In 1970, LEDs were found to have a better life than long life, that is, they can be switched at very fast speeds, and the switching speed can reach KHz. By modulating the receiver's amplifier to the transmitter's modulation frequency, it can only amplify the optical signal that vibrates at this frequency.
We can compare the modulation of light waves to the transmission and reception of radio waves. By tuning the radio to a station, you can ignore other radio wave signals. A modulated LED transmitter is similar to a radio wave transmitter, and its receiver is equivalent to a radio.
People often have a misconception: Because the infrared light emitted by the infrared LED is invisible, the energy of the infrared light will definitely be strong. The amount of energy of the modulated photosensor has little to do with the wavelength of the LED lightwave. An LED emits very little light and is modulated to make it very energetic. An unmodulated sensor only by using a long focal length
The mechanical shielding of the lens allows the receiver to receive only the light from the transmitter in order to make its energy very high. In contrast, a modulated receiver can ignore ambient light and respond only to its own light or light with the same modulation frequency.
Unmodulated sensors are used to detect ambient or infrared radiation, such as red hot bottles that have just been released. In this application, if other sensors are used, they may malfunction.
If a metal emits much more light than the surrounding light, it can be reliably detected by the surrounding source receiver. The ambient light receiver can also be used to detect outdoor light.
However, it does not mean that the modulated sensor must be free from interference from ambient light, which can be problematic when used in a strong light environment. For example, an unmodulated photosensor can operate normally when it is pointed directly at the sun. We all know that when a piece of magnified glass is used to concentrate sunlight on a piece of paper, it is easy to ignite the paper. Imagine replacing the glass with the lens of the sensor and replacing the paper with a phototransistor, so it's easy to understand why it doesn't work when the modulated receiver is pointed at the sunlight, which is saturated by the surrounding light source.
The modulated LED improves the design of the photosensor, increases the detection distance, and expands the angle of the beam. This reliable and easily aligned beam is gradually accepted. By 1980, unmodulated photoelectric sensors gradually withdrew from the historical arena.
Infrared LEDs are the most efficient beam and the closest beam to the phototransistor in the spectrum.
However, some sensors need to be used to distinguish colors (such as color code detection), which requires the use of visible light sources.
In the early days, color-coded sensors used incandescent lamps as light sources, using photocell receivers, until later invented efficient visible LEDs. Today, most color-coded sensors use modulated LED emitters of various colors. Modulated sensors often sacrifice response speed to obtain longer detection distances because the detection distance is a very important parameter. Unmodulated sensors can be used to detect small objects or very fast moving objects, where the response speed is very fast. However, today's high-speed modulation sensors can also provide very fast response speeds for most inspection applications.
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