Light Sensors
Light sensors have become very popular in many practical applications, perhaps more so than other physical sensors such as temperature, strain, and pressure sensors. They are also becoming very important in many biosensor applications, commonly used in co
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Light Sensors
Light sensors have become very popular in many practical applications, perhaps more so than other physical sensors such as temperature, strain, and pressure sensors. They are also becoming very important in many biosensor applications, commonly used in conjunction with fluorescent dyes. In this chapter we will examine a group of light sensors that are made out of semiconductors, namely photoresistor, photodiode, and phototransistor. