Shulou(Shulou.com)11/24 Report--

Human facial features are very complex and sensitive "sensors". However, most of people's facial sensations can only make "qualitative" perception of external information, but can not make quantitative perception. Moreover, there are many physical quantities that can not be felt by people's facial features, such as the visible part of vision, but not the non-visible spectrum with a wider frequency domain, such as infrared and ultraviolet spectrum. however, human beings are "blind". With the help of infrared and ultraviolet sensors, these invisible lights can be sensed.

Ultraviolet sensor, also known as ultraviolet photosensitive tube (UV tube for short), is a kind of phototube using photoelectron emission effect. It only responds to ultraviolet radiation below 300nm, and has the characteristics of high sensitivity, high output, high response speed, strong anti-interference ability, stable and reliable, long life and low power consumption, so it has extensive application value in current safety protection and automatic control.

With the wide application of electronic computers, all kinds of sensing technologies for computers have been paid more and more attention. The ultraviolet sensor can detect the ultraviolet rays imperceptible to the human senses, and can avoid the interference of sunlight, light and other common light sources. It is very useful for the discovery and flameout protection of the fire, and the photoelectric control of special places.

The structure classification of ultraviolet sensors at home and abroad at present, the valuable UV tubes at home and abroad can be divided into spherical, wire and flat structures according to the shape of the cathode, all of which are the electrode structure of diodes, and the shape and material types of the shell are designed for use. From the point of view of the working state, it is more appropriate to analyze the electrode shape classification.

1. Filamentous electrode structure the electrode of this kind of pipe is generally composed of two or more symmetrical wires, which is an early structural form of ultraviolet tubes, which often uses tungsten or platinum wires with high purity, and the parallel line close to each other is the working area.

Because the UV tube depends entirely on the photoelectron emission effect on the electrode surface, and then the strong signal is obtained by gas multiplication, the spectral response range depends on the escape work of the cathode material.

In the process of photoelectron emission, the shorter the wavelength of the photon, the higher the energy, even if the amount is small, it can excite the electron to overcome the escape work and fly out of the cathode surface. Even a large number of low-energy photons cannot excite electrons on the surface of the cathode. In the UV tube, the surface purity of the cathode material is very high, otherwise it will affect the range of the spectrum and lose its use value. The use of symmetrical filamentous structure is for the convenience of process treatment, as far as possible to avoid the pollution of other substances to the electrode.

The characteristic of this kind of tube is that it can work in AC state, the working current is large, the circuit is simple, and the impurity on the electrode surface can be removed by appropriate process, but the visual angle sensitivity fluctuates greatly. the work area is easy to produce uneven emission.

2. The spherical cathode structure in order to fully avoid the tip effect and make the photoelectron emission more stable and uniform, it is necessary to fix the working area on the cathode, because the UV tube transforms the optical signal into electrical signal and amplifies it by photoelectron emission and gas multiplication. Generally, the utilization rate of light emission is the highest in the area near the electrode, from which the UV tube with point structure spherical cathode is calculated, as shown in the figure.

No matter from which angle the photon radiates to the hemispherical cathode, the discharge region is always on the hemispherical vertex near the anode. Because the effective area of the cathode is small, the working current of the tube is generally less than 0.3mA, but its viewing angle is wide and the viewing angle sensitivity is relatively uniform, so it is especially suitable for fire prediction, and the focusing method can be used to improve the sensitivity.

The anode is made into a hemispherical reflector in the tube, such as the 540 °C UV photosensitive tube in the United States. The sensitivity of the tube is improved by reflecting the received ultraviolet radiation to the central cathode, because the far ultraviolet radiation has the same linear propagation and reflection effect as visible light, as shown in figure 2B.

3. Flat cathode structure

The sensitivity of the UV tube depends on the number of photons of far ultraviolet radiation received on the cathode. The larger the cathode area, the higher the receiving probability, so that more electrons escape from the cathode. Under the action of an external high voltage electric field, the gas molecules are accelerated and collided with the gas molecules in the tube, and the electrons produced after ionization collide with the gas molecules again, so that the cyclic motion will eventually cause the gas in the tube to discharge. The chance of this avalanche discharge depends on the photoelectron emission effect on the cathode. In order to improve the sensitivity, a kind of UV tube with flat cathode structure has been developed in recent years.

