ASi and VOx thermal imaging technologies
We look forward to welcoming our visitors with a new contribution! This time we would like to express our opinion on some statements about ASi and VOx thermal imaging technologies that can often be read on the Internet. We are not concerned with the question "which is better?", But with the explanation of the facts as they come closest to the truth from our point of view.
As to the statement: "The European and Eastern European market is dominated by manufacturers who install ASi detectors, while in the USA VOx technology is dominating the market."
This rather courageous assertion can be doubted above all because VOx sensors and other components as well as finished VOx thermal imaging devices have recently been emerging from China on the European market. Here you will find your buyers first of all because of their low price and mostly good quality.
The uncomplicated imports and price flexibility of the suppliers mean that VOx technologies quantitatively gain the upper hand on the European market. On the other hand, this fact leads to the assumption that the widespread claims about “clear” advantages of VOx compared to ASi technology can be attributed at least in part to this fact.
As to the statement: “The ASi detector production is easier and therefore cheaper. Vanadium as a raw material is rarer and therefore more expensive than silicon. ”
In this context, we first turn to the controversial issue of manufacturing costs. Customers who are about to buy a device with ASi technology are often suggested that the devices are completely overpriced, because the VOx would be much more expensive to procure the necessary raw materials for the thermal view and would therefore have to be better. The impression is given that retailers and manufacturers are cheating on overpriced devices with ASi technology.
The argument that is often cited that amorphous silicon (ASi) is more expensive than vanadium oxide (VOx) as a starting material is simply not tenable.
The extremely thin layer, which is supposed to improve the detection properties, is so "cheap" in terms of material technology that it is hardly reflected in the price of a single bolometer. It can also be said that intermediate layers made of heat-conducting elements are used in devices with ASi technology, which are in no way inferior to the VOx in terms of costs.
You also have to keep in mind that the technologies for manufacturing both types of sensors are very similar! Almost identical materials are used, among other things, for the membrane - a silicon plate with extremely small holes. In all of these matters, ULIS from France (a bolometer manufacturer) is nothing short of the American and Chinese VOx bolometers.
Sure, labor costs in China are lower than in Europe, of course. On the other hand, the most expensive smartphones are also made in China, so this argument is not really valid, especially since the production of bolometers is very capital-intensive.
If we return to the topic "Comparison between ASi and VOx", we will find that the platelets to which the amorphous silicon (ASi) or the vanadium oxide (VOx) are applied are also made from the silicon that is strongly criticized by the VOx supporters ... consist! Why individual “specialists” also emphasize the extraordinary hardness of vanadium is completely unclear, because vanadium oxide is a powder that can be dissolved in light acids and even water.
To the statement: "All US armament programs use VOx detectors, this is often an indication that it is the technically better choice. However, US export restrictions limit the use of US VOx sensors. "
We will not discuss the topic of export restrictions and patents at this point, especially because of the origin of the VOx-based devices that are currently offered on the thermal imaging technology market in Europe. If one follows the logic of the above statement to the end, one could at least assume that the export of ASi technology and the sensors and devices based on it from Europe to the USA should also be subject to similar restrictions. The same also applies to patent law, because development work to improve the performance of amorphous silicon (ASi) with different additives, catalysts and alloying elements has been going on for a long time, and their results in some cases far exceed the characteristics of VOx devices.
But let's get to the facts, which are probably deliberately distorted. Here is a quote:
«VOx has a better thermal conductivity than ASi and can therefore transfer photons faster, which means it has better performance.»
The authors' endeavor to describe such a complex technological problem with such greatly simplified theses can only be explained with the wish to provide the inexperienced reader with the "information content" that is necessary for the author and is inexpensive in scope. Well, let's clear it up.
A material that is highly sensitive to infrared radiation, which forms the detection layer, is the essential, if not the most essential, element of a sensor (bolometer). This is precisely the layer that forms the electrical signal required for imaging, which is generated when the temperature difference. This material must have the lowest possible electrical resistance so that a constantly present "noise" (Johnson noise) is as low as possible. In addition, the low resistance makes it easier to read the information for further signal processing. At the same time, this layer must also have a very high thermal resistance. The higher this coefficient, the higher resistance is generated in the material with the slightest temperature changes - which in turn is easier to read! A similar property is the high activation energy of the material of the detection layer.
Indeed, VOx-based bolometers have a high coefficient of temperature resistance. Unfortunately, vanadium oxide is a material that does not go well with silicon - as we already know, VOx technology cannot do without silicon. For this reason, the plates made of silicon, with layers of vanadium oxide applied to them, rely on a very low curing temperature, otherwise any attempts to read electrical signals from them (optical transmission) will fail.
Hydrogenated, amorphous silicon, which has a high activation energy and a high thermal conductivity, is used as the detection layer based on ASi. Unfortunately, this material has a “side effect”: a slightly higher electrical resistance. In order to get rid of this undesirable side effect, a mix of amorphous silicon, catalysts and other oxides is used, which, in addition to lowering the electrical resistance, increase the activation energy and the thermal conductivity to and above the level of VOx technology.
To the statement: "If we compare an ASi with a VOx optics that have identical lenses (f number) and resolution, we find (also arithmetically) that the VOx optics have 3 times higher temperature sensitivity. (VOx = 0.039 Kelvin to 0.1 Kelvin = ASi, at 25C ° and f = 1). "
Here it is difficult to clarify how outdated the data listed here that contributed to this outgoing invoice is. Today, the "heat noise", ie the level of a level from which the ASi bolometer can measure the temperature change, has reached approx. 0.05 Kelvin (K) or 50 mK (at room temperature). In blogs and articles, the term NETD can sometimes be found (the size that represents the smallest detectable temperature change), which is called 2-3 times larger especially for ASi bolometers than is supposed to be the case for VOx bolometers . This is definitely wrong, at least for the ASi sensors from well-known manufacturers. The actual difference is about 25% - 40%, which is not at all in line with statements made by some European bloggers. In addition, anyone who can convert Celsius to Kelvin is able to check how small and uncritical this difference is. When comparing the NETD data, it is important (correct!) To have the same lens diameter. The larger the lens on the lens - the better the detection properties (sometimes even drastically better!).
What trends have we observed in recent years and which key figures of the devices that use ASi and VOx bolometers are increasing? The evolution is clearly evident - the increase in the number of pixels and the decrease in the pixel size, the enlargement of the OLED display and an improvement in its resolution, the "heat feedback" is faster and the imaging itself is increasing rapidly. Incidentally, the “heat feedback” is usually significantly better with ASI bolometers than with the other devices.
Finally, I would like to comment on the clear and irrefutable advantage of ASi technology over VOx technology. The latter is known to many users from these disappointing moments when the picture “freezes” over time. This annoying disadvantage arises from the low stability of the pixel calibration. The linearity of the calibration for ASi-based devices is extremely high, sometimes an intermediate calibration is necessary without being noticed by the user - in VOx-based devices, a regular, time-consuming (and still quite loud) intermediate calibration is imperative - every few minutes. Otherwise, the linearity of the calibration drops, that is - the quality of the displayed image information - very quickly until you can no longer see anything.
Therefore, statements that claim that VOx technology is clearly better than ASi technology simply cannot be taken for full. Each user selects their device very carefully, and I hope this critical and important information will help them do this. Unfortunately, the blogs have so far shown a rather one-sided, incomplete and tendentious dissemination of information, which should be treated with caution.