Ultraviolet (UV) sensors are a type of sensor that uses photosensitive elements to convert UV signals into measurable electrical signals through photovoltaic and photoconductive modes. Early UV sensors were based on pure silicon; however, according to the National Institute of Standards and Technology (NIST), pure silicon diodes also respond to visible light, generating unwanted electrical signals and resulting in low accuracy. GaN -based UV sensors, on the other hand, offer significantly higher accuracy than single-crystal silicon, making them a commonly used UV sensor material.
With the widespread application of computers , various sensing technologies serving computers are receiving increasing attention. Ultraviolet (UV) sensors can detect UV rays imperceptible to the human senses and avoid interference from sunlight, artificial light, and other common light sources . They are very useful for fire trap detection and fire suppression systems, as well as photoelectric control in special locations.
Structural classification of ultraviolet sensors
Currently, ultraviolet tubes with practical value both domestically and internationally can be classified into spherical, wire , and flat structures according to the shape of their cathodes. These are all electrode structures of diodes , and the shape and material of their outer shells are designed for application requirements . From the perspective of working conditions , it is more appropriate to analyze them by classifying them according to their electrode shapes.
1. Filament electrode structure
The electrodes of this type of tube are generally composed of two or more symmetrical metal wires . This is an early structural form of ultraviolet tubes , which often uses high-purity tungsten or platinum wires . The closely spaced parallel lines form the working area.
Since ultraviolet (UV) tubes rely entirely on the photoelectron emission effect at the electrode surface , and then utilize gas multiplication to obtain a strong signal, their spectral response range depends on the work function of the cathode material. During photoelectron emission, the shorter the wavelength of the photon, the higher its energy ; even a small number of photons can excite electrons to overcome the work function and escape from the cathode surface. Low-energy photons, even in large numbers, cannot excite electrons at the cathode surface. The purity of the cathode material surface in UV tubes is extremely important; otherwise, it will affect the spectral range and render the tube unusable . A symmetrical filamentary structure is used for ease of processing and to minimize contamination of the electrodes by other substances.
These types of tubes are characterized by being able to operate under AC conditions, having a relatively large operating current, and using simple circuitry. They can also have impurities removed from the electrode surface using appropriate processing techniques . However, their viewing angle sensitivity fluctuates significantly , and they are prone to uneven emission in the operating area.
2. Spherical cathode structure
To fully avoid the effect and make photoelectron emission more stable and uniform, the working area needs to be fixed on the cathode. This is because the ultraviolet tube relies on photoelectron emission and gas multiplication to convert light signals into electrical signals and amplify them . Generally, the light emission utilization rate is high in the area closer to the electrode . Therefore, a point-structured spherical cathode ultraviolet tube was designed.
Regardless of the angle from which photons radiate onto the hemispherical cathode, the discharge region is always located at the apex of the hemisphere closest to the anode. Because the effective area of the cathode is small , the operating current of the tube is generally less than 0.3 mA , but it has a wide viewing angle and relatively uniform viewing angle sensitivity , making it particularly suitable for fire prediction applications . Focusing techniques can also be used to further enhance sensitivity.
The anode inside the tube is made into a hemispherical reflective surface , such as the ultraviolet phototube in the United States that can withstand temperatures up to 540 ° C . This causes the received ultraviolet radiation to be reflected back to the central cathode , increasing the tube's sensitivity , because far-ultraviolet radiation has the same rectilinear propagation and reflection effect as visible light.
3. Flat plate cathode structure
The sensitivity of an ultraviolet (UV) tube depends on the number of far-ultraviolet photons received at the cathode. A larger cathode area results in a higher reception probability , leading to more electrons escaping from the cathode. These electrons are accelerated under a high-voltage electric field and collide with gas molecules inside the tube , ionizing them. The resulting electrons then collide with more gas molecules , and this cyclical process eventually causes the gas inside the tube to discharge. The probability of this avalanche discharge depends on the photoelectron emission effect at the cathode. To improve sensitivity, a flat-plate cathode structure has been developed and researched in recent years for UV tubes.