The ultraviolet tube is a cold cathode discharge diode, which uses the photoelectron emission effect of the cathode just like the phototube. The special gas is sealed in the tube as the working medium of the discharge tube. When a voltage is applied between the photocathode and the anode, which is only sensitive to ultraviolet light, the ultraviolet ray irradiates to the cathode through the glass shell, and the cathode emits photoelectrons.

Due to the action of the electric field, the photoelectrons are attracted to the anode, but the output current is very weak. If the applied voltage is increased, the electric field becomes very strong, the photoelectrons are fully accelerated and collide with the gas molecules in the tube to ionize the gas molecules, and the electrons produced by the gas ionization collide with the gas molecules again, and finally cause the discharge phenomenon. To get a large signal current. Working under DC voltage, the discharge will continue if a special damping circuit is not formed. These are the main working principles of UV tubes. The main parameters such as sensitivity, viewing angle, spectral response range, noise and reliability are described below.

1. The sensitivity of UV tube can be expressed in two forms: output pulse and output voltage. Different lines and measurement standards can get different data. Generally, there are four items that need to be determined: standard ultraviolet radiation light source, distance, circuit and measuring instrument.

Among the UV tubes with various structures, the sensitivity of the flat cathode is the highest, especially the long-distance detection is the most obvious, and the noise of the spherical cathode structure is very low, so it can still effectively distinguish the distant ultraviolet radiation.

In order to improve the sensitivity, the surface treated aluminum metal coated with magnesia can also be used as a focusing mirror to focus the scattered photons on the cathode to improve the light emission probability. It can also be focused on the front of the probe with a convex lens of transparent purple material.

Improving the sensitivity of the circuit will make the noise signal rise at the same time, so we must try our best to ensure both high sensitivity and low background noise in the production process. Using the output voltage to distinguish the sensitivity of the tube is often suitable for the application of flameout protection. There is an output of 10 to 30 volts from a candlelight 20CM. The stability of the output voltage can make the monitor work more reliably. The sensitivity of the UV photosensitive tube attenuates very little in the course of use, and the sharp decrease in sensitivity is rarely found in the life span of 10,000 hours, so the phenomenon of non-reporting of fire will not occur in the fire alarm.

2. Angle sensitivity is very important in fire alarm in large space, because the sensitivity of different angles is different because of the difference of electrode shape.

Under the DC working condition of the filamentous cathode, the sensitivity of the vertical cathode at 30 °to 60 °is the highest, indicating that the electrons in this region are most likely to escape from the cathode surface under the action of electric field strength.

Due to the inflatable pressure and the minimum cathode distance required for dry discharge with a small pd value, the light emission near the cathode and anode decreases. The angular sensitivity curve changes symmetrically along the parallel direction of the electrode.

It is the highest when facing the tube at 0 °, but only 60% of the highest sensitivity. If the tube works in the AC state, the same sensitive area will appear on the other electrode, showing a symmetrical bilobate pattern. The angle sensitivity uniformity of filamentous cathode is stronger than that of plug for UV detection with larger space.

The spherical anode has a uniform sensitivity within 160 °on the front. When the ultraviolet source moves vertically in the direction of the anode, the trough area appears due to the blocking of the anode to the radiation.

When the horizontal anode rotates, the peak area appears at 60 °due to the absence of the anode. The front 160 °conical surface shows a sensitivity of more than 60%. Therefore, the spherical cathode is more suitable for fire subsidence detection in a large area.

The viewing angle sensitivity of the flat cathode is uniform and symmetrical, and there is basically no valley area (Fig. 4C). This is because the area of the cathode is large, the precision mesh anode forms a uniform occlusion state on the cathode, and a fairly uniform sensitivity curve is formed in the viewing angle range of 160 °, which is very suitable for fire monitoring with large viewing angle.

