Photonics for Security, metrology & sensors
“Metrology & sensors“
Automatic or security systems must be able to “see” their surroundings. This need leads to the creation of sensors. A sensor is a device able to detect a change of a parameter in a situation. Depending on the situation, many parameters can be considered and different sensors can be used. Photonics allows to detect lots of different parameters.
Chemical reaction can create change of color. For instance the pH of a solution can be measured with pH-paper which color changes depending of the pH. It can help determining the quality of a solution.
The current delivered by a resistance changes with the temperature so a measure of current can be used to deduce the temperature. For instance thermic cameras are made of very small resistances that are used as pixels and the image formed shows the different temperatures.
Thermic cameras can be used for Health applications, building isolation or security (prevent fire or explosions).
Other materials can detect the change of strength of a pressure. They may be used for example as an on/off button or placed in a newly made wall to make sure it does not move.
Finally Photodiodes are often used as displacement or position sensor. They detect changes of light and the place where there is less light is normally where the person is localized. For instance automatic lamps emit IR light which is not visible for a human eye and measure the reflection. When the reflection changes the visible light is activated because a human is walking near the lamp.
“Security & safety”
Safety regulations in quality controls or security applications require fast and accurate sensors to get reliable results.
Food inspection must determine if foodstuff are eatable, toxic, rotten… the most common test is to take samples, dilute them in water and place drops in a petri box to observe the development of bacteria. Depending on the kind of bacteria created and their size, biologists can deduce the quality of the food. However it takes too long for a biologist to check himself all the samples and count the clusters of bacteria so an automatic detection device is needed. The first possible solution is to use a camera:
Light goes through the empty space but bacteria reflect or absorb it so the camera will see black spots where the clusters are. A software then detects, measures and counts the clusters. This method is fast and efficient but this kind of detection often requires to be thin and endowed with a large field of view which is difficult with a camera (it needs at least a few centimeters of thickness to see the whole box). In addition the cost can be high.
Another method is to place the bacteria in water:
Light is injected in the water and photodiodes measure reflected and transmitted light. The measures give optical properties of the bacteria and biologists easily deduce the kind of bacteria this is. A similar method is to add a polarizer to measure the optical rotation i.e. the turning angle of matter to get linearly polarized light. The water analysis method is potentially cheaper than using cameras but it is hard to distinguish different bacteria.
Thanks to these methods of detection and the new knowledge about diseases, the number of food poisoning has drastically been dropping over the past century.
Air quality surveillance
Pollution is one of the most important problem of our century for it causes great damages to the environment and can also harm people. Air quality control is needed to prevent major problems or epidemics. All over the world, samples of air are taken and tested to detect and measure the quantity of molecules such as carbon monoxide, ozone or sulfur dioxide. Photonics provides some detection systems:
Air sample are enlightened with Infrared (IR) or Ultraviolet (UV) light and transmitted light is measured to determine the absorption of the sample.
– Carbon monoxide quantity is determined with the IR absorption;
– Ozone quantity is determined with the UV absorption;
– Sulfur dioxide emits visible light when enlightened with UV so measure of visible light gives information about the quantity of sulfur dioxide.
Another test often made is the luminol test: nitrogen oxide reacts with luminol and it creates light that is detected by the photodiode.
To enhance security for entrance clearance or other security systems, new secure ways of personal identification are required. Human body information can be used for all bodies are different and some data are hard to fake. This information is called biometric data.
Fingerprints are the most known biometric and the oldest. It has been used by the police for more than hundred years. At first ink was used and policemen compared themselves the fingerprints of suspects. The quality was poor and it took a lot of of time to identity one. Now ink can be used but most of fingerprints images (like in a passport) are made with cameras:
Finger placed on a glass panel and light reflects on fingerprints to go the camera that gives a clean image if the resolution is high enough.
Fingerprints are easy to read and systems can be integrated in small devices but it is too well-known and people put fingerprints everywhere so criminals can find them and they have ways to fake fingerprints. In addition a sensor where everybody places their fingers is not hygienic. That’s why other biometrics were studied.
Eyes also have interesting patterns that can be used for identification.
The first one is the pattern of the iris:
Figure 16 shows the kind of pattern sought. On figure 17 we see that the devices send light to the eye and image the reflected light. The image is processed on computer to extract the pattern and compare it to the data bank.
It is also possible to do a retinal scan to get the pattern created by the veins:
Operation is basically the same as iris recognition but IR light is used to go through the eye and reflect on the veins. The vein pattern image is processed to be extracted and then compared to the data bank.
Both techniques are normally touchless and fast but they require the subject neither to move nor to blink. In addition heavy image processing is needed. These techniques are fine for fixed devices but portable devices have to be light and have low power consumption.
Last possibility is to observe vein pattern from fingers. Indeed it has been proved that even homozygote twins have different patterns so it can used as biometrics. This method is recent and only few devices have been created yet. Two general designs are possible:
Both designs use IR light that go through (only 40% of reflection) flesh and bones but is absorbed and reflected by veins. As a consequence veins appear darker than flesh on the image. The reflection design is useful to get thin devices but transmission design can also be used even if more light is needed for the transmission of finger is very low (less than 10%). The image is then processed and compared to a data bank. The texture and color of skin has to be considered too because optical properties might change depending on the person using the device.
It is a new biometrics and it requires more studies to be used. For example the texture and color of skin has to be considered too because optical properties might change depending on the person using the device. Still it is a non-invasive method of identification and much easier and comfortable to apply than eye reading.