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 Optical lens-ultraviolet imaging technology

Introduction:

With the maturity of lens applications such as digital cameras, smart phones, and security monitoring, emerging fields such as automotive imaging and infrared thermal imaging will become new forces for the growth of the optical lens industry. The gradual implementation of cutting-edge technologies such as artificial intelligence and the Internet of Things is applying optical lenses to broader areas of the national economy. Optical lenses will become the eyes of the future information world. No matter what kind of lens, its basic purpose is to collect the light scattered or emitted by the object and reproduce the image of the object on the image sensor. The refractive index of light of different wavelengths is different, and the light of different wavelengths emitted from the same point cannot be converged into one point when imaging, resulting in chromatic aberration; the larger the wavelength span, the more obvious the chromatic aberration. When designing an optical system, different wavelength ranges are required. Therefore, optical imaging lenses can be divided into ultraviolet imaging lenses, visible light imaging lenses, and infrared imaging lenses according to the wavelength of their application. In this issue, let’s talk about UV lenses with shorter wavelengths.

1. Ultraviolet rays

Objects whose temperature is greater than absolute zero can emit electromagnetic waves; electromagnetic waves range from low to high frequencies, including radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays. Light is also an electromagnetic wave. Ultraviolet rays usually refer to electromagnetic waves with a wavelength between 100 and 400 nm, which are invisible to the human eye.

For sunlight ultraviolet rays with a wavelength in the range of 190-285nm, in the process of passing through the atmosphere, it is almost completely absorbed by the earth's ozone layer; and even in the atmosphere below the ozone layer, the scattering effect of other components and the surface ozone are also It has an absorbing effect, that is, the background radiation of this band of ultraviolet rays on the ground surface is almost zero, which makes a natural "dark room" formed near the ground surface-where the naturally occurring ultraviolet rays of this band are almost completely undetectable, also known as "Sunblind" ultraviolet rays. If solar-blind ultraviolet rays appear on the earth, there are usually only three situations:

One is natural abnormal weather, such as strong lightning.

Second is unnatural danger signals, such as gun fire, gunpowder explosion, fire, and corona caused by leakage of high-voltage transmission lines. Etc.;

The third is a sun-blind ultraviolet light source manufactured artificially. This means that if solar-blind ultraviolet signals are detected in the "dark room", it means that a specific event has occurred.

2. Ultraviolet imaging

In the solar-blind ultraviolet spectral region, the background noise is extremely low. Compared with infrared detection technology and visible light detection technology, the solar-blind ultraviolet imaging technology has the following advantages:

1) All-day, all-weather: the daytime detection is completely unaffected by strong sunlight, and it is also unaffected by environmental changes (fog, haze, day and night changes) and other high-temperature interference sources;

2) Intuitive, without image processing: The total blind ultraviolet image has no background interference, and the imaging is intuitive. It does not require complex image processing. For example, if a missile strikes, there is no need for complex image processing at all. "Discovery is the target";

3) High precision: strong anti-interference ability, suitable for various complex environments (night, bad weather, electromagnetic interference, etc.), and can achieve centimeter-level high-precision positioning;

4) High sensitivity: the response time including imaging reaches 10ms level;

5) Wide monitoring range: real-time detection can be carried out several kilometers away, and a large area can be covered by means of erecting a cloud platform or helicopter observation;

6) Simple structure: no need for low-temperature cooling, no scanning, small size, light weight and low power consumption of the detection imaging system.

The sun-blind UV imaging is based on sun-blind technology, in the areas of aircraft fog-breaking blind landing, ship fog-breaking pilotage, missile warning, space-based warning, forest fire warning, power grid security monitoring, maritime search and rescue, satellite navigation, and criminal investigation. Widely used gradually. The power system and high-speed rail system use daily blind ultraviolet imaging technology to achieve high-sensitivity corona and arc detection to find early local defects and provide early warning; and can monitor the operation of equipment to avoid equipment running with diseases and avoid major accidents. Using the principle that forest fires can send out solar-blind ultraviolet signals, it can monitor forest fires in real time from a long distance, while shielding other high-temperature objects from interference, solving the problem of high false alarm rates of existing infrared and visible fire alarms.

In addition to long-distance imaging, UV imaging can also be used for close-range criminal investigation photography, mainly to search for potential fingerprints, footprints, hide bloodmarks, fibers and other physical evidence, that is, use UV imaging to capture some potential, weak contrast or interference by background The handprint traces are suitable for taking pictures of handprints left on the surface of objects such as glass, plastic, enamel, paint, photos, and sticky layer of tape. The glass is black due to the strong absorption of 254nm short-wave ultraviolet rays, and the sweat fingerprint is white due to the reflectivity of 50%. The contrast between black and white is very large, so the ultraviolet imaging can express the subtle features very clearly.

3. Ultraviolet imaging lens

Although the general visible light lens can also transmit ultraviolet light as long as the ultraviolet light is strong enough and the exposure time is long enough, the general optical glass has a natural absorption band in the ultraviolet region, so professional ultraviolet imaging lenses must use special ultraviolet optical materials ( UV glass or crystal) to make lenses. UV optical systems also need to perform chromatic aberration correction to ensure that the system has good performance. However, due to the limited selection of UV optical materials and small dispersion coefficients, the chromatic aberration correction of the UV optical system has a certain degree. The difficulty. Some UV lenses will adopt a reflective optical system design, which can better perform chromatic aberration correction than a refractive optical system. In addition, during day and night, the ratio of visible light from skylight background radiation is much greater than that of ultraviolet light. In order to improve the signal-to-noise ratio of the system, a UV lens with a large relative aperture is required; Optical materials, and ultraviolet optical materials have a low refractive index (n<1.46), which leads to an increase in the number of ultraviolet lenses or an increase in lens thickness, and thus reduces the transmittance of the optical system, which partially offsets the large pass obtained by the large relative aperture. The effect of the amount of light. Therefore, the optical lens group generally has the following characteristics:

1) No coating or little coating;

2) No glued lenses, even if there are non-fluorescent glue;

3) The smaller the number of lenses, the better.

As an extension of infrared lens and visible light lens, ultraviolet lens has potential and broad application prospects and application requirements. It will play an increasing role in the fields of ultraviolet astronomy, sky communications, fire monitoring, environmental monitoring, and military early warning.