3. The spectral response range of the UV tube can be shown as a "solar blind" state that does not respond to sunlight, which is very necessary for its application in special places.

Usually, the escape work of the cathode material determines the long wavelength cutoff end of the spectrum, and the shell or window material determines the short wavelength cutoff end. For example, the fused quartz is 165nm and the transparent glass is 185nm, but the same electrode material and treatment process can not guarantee that the spectral wavelength cutoff end is exactly the same, which is due to the inconsistency of the final cathode surface purity.

Because of the high purity of the material and avoiding the tip effect in the spherical cathode, the cutoff wavelength is generally within 245nm. The smaller the cathode area is, the easier it is to control the cutoff wavelength, but if the area is too small, the sensitivity will be reduced.

The spectral range of the flat cathode can also be controlled at 245nm, but due to the large area and high sensitivity of the cathode, the noise level at the long wavelength increases accordingly.

Comparing the ground sunlight with monochromatic light with a tube with a wide light range, it is proved that no radiation lower than 280nm is found even under intense sunlight in summer.

4. Noise (background) UV tube can convert optical signal into electrical signal, and it also has background and environmental noise. Because the metastable electron transitions excited by cosmic rays or tubes often produce false counts in frequent pulses, a small amount of noise signals will appear even in complete darkness, but with a long time interval. The noise signal of the tube with poor quality will rise, and sometimes it will be not far from the signal, resulting in false alarms of the whole machine, some of which can be self-excited and can not be used in visible light. Therefore, the UV tube must be aged for a long time (hundreds of hours) to ensure the safety and reliability of use.

The background of the spherical cathode is suppressed because the effective area of the cathode is small, and the spectral range is narrow, so the background noise of the tube is very low. The noise of the flat cathode is high, about 1-4 times per minute, but because of its high sensitivity, the signal-to-noise ratio is still very high, and weak ultraviolet rays can be clearly distinguished from noise.

Hydrogen in the tube can purify the electrode in the discharge. in the process of flameout protection, the long-term discharge does not increase much noise and can run reliably for 10,000 hours. The incorrect use of the tube will enlarge the noise and shorten the service life, so there are strict requirements for the circuit of the ultraviolet sensor. As the best condition is that the cathode working area is full of glow and the average working current is small, the tube in the line must have sufficient quenching time (such as 5~10ms) to make the gas in the tube have sufficient deionization time, which is determined by the use of R-C damping circuit in the line. Large current and small glow surface will cause local pollution in the cathode area, expand noise and reduce service life.

5. The reliability of UV sensor the UV tube is vacuum sealed, and the climatic factors such as air pressure, temperature and humidity have little influence on its use. The application of 150 ~ 200 °C will make the harmful substances on the tube glass shell and lead evaporate to the electrode due to thermal movement, so that the spectral range will be expanded, but the discharge purifies the electrode, so that it can still meet the requirements of fire monitoring, and the total life will be reduced. Strong vibration or shock will also affect the parameters of individual UV tubes, which is due to the expansion of the spectral range due to the return of harmful substances to the cathode in the process. The requirement of process treatment in UV tube is very strict, so pollution should be reduced as much as possible.

The UV tube can be stored for a long time, and the parameters of the tube stored for more than 8 years have only a small change, which still does not affect the use. Structurally, the parameters of the spherical cathode are easy to control because its discharge area is small and stable.

The Application of Ultraviolet Sensor Ultraviolet is the general term for the radiation of wavelength from 10nm to 400nm in the electromagnetic spectrum. According to the different wavelengths, ultraviolet rays are generally divided into three bands A, B and C. the details are as follows: UVA is 400,315 nm,UVB, 315,280 nm,UVC and 280,100 nm. Corresponding to different wavelengths, the specific applications are different. In the UVLED market application, UV-A has the largest market share, up to 90%. Its main application market is curing, which involves manicure, dental, ink printing and other fields. In addition, UV-An also introduces commercial lighting to make white clothes look whiter. As for UV-B and UV-C, they are mainly used in sterilization, disinfection, medical phototherapy and so on, of which UV-B is mainly medical treatment and UV-C is sterilization. In addition, UV LED has been applied from banknote identification to optical resin hardening, insect capture, printing, and other markets to develop biomedical, anti-counterfeit identification, air purification, data storage and military aviation, focusing on special lighting.

