d expected it to be about one and a half times smaller. No, of course I’d seen the photos and dimensions, but seeing it is one thing; holding it in your hands is quite another.


For comparison, here are the dimensions of the previous thermal imager, the new one, and a regular smartphone with a 6-inch screen.


The front has a 3.2-inch screen and 6 buttons. The back lists the basic specs and serial number, along with a couple of screws, one of which is covered by a seal that comes off easily.


1. On the left is a cover for the microUSB port.
2. On the right is the “photo” button. Press it again to confirm saving a photo.
3. The cover is very awkward to open. If you don’t have fingernails, you’ll need something thin.
4. The device weighs just over 200 g, which is comparable to the weight of a smartphone.

The case is made of some kind of plastic that isn’t very rigid—or perhaps slightly soft. It’s hard to explain.


At the top of the back are a regular camera, an IR camera, and the flashlight LED. The LED is noticeably brighter than on my old model, where it was more of a dummy. It turns on and off by holding down the “photo” button, just like on the old model, which suggests they’re related.


I didn’t take it apart for two reasons:
1. Someone has already posted a teardown online, so I’d already satisfied my curiosity.
2. Taking the case apart involves more than just removing a couple of screws: another 6 are under the front panel, which is glued in place.

I’ll have to take it apart sooner or later, though, because there’s some kind of speck inside, at the top of the screen, and it’s annoying. I first tried to shake it loose by tapping the device, but that didn’t work. I tried to take it apart, but nearly broke it. Only afterward did I find out that the front panel has to be peeled off.

Here’s a teardown video:


I did find some high-quality photos of the circuit board online. They’re from the HT-A1, but a teardown of the HT-A2 showed that the boards are identical.

The device is based on an AllWinner A33 SoC and is essentially a simplified tablet with custom software. It has 4 GB of flash memory and 512 MB of RAM (256M × 16).
Power and battery charging are handled by an AXP223 chip, which you’ll also find in inexpensive tablets.




The HT-A1 IR camera module. I assume the only difference in the A2 is the sensor itself. I also found a mention that the SeekThermal uses the same controller.


There are six control buttons on the bottom:
Menu, confirm, and thermal imager power on/off.
Four navigation buttons. In viewing mode, left/right switches display modes, while up turns the bottom status line on or off.
Enter selects an option in settings and cancels when prompted to save a photo.


Before describing the menu, a few words about power consumption and charging.
The thermal imager has a built-in 2000 mAh, 3.7 V battery, size 654060. According to other users, a charge lasts about 3 hours. I didn’t measure the actual runtime, but subjectively it seemed closer to 2.5 hours. The battery indicator is pretty much random: for example, when it showed the battery as empty (the last red bar), the thermal imager kept running for another 40 minutes or so.

It charges at 1 A and takes about 2.5 hours. Even when fully charged, the device continues to draw around 200 mA.


1. A splash screen appears at startup, and booting takes noticeably longer than on my old model: 15–16 seconds versus 7.
2. The screen shows the thermal image with markers for the maximum and minimum temperatures, as well as the center point. This model also displays the temperature value. The maximum/minimum readings are repeated at the bottom, with the current time shown to the right; this line can be turned off. The scale on the right maps the image to the full temperature range.
3. It shuts down much like my old model, with a countdown.
4. Pressing the Menu button while it’s on opens the settings.

The display is bright enough to use outdoors, but switching it to maximum brightness makes it more comfortable to view. Viewing angles are good horizontally and from below, but the image gets worse when viewed from above.

The buttons are stiff, but I think that’s the trade-off for a more rugged design.


Menu.
1. View saved photos.
2. Thermal image display modes: three color options and two black-and-white ones. Which one you choose depends on your preference and how you use it; the first one works for me.
3. Emissivity adjustment, four presets plus manual setting.
4. Advanced settings menu.
5. Auto power-off: off, 5 or 20 minutes.
6. Display brightness: low, medium or high. I ended up setting it to maximum.
7. Language: Russian isn't available, but I'm actually more comfortable with English than with "Chinese Russian".
8. Temperature display in degrees Celsius or Fahrenheit.
9. Time display in 12- or 24-hour format.
10. Date and time settings.
11. Show points with the maximum/minimum temperature.
12. Firmware version information.


Naturally, the first thing I wanted to do was see how it takes pictures. I started by going out onto the balcony and taking a couple of photos of the neighboring building, where the spots losing heat are clearly visible. Unfortunately, although it was relatively cool outside, the heating had already been turned off, so the heat leaks weren't very noticeable.
The third photo shows the kitchen window, but what impressed me more was the flower on the windowsill, which "glows" in IR.


Last time, I also complained that the images from the regular and IR cameras were offset. That's understandable, since they're different cameras, so you can only align them perfectly at a particular distance.
This problem is fixed in software here. Select the top menu item (Image registration) and four arrows appear. Pressing the up/down and left/right buttons lets you "synchronize" the images, but there's a catch: the more you adjust the alignment at, say, 0.5 m, the more it will be off at, say, 5 m. So you can either leave it as is or adjust it for your particular use case.


As an example, here's a thermal image of a heated battery: the left image is the original, and the right shows what you can do. The adjustment is very fine, but holding down a button triggers auto-repeat, which makes it easier to set.


