The difference between infrared thermal detectors, vanadium oxide (VOX), and polysilicon

The infrared thermal imager is mainly composed of three parts, the detector, the signal processor, and the optical system. The core component of this is the infrared detector.

Infrared Detectors

The main technologies of infrared detectors are American vanadium oxide (VOX) and French polysilicon. From the picture of the two, VOX is better than polysilicon, the picture is delicate and layered. There will be slight vertical stripes on the polysilicon screen. As for the effect of use, the same level of polysilicon that vanadium oxide is expected to see can be used. However, in terms of price, it is generally possible to obtain 90% performance of polysilicon with a vanadium oxide movement at 60% price. This is the advantage of polysilicon.

The detector used by FLIR is a self-developed vanadium oxide infrared detector. In China, due to the restriction of technology introduction, the cost of vanadium oxide detectors is relatively high and the openness of technology is low. Domestic companies have independently developed detectors, but they are not as good as imported equipment in terms of effect and quality. The cost of the detector determines the development of the infrared thermal imaging industry.

The infrared thermal imaging lens

The infrared camera lens is usually made of germanium glass. The refractive index of this glass is very high. It is only transparent to infrared light, but opaque to visible light and ultraviolet light, so it can separate plants and animals in a very dark environment. The lens of an ordinary camera is made of optical glass, and its main function is to refract light, which can also be said to be magnifying objects. General optical glass is artificially synthesized. Infrared thermal imaging camera lenses are compared with common germanium scattered metal elements used in ordinary camera lenses. Although the reserves in nature are not low, the extraction of high-concentration germanium is very difficult. Therefore, the production cost of germanium lenses is higher, and they are sold on the market. The price is often higher than that of ordinary camera lenses.

Important parameters of infrared thermal imaging

As typical high-end application equipment, with the continuous improvement of the manufacturing process, the performance of all aspects of the infrared camera has been significantly improved today. Here is a systematic introduction to the main parameters of the infrared camera.

Detector type: The main technology of the movement is the American vanadium oxide (VOX) and French polysilicon. From the picture of the two, VOX is definitely better than polysilicon, the picture is delicate and layered. There will be slight vertical stripes on the polysilicon screen. As for the effect of use, the same level of polysilicon that vanadium oxide is expected to see can be used. However, in terms of price, it is generally possible to obtain 90% performance of polysilicon with a vanadium oxide movement at 60% price. This is the advantage of polysilicon. Pixel pitch: The smaller the pixel pitch, the more pixels there will be, and the larger the field of view will be when the lens with the same focal length is used. On the whole, 17UM is the smallest, the effect is the best, followed by 25UM, and 35UM. Generally, 25UM is sufficient. The smaller the pixel pitch, the higher the thermal sensitivity.

Thermal sensitivity

It can be simply defined as the minimum temperature at which the instrument or the observer can accurately distinguish the target radiation from the background. The smaller the thermal sensitivity value, the higher the thermal sensitivity. In addition, the higher the thermal sensitivity, the brighter the picture.

Field of view

It is the abbreviation for the field of view of the optical system, which represents the spatial range that can be imaged in the field diaphragm of the image plane of the optical system. When the object is located at any point (within a certain distance) in the cone with the optical axis as the axis and the apex angle as the field of view, it can be found by the optical system, that is, the image is imaged in the field diaphragm of the image plane of the optical system. The maximum opening angle of the object space that the body can image in the thermal imaging camera is called the field of view, which is generally the field of view of the ao×βo matrix. Similar to the concept of effective pixels of a CCD camera. The larger the field of view, the higher the image clarity.

Spectral response

This refers to the response range of the thermal imaging camera to the infrared spectrum. There are usually two response ranges of the mid-infrared spectrum and the far-infrared spectrum. The far-infrared spectrum (8.0-14.0μm) is better, because the wavelength of the mid-infrared spectrum (3.0-8.0μm) is relatively short, and it is not easy to penetrate some materials, making it impossible to image normally in the end.