1. Application fields of light curing system: the typical applications of UVA band are UV curing and UV inkjet printing, representing 395nm, 365nm UV LED UV curing applications including UV adhesive curing in display screen, electronic medical, instrumentation and other industries; UV coating curing in building materials, furniture, household appliances, automotive and other industries; UV ink curing in printing, packaging and other industries. Among them, the UV LED decoration panel industry has become a hot spot, the biggest advantage is the production of formaldehyde-free environmental protection plate, and energy saving 90%, large output, resistance to coin scratching, comprehensive economic benefits and other advantages. This means that the UV LED curing market is an omni-directional and full-cycle application product market.

Microelectronics industry-UV UV-curing applications:

Mobile phone components assembly (camera lens, handset, microphone, housing, LCD module, touch screen coating, etc.), hard disk head assembly (gold wire fixing, bearing, coil, chip bonding, etc.), DVD / digital camera (lens, lens bonding, circuit board reinforcement), motor and component assembly (wire, coil fixing, coil end fixing, PTC / NTC component bonding, protection transformer core) Semiconductor chips (moisture protection coating, crystal element mask, crystal element contamination inspection, UV tape exposure, crystal element polishing inspection), sensor production (gas sensors, photoelectric sensors, optical fiber sensors, photoelectric encoders, etc.).

PCB industry LEDUV UV-curing applications:

Components (capacitors, inductors, various plug-ins, screws, chips, etc.) fixed, moistureproof filling and core circuit, chip protection, anti-oxidation coating protection, circuit board preservation (angle) coating, ground wire, flying wire, coil fixed, wave soldering through hole mask.

Optical resin hardening applications:

UV-curable resin is mainly composed of oligomers, cross-linking agents, diluents, photosensitizers and other specific auxiliaries. It irradiates the polymer resin with ultraviolet light to make it cross-linked and solidify instantly. Under the irradiation of UV LED UV curing machine, the curing time of UV curing resin does not need as long as 10 seconds, basically 1.2 seconds can be cured, which is much faster than the traditional UV mercury light curing machine. At the same time, the heat is also better than that of UV mercury lamp. Through the different blending of the components of UV-curable resin, the products that can meet different requirements and uses can be prepared. At present, UV-curable resin is mainly used for wood floor coating, plastic coating (such as PVC decorative board), photosensitive ink (such as printing of plastic bags), electronic products coating (marking and circuit board printing), printing glazing (such as paper, playing card glazing), metal parts (such as motorcycle parts) coating, optical fiber coating, photoresist and precision parts coating, etc.

2. Medical field: skin treatment:

An important application of UVB band is the treatment of skin diseases, that is, ultraviolet phototherapy. Scientists have found that ultraviolet light with a wavelength around 310nm has a strong dark spot effect on the skin, which can accelerate skin metabolism and improve skin growth, so it can effectively treat light-induced skin diseases such as vitiligo, pityriasis rosea, pleomorphic sunrash, chronic actinic dermatitis, photolinear prurigo, etc., so ultraviolet phototherapy has been more and more used in the medical industry.

Compared with the traditional light source, the spectral line of UV-LED is pure, which can ensure the therapeutic effect to the greatest extent. UVB band can also be used in the field of health care. Irradiation in UVB band can cause photochemical and photoelectric reactions of the human body, making the skin produce a variety of active substances, which are currently used to regulate advanced neurological function, improve sleep, reduce blood pressure and so on.

In addition, studies have shown that UVB band can accelerate the production of polyphenols in some leafy vegetables (such as red lettuce), which are claimed to have anti-cancer, anti-cancer spread and anti-cancer mutation properties.

