Photosensitivity is a special way of skin sensitization. It is characterized by the non-toxic effect of skin contact alone, which needs to be partially or systemically toxic after being reflected by light of a specific wavelength. There are usually light allergies and phototoxic reactions.
(1) Fundamental concepts
Photosensitization (photosensitization, also known as photodynamic or photomechanical action, refers to the oxidation of molecules in living organisms under the reflection of visible light (the absorbed light of the pigment) in the presence of both oxygen and pigment in the living body. Commonly used pigments are Methyl blue and pod nitrogen, etc., can cause damage to nucleic acids and proteins. It is generally believed that this is mainly due to the easy injury of bird's foot and histidine, but the detailed process is not very clear.
(2) Common photosensitive materials
The photosensitive substance refers to a substance that is good at absorbing long-wave ultraviolet rays in sunlight.
1 Exogenous photosensitive substances: such as asphalt, tar, cosmetics, detergents, dyes, food additives, preservatives, eucalyptus, pyridine, haloanilide, bleach, musk musk and methyl coumarin .
2 photosensitive food: such as citron, coriander, celery, rape, mustard, spinach, snow, horse chestnut, malan head, carrot, fennel, buckwheat, lettuce, sassafras, dragon bud, fig, lemon, gray dish, Purple cabbage, dried fungus, yellow mud snail, pimp shrimp, crab and so on. If you eat a lot and sun, you will get a sun rash on vegetables, a solar rash on the fruit, and a sun rash on the yellow mud snail.
3 photosensitive drugs: chlorpromazine, promethazine, chlorothiazide, hydrochlorothiazide, naphthyridine and the like.
Some diodes have a photosensitive effect and are called photoreceptor tubes, also called photodiodes.
(1) Introduction to Photodiodes
The photosensitive receiving tube is similar in construction to the semiconductor diode, and the die is a PN junction having a photosensitive characteristic, and has a single conductive property, so that a reverse voltage is applied during operation.
(2) Working principle
A photodiode is a semiconductor device that converts an optical signal into an electrical signal. Its central part is also a PN junction. Compared with ordinary diodes, the difference in structure is that the PN junction area is made larger as much as possible to avoid the incident illumination. The electrode area is as small as possible, and the junction depth of the PN junction is very large. Shallow, usually less than 1 micron.
The photodiode operates under a reverse voltage. When there is no light, the reverse current is very small (normally less than 0.1 microamperes), called dark current. When there is light, the photon carrying the energy enters the PN junction, and transfers the energy to the constrained electrons on the covalent bond, causing the local electron to break free from the covalent bond, thereby generating electron-hole pairs, called photo-generated carriers. .
They participate in the drift motion under the action of the reverse voltage, so that the reverse current becomes significantly larger, and the greater the intensity of the light, the larger the reverse current. This property is called "photoconductive". The photodiode is a photocurrent generated by the illumination of ordinary illumination. If a load is connected to an external circuit, an electrical signal is obtained on the load, and this electrical signal changes correspondingly as the light changes.
Photosensitive receiving tubes and phototransistors are photosensitive devices commonly used in electronic circuits. The photodiode has a PN junction like a normal diode. The difference is that there is a transparent window on the outer surface of the photodiode to receive the light reflection, and the photoelectric conversion is completed. In the circuit diagram, the character symbol is generally VD. In addition to the function of photoelectric conversion, the phototransistor also has an amplifying function, and the character symbol is generally VT in the circuit diagram. Since the phototransistor is an optical signal because of the input signal, it usually has only two pin lines of the collector and the emitter. Like the photodiode, the phototransistor housing also has a transparent window to receive the light.
(3) Fundamental characteristics
1 spectral characteristics
2 volt-ampere characteristics
3 lighting characteristics
4 temperature characteristics
5 frequency response characteristics