Field temperature range

The field temperature range refers to the highest temperature that the thermal imaging camera can sense imaging. Once the object exceeds this temperature, the camera will not be able to give edge images. This is similar to the light oversaturation phenomenon of ordinary CCD cameras. The higher the temperature range, the wider the image dynamic range of the thermal imaging camera.

In order to facilitate the readers to identify the above parameters, let’s take the parameters of a certain series of FLIR thermal imaging cameras as an example:

Detector type: uncooled vanadium oxide; pixel pitch: 17 or 25μm; thermal sensitivity: <50mk;

Field of view (array format): 320×240;

Spectrum response (spectrum response): 7.5-13.5μm, far-infrared induction (LWIR); scene temp range: up to 150°C, up to 560°C (optional);

Other important parameters

1. Frame rate The frame rate refers to the number of images that can be captured, processed, and displayed by the thermal imager within 1 second. The faster the sensor response and the higher the processing speed of the internal circuit, the greater the frame rate that can be achieved. The thermal imager with high frame rate is suitable for capturing the temperature field distribution of high-speed objects. It is more suitable for scientific research and military research.
2. Pixel array and pixel pitch The current infrared thermal imager detectors are uncooled focal plane detectors. During the production process, an array of sensor units are processed on vanadium oxide or polysilicon materials. There is an array of sensor units between each unit. A certain spacing.
3. Temperature measurement accuracy Accuracy refers to the percentage of the ratio of the maximum error of the infrared thermal imager’s temperature measurement to the instrument range when the thermal imager is corrected for environment, temperature, humidity, distance, and emissivity.
4. Display mode This point, according to professionals, usually refers to whether the display on the thermal imager screen is black-and-white or pseudo-color.
5. Temperature measurement range For thermal imaging cameras, during normal operation, there will always be a certain temperature measurement range, which refers to the range of the minimum and maximum temperature values ​​for the measurement temperature. 6. Temperature resolution The temperature resolution specifically refers to the important parameter index of the infrared thermal imager. The temperature resolution refers to the sensitivity of the detector to the temperature change of the measured object. The smaller the temperature resolution, the better. The measurement and measurement of temperature resolution are done under specific conditions.

Today I mainly explain to you the main technology of the movement is the difference between American vanadium oxide (VOX) and French polysilicon.

The uncooled thermal imager using the vanadium oxide focal plane detector is a relatively new type of thermal imager in China. Compared with polysilicon, the vanadium oxide focal plane detector has the better image quality and sensitivity, and can better meet different requirements. In the past, domestically produced thermal imaging cameras mainly used polysilicon bolometers.

Features of polysilicon infrared detector
Polysilicon is a form of elemental silicon. When molten elemental silicon solidifies under supercooling conditions, silicon atoms are arranged into many crystal nuclei in the form of a diamond lattice. If these crystal nuclei grow into crystal grains with different crystal plane orientations, these crystal grains combine to crystallize into polysilicon. Polysilicon is known as the “cornerstone” of the microelectronics industry and the photovoltaic industry. It is a high-tech product that spans multiple disciplines and fields such as the chemical industry, metallurgy, machinery, electronics, etc. It is an important foundation for the semiconductor, large-scale integrated circuit, and solar cell industry. Raw materials are extremely important intermediate products in the silicon product industry chain. Its level of development and application has become an important indicator of a country’s comprehensive national strength, national defense strength, and modernization level. It is understood that there are currently very few domestic manufacturers of polysilicon products, which are far from being able to meet the rapid development of the domestic microelectronics industry and solar cell industry. With the development of my country’s integrated circuit, silicon wafer production, and solar cell industries, polysilicon is in huge demand in the domestic and international markets, and the price of polysilicon continues to rise.