Medical devices:

UV glue bonding makes it easier to assemble medical devices economically and automatically. Now, the advanced LED UV light source system, which can solidify the solvent-free ultraviolet glue for a few seconds, as well as the dispensing system, is an effective and economical way to form a consistent and repetitive bonding in the medical device assembly process.

The optimization and control of UV light source is very important for the manufacture of reliable medical devices. The use of UV curing glue provides many advantages, such as lower energy requirements, saving curing time and location, increased productivity, and easier automation.

UV glue is generally used to bond and seal medical devices, which require very high quality and the best reliability. UV glue curing is typically used in medical device assembly, such as the need to bond 1) different materials (or different mechanical properties) 2) the material is not thick enough to use welding methods 3) pre-production of sub-parts.

3, the field of sterilization and disinfection: because of its short wave length and high energy, UVC band ultraviolet can destroy the molecular structure of DNA (deoxyribonucleic acid) or RNA (ribonucleic acid) in microbial cells (bacteria, viruses, spores and other pathogens) in a short time, cells can not regenerate, bacteria and viruses lose the ability of self-replication, so UVC band products can be widely used in sterilization such as water, air and so on.

As UV-LED has the advantages of small size, it can be used as the light source of complete sets of UV (ultraviolet) sterilization equipment, suitable for all kinds of shapes and structures, various materials of a large number of products before packaging process; matching as indoor air sterilizer UV (ultraviolet) light source: suitable for household, public places and other indoor air sterilization; used in disinfection cabinets, microwave ovens and other household appliances.

At present, some deep ultraviolet application products on the market include LED deep ultraviolet portable disinfector, LED deep ultraviolet toothbrush sterilizer, deep ultraviolet LED contact lens cleaning sterilizer, air sterilization, clean water sterilization, food and object surface sterilization. With the improvement of people's awareness of safety and health, the demand for these products will be greatly increased, thus creating a large-scale market.

4. Flame detection field: ultraviolet flame detector is commonly known as ultraviolet flame detector. Ultraviolet flame detectors detect fires by detecting the ultraviolet rays produced by the combustion of substances. In addition to ultraviolet flame detectors, there are also infrared flame detectors on the market, that is, linear beam smoke fire detectors. Ultraviolet flame detector is suitable for places where open fire is easy to occur when a fire occurs. Ultraviolet flame detector can be used in places with strong flame radiation or no smoldering stage. Flame detection ultraviolet sensor requires the sensor itself to withstand high temperature and high sensitivity.

5. Arc detection field: high voltage equipment will produce arc discharge due to insulation defects, which will be accompanied by a large amount of light radiation, which is rich in ultraviolet light. By detecting the ultraviolet radiation produced by arc discharge, the safe operation status of high voltage power equipment can be judged. Ultraviolet imaging is an effective method for arc discharge detection, which is intuitive and has good detection and positioning ability, but there is still some difficulty in detecting the weak ultraviolet signal.

6. Banknote recognition: ultraviolet recognition technology mainly uses fluorescence or ultraviolet sensors to detect the fluorescent printing anti-counterfeiting signs and the matte reaction of banknotes. This kind of identification technology can identify most counterfeit notes (such as washing, bleaching, pasting, etc.). The development of this technology is the earliest, the most mature and the most widely used. It is used not only in the deposit identification of the ATM machine, but also in the financial instruments such as banknote counting machine and banknote detector. In general, fluorescence and purple light are used to detect the reflection and transmission of banknotes in all directions. According to the different absorptivity and reflectivity of ultraviolet rays between banknotes and other papers, the authenticity can be distinguished. The paper money with fluorescent imprint can also be quantitatively identified.

With the development of the new mechatronics technology, the performance of the ultraviolet sensor will be further improved, the detection result will be more accurate, the detection distance will be longer, and the dynamic detection performance will be better. The application prospect of ultraviolet sensor will be broader.

This article is from the official account of Wechat: sensor Technology (ID:WW_CGQJS). Author: Faraday.

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