The thermal imager is quite sensitive, meaning it can distinguish areas with even small temperature differences, automatically adjusting the image.
For example, I pressed my hand against the wall for just a few seconds, and its thermal imprint remained visible for a while afterward.
You can do the same on the floor and then see your footprints; they're easy to spot even a minute later.


But sometimes the automatic adjustment makes things worse. I saw this complaint on the forum too, in the thread about this thermal imager.
The thing is, the thermal imager has a certain range of colors it can display (so to speak), a scale, and it fits the entire image into that range, from the minimum to the maximum temperature.

I noticed this when I took the thermal imager to work, just out of curiosity. When I looked at the building and the car, I saw that the whole image was tinted red.
It turned out the thermal imager was reading the sky at -35 degrees, so naturally everything else looked warm to it.

Ideally, it would be good to have the option to narrow the range manually—for this example, say, to 5–25 degrees.


A couple more outdoor photos where the thermal imager includes the sky. The image detail is preserved, but warm objects are still rendered in almost a single color.
In other words, the smaller the temperature difference in the frame, the easier it is to make out details. Since this thermal imager is sold here as a building energy-audit tool, you should keep this limitation in mind; otherwise, you may end up with complete nonsense instead of useful results.


Here's a photo of a dirt road. The sky isn't visible, so the temperature differences are more obvious. I'll also note that the regular camera takes pretty good pictures, and judging by the characteristic clicks, it also has autofocus.
Here I took photos with IR off, with a 50% overlay, and with 100% IR. Since the IR resolution is quite good, you can skip the regular camera overlay for many applications.


And here's what people look like in the photos :) There was also a photo of a cat, but it didn't turn out very well.


The regular camera's sensitivity also seems pretty good to me, allowing it to take pictures even in street lighting.


Display options with only the IR camera enabled:
1. Rainbow,
2. "Iron red"
3. Cool


1. Black and white
2. White and black


Overlay options for images from the regular and IR cameras, in "rainbow" mode.
25, 50 and 75%


And 0 and 100%, respectively.
This also shows how the thermal imager sees a voltage converter. A nasty trick can be played by the fact that power components are often soldered to a circuit board that acts as a heatsink. As a result, the thermal camera colors not only the component itself, but also the surrounding area. But it’s worth saying right away that both ambient lighting and the fact that it’s much easier to tell what’s heating up in a video than in a still photo make a big difference.
I actually prefer the IR-only view, and besides, in that mode there’s no need to “synchronize” the cameras.


But in some situations it’s hard to do without the regular camera. When there’s a very large temperature difference between the object and the surrounding air, the image can end up with too much contrast. The thermal camera sees anything at room temperature as “cold.”



Comparison photos taken with the old and new thermal cameras from the same distance.
1. A photo of a wall with a refrigerator behind it.
2. A window
3. A stove with a heated kettle.



And here’s an example of photos that are closer to the tasks I actually have in mind.
1, 2. Heating on the balancer board. In the first photo, the heating of some components masks that of others. In the second, the board has been modified, the temperature has evened out, and things look better.
3. Testing one of the batteries. You can clearly see both the battery itself heating up and the wires heating up as 40 A flowed through them during the test.


I can already hear the obvious question: why is the image still kind of blurry, and can it be improved?
Yes, it can. This is usually done by adding a lens, but as you can imagine, an ordinary lens won’t work; you need a special one.


And here’s a comparison of images posted on one of the forums: the one on the left is without a lens. More details here.


That’s probably enough examples for now, although I actually took a lot more shots—I got a bit carried away :)

Now for the conclusions.
Overall, I liked the thermal camera. The higher resolution of the IR sensor is definitely noticeable, as is its significantly greater sensitivity. If the temperature across the field of view is uniform, the thermal camera can detect a difference of just 1–1.5 degrees. Another plus is the ability to align the images from the regular and IR cameras.
It has a large display, a sturdy, rugged housing, a battery, and 3 GB of built-in memory. With file sizes of 60–180 KB, that’s enough for a really large number of photos.
The display is not only larger, but better too, both in image quality and viewing angles. The flashlight actually looks like a flashlight, rather than just an indicator that it’s on.

There are some drawbacks, too. The housing is compact, which is convenient, but taking photos is easier when the body is shaped like a “pistol.” After using it for a while, I found out that pressing the “photo” button again confirms saving the image, so there’s no need to shift your grip.
The old model displayed the photo date on screen as well; this one shows only the time. On the one hand, the date isn’t always that important, but on the other, when you’re sorting through a pile of photos, it’s handy to have the date right there on screen. On the model reviewed here, the photo’s date and time are included in the filename.
On the plus side, working with the internal memory is much more convenient. On the old model, after transferring the photos, new ones started numbering from zero, which was annoying when I needed to transfer more photos to the same folder because the filenames would clash.
I have some concerns about the design of the rubber USB port cover. For some reason, I don’t think it will last long. Wi-Fi or Bluetooth would have been nice, too.

Overall, I’m happy with it so far. It’s also convenient that the old and new models have almost identical controls, so there’s no need to get used to a new layout. The next step is experimenting with lenses.

That’s all for today. I hope this was useful, and as always, I’m looking forward to your questions.