Features of vanadium oxide infrared detector

(1) Adopt advanced uncooled bolometer (vanadium oxide) staring focal plane detector;

(2) The size of the detector is 324×256, which can better meet the requirements of the PAL TV system;

(3) The area of ​​a single photosensitive element of the detector is small (38μm pitch), which can reduce the volume and weight of the thermal imaging system;

(4) The sensitivity of the detector is higher. At f/1.6, the NETD can reach 85mK. At f/1.0, its NETD is equivalent to 35mK, which is close to a general refrigerated detector. Therefore, the thermal imager has a higher detection and recognition distance;

(5) A good non-uniformity compensation circuit is used inside, so there is no need to use a thermoelectric cooler (TEC) to stabilize the working temperature of the focal plane. In the working temperature range of (-40~+75℃), the thermal image The instrument has good image uniformity and dynamic range;

(6) Because the TEC is not used, the thermal imager has the two major characteristics of fast start-up and low power consumption. With a driving time of 2s, the thermal imager can be turned on and used at any time without waiting. The power consumption of the thermal imaging camera core can be reduced to 1.5W, which extends the working time of the battery;

(7) With 2x digital zoom function;

(8) The new image processing capability improves the clarity of the image and can output pseudo-color images

In short, the main advantage of vanadium oxide detectors is that they have higher photoelectric conversion efficiency for infrared light. Compared with polysilicon detectors, they have a higher signal-to-noise ratio and strong light protection. The vanadium oxide detector has good temperature stability, long life, and small temperature drift.

The vanadium oxide movement has better image quality and sensitivity than the polysilicon movement so that the thermal imager has a higher detection and recognition distance. Low power consumption, quick start-up, and ready to use after booting. The image definition is 3 times that of a polysilicon movement. The temperature detection sensitivity of vanadium oxide can reach 0.03℃, while the polysilicon movement can only reach 0.1℃. At the same time, the vanadium oxide movement is longer and more durable than the polysilicon movement. The uncooled thermal imager using the vanadium oxide focal plane detector is a relatively new type of thermal imager in China. Compared with polysilicon, vanadium oxide material has the better image quality and sensitivity, and can better meet different requirements. In the past, domestically produced thermal imaging cameras mainly used polysilicon.

However, many manufacturers use indicators to mislead consumers. Because vanadium oxide detectors are generally 336X256 pixels, while polysilicon detectors are generally nominally 384X288 pixels, so in terms of product indicators, polysilicon detectors have higher indicators than vanadium oxide detectors. But in fact, judging from the above comparison effect, the vanadium oxide detector with the same lens is far better than the polysilicon detector.
　　

For example, dual-band thermal imaging dedicated to temperature measurement and warning for forest fire protection is generally placed in the forest and other field environments, so it has higher requirements on the quality and effect of the movement. If a low-cost polysilicon movement is used, it may not be suitable for the wild The severe environment will change, and the fire source cannot be found in time when the temperature difference is not large, which will cause heavy losses in the forest area. Therefore, the vanadium oxide movement is finally used to solve these problems.
　

In addition to these compared with polysilicon, the vanadium oxide movement itself also has some unique advantages: a good non-uniformity compensation circuit is used inside, so there is no need to use a thermoelectric cooler (TEC) to stabilize the working temperature of the focal plane. In the working temperature range of (-40~+75℃), the thermal imager has good image uniformity and dynamic range; because the TEC is not used, the thermal imager has the two characteristics of fast startup and low power consumption. With a driving time of 2s, the thermal imager can be turned on and used at any time without waiting. The power consumption of the thermal imaging camera core can be reduced to 1.5W and has a 2x digital zoom function.

With the maturity of thermal imaging technology, people’s requirements are getting higher and higher, and there are almost stringent requirements for the temperature difference, the durability of the movement, and the clarity of the image. The traditional polysilicon movement has been used in security equipment. It is not dominant, and vanadium oxide can make up for the shortcomings of polysilicon movement and has its own unique advantages. It has already occupied a pivotal position in the security market.