UV Optical sensing – UV Sensor

UV sensing (UV Sensor) is a form of optical sensing, specified to the ultraviolet spectrum (10 to 400 nm). The same principles are applied that are used for other forms of optical sensing. A limitation is however that the short wavelengths of the UV spectrum are easily absorbed by most materials.
Applications

There is virtually no limit to the applications for UV sensing, ranging from very advanced to very simple. An example of a simple and inexpensive application is a device called ‘solar meter’. It measures the intensity (of certain wavelengths that are specifically found in sunlight) and gives feedback on how much UV exposure one has had in a day. It can be inexpensive because it works with a simple LDR or diode. And furthermore the requirements for the coating are not too high: Shorter wavelengths (UV-C) can’t be transmitted by most materials, but they won’t be found in sunlight as it reaches the earth at ground level.

An application that uses the absorption properties of short waves by most materials, is a fire detector. When smoke or gasses fill the gap between a UV transmitter and a sensor, the measured intensity will be lowered. If the intensity passes a certain threshold, the alarm goes off.

The transmission of specific wavelengths is also used in digital photography which is a form of optical sensing as well. Millions of little sensors on a chip measure intensities behind different color filters to create a digital color image.

Working principle
There are many ways to measure light intensity, but the basic lay-out is mostly the same. Photons hit a certain material, it reacts by emitting electrons or by changing its electrical resistance, the fluctuations in current are processed, and the radiation can be interpreted. The most common examples of photo sensors are photo resistors (LDR), photo diodes, photo transistors and photo CMOS sensors.
 
Most of the photoelectric materials are not applied without a protective layer, because many conditions can influence the values that the sensor is giving, for example moisture and temperature. They are dipped in glass or plastic, or they can have a coating. Recently a coating has been developed that allows for wavelengths down to 200 nm (UV-B) to be transmitted, opening a new field of applications. Fiber optics are often applied in combination with the sensor. This allows for measurements on larger distances, difficult angles and austere conditions. Some types of optical cables can transmit UV light as well.

Design guidelines
1. When designing with UV sensors, first of all make sure that the sensor can measure the proper wavelengths.
2. Consider the absorption properties of the materials between the source and the sensor. This may limit the distance over which the radiation is detectable.
3. Keep in mind that some materials tend to degrade under exposure of short wave radiation.

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