Infrared principle and use of LED lighting
Infrared spectroscopy
The light that people can see with the naked eye is called visible light, and the wavelength of visible light is 380-750nm. The order of the wavelength of visible light from short to long is purple light→blue light→cyan light→green light→yellow light→orange light→red light. Light with a longer wavelength than red light is called infrared light, or infrared light (infrared). Infrared light is light that people cannot see with the naked eye.
The wavelength distribution of part of the light is as follows:
Violet light (O.40～0.43μm); blue light (0.43～0.47μm); cyan light (0.47～0.50μm); green light (0.50～0.56μm); yellow light (0.56～0.59μm); orange light ( 0.59～0.62μm); red light (0.62～0.76μm); infrared (0.76～1000μm); infrared light can be divided into:
1. Near-infrared (760～3000nm); 2. Mid-infrared (3000～60000rim); 3. Far-infrared (6000～150000nm).
Any object with temperature in nature will radiate infrared rays, but the wavelengths of the radiated infrared rays are different. According to experiments, the infrared (energy) wavelengths radiated by the human body are mainly concentrated at about 10,000 nm. According to the characteristics of the infrared wavelength of the human body, if a detection device is used, the infrared radiated by the human body can be detected and other unnecessary light waves can be removed.
The purpose of detecting human activity information can be achieved. Therefore, a sensor product that detects infrared rays of the human body has appeared. The infrared sensor of the human body is made according to the principle of pyroelectricity.
Pyroelectric principle
The human body infrared sensor is a kind of sensing product made by using the principle of pyroelectric effect. What is the pyroelectric effect? It is a phenomenon in which electric charges are generated due to changes in temperature.
In order to clearly illustrate the pyroelectric effect also appears. Illustrate with a diagram.
Figure 1 is a schematic diagram of a temperature change curve: Figure 2 is a schematic diagram of a sensor surface charge change state curve caused by a temperature change; Figure 3 is a schematic diagram of a voltage change output curve caused by a sensor surface charge change.
At the beginning stage (T) of Fig. 1, the temperature of the pyroelectric infrared sensor does not change without infrared radiation, the charge on the surface of the sensor is in a neutralized state, and the positive and negative electrons are equal (A). At this time, the sensor has no output ( 0). Figure l The second stage (T+△T), when there is a temperature change. Under the infrared radiation of the human body, if the temperature of the pyroelectric infrared sensor rises by △T, the charge on the surface of the sensor will change accordingly as shown in Figure 2(B). If the temperature change is △T, the corresponding charge change will produce a △V change. Therefore, the sensor outputs △V. As time goes by, the surface of the sensor will re-adsorb ions in the air and cancel each other out to reach the neutralized state as shown in Figure 2c. At this time, the sensor returns to no output (O). As shown in Figure 3.
When the temperature drops, the temperature returns to its original state (T), and its free polarization state is shown in Figure 2D. Since the process of temperature decrease and change (relatively speaking) is opposite to temperature increase, the change process of charge on the surface of the sensor is just the opposite of the change process when it rises, which is an inverse process.
Therefore, the output signal of the sensor is a △V, as shown in Figure 3. In the same way, as time goes by, the surface of the sensor will re-adsorb ions in the air, and the output signal of the sensor will be zero again.
The sensor's output signal for the whole process of sensing human activity information is shown in Figure 3. It is not difficult to see from the sensor output diagram that the signal output by an action of the sensor to human activities is a complete waveform. In the experiment. If the signal is amplified by an amplifier, and then observed with an oscilloscope, it will be a positive pulse and a negative pulse. In other words, a movement signal sensed by the sensor output is similar to a complete l Hz pulse signal.
Infrared sensor
In the pyroelectric sensor, the one-element sensor was used in the past. Because the one-element sensor is more affected by factors such as stray light, the application effect is relatively poor. Therefore, dual-element sensing units are now commonly used. This type of sensor has the following advantages:
1. It has the characteristics of high sensitivity.
2. The two unit devices are connected in reverse. Therefore, the infrared rays input at the same time will cancel each other, and there is no output. This increases the stability to external stray light, environmental temperature changes and external vibration effects (see Figure 5).
Due to the extremely high input impedance of the pyroelectric infrared sensor, it is very easy to introduce noise.
Therefore, it is necessary to perform electromagnetic shielding treatment on the sensor, so the metal package is adopted and the shell is grounded. In this way, the purpose of shielding clutter noise can be achieved.
In nature, the heat energy radiated by all objects is proportional to their own temperature. The higher the temperature of an object, the shorter the peak wavelength of its radiated heat energy. A human body with a temperature of 36-37°C radiates infrared rays with a peak heat energy of about 900-1000nm. Therefore, the presence or absence of the human body can be detected by a pyroelectric infrared sensor.
In order to avoid the influence of sunlight and lighting lights in the process of monitoring the presence or absence of the human body, a filter is usually added to the surface of the pyroelectric infrared sensor. At the same time, because the human body moves slowly, it is also necessary to carry High-efficiency, focusing Fresnel lens and other accessories can meet the actual needs of use.
