Solar-blind technology
Solar-blind technology is a set of technologies to produce images without interference from the Sun. This is done by using wavelengths of ultraviolet light that are totally absorbed by the ozone layer, yet are transmitted in the Earth's atmosphere. Wavelengths from 240 to 280 nm are completely absorbed by the ozone layer. Elements of this technology are ultraviolet light sources, ultraviolet image detectors, and filters that only transmit the range of wavelengths that are blocked by ozone.[1] A system will also have a signal processing system, and a way to display the results (image).[2]
Ultraviolet sources
Ultraviolet illumination can be produced from longer wavelengths using non-linear optical materials. These can be a second harmonic generator. They must have a suitable birefringence in order to phase match the output frequency doubled UV light. One compound commercially used is L-arginine phosphate monohydrate known as LAP.[3] Research is underway for substances that are very non-linear, have a suitable birefringence, are transparent in the spectrum and have a high degree of resistance to damage from lasers.[4]
Optical system
Normal glass does not transmit below 350 nm, so it is not used for optics in solar-blind systems. Instead calcium fluoride, fused silica, and magnesium fluoride are used as they are transparent to shorter wavelengths.[2]
Filters
An optical filter can be used to block out visible light and near-ultraviolet light. It is important to have a high transmittance within the solar-blind spectrum, but to strongly block the other wavelengths.[2]
Interference filters can pass 25% of the wanted rays, and reduce others by 1000 to 10,000 times. However they are unstable and have a narrow field of view.
Absorption filters may only pass 10% of wanted UV, but can reject by a ratio of 1012. They can have a wide field of view and are stable.
Ultraviolet detectors
Semiconductor ultraviolet detectors are solid state, and convert an ultraviolet photon into an electric pulse. If they are transparent to visible light, then they will not be sensitive to light.[2]
| name | limit nm | efficiency | ref |
|---|---|---|---|
| hexagonal boron nitride | 300 | [5] | |
| hydrogenated amorphous silicon | 187 | 20% | [6] |
| amorphous silicon carbide | 187 | 20% | [6] |
| gallium nitride | |||
| zinc oxide | |||
| diamond | |||
| photomultiplier tube | 300 | 30% | [7] |
Use
Solar-blind imaging can be used to detect corona discharge, in electrical infrastructure. Missile exhaust can be detected from the troposphere or ground. Also when looking down on the Earth from space, the Earth appears dark in this range, so rockets can be easily detected from above once they pass the ozone layer.[2] Israel, People's Republic of China, Russia, South Africa, United Kingdom, and United States are developing this technology.[8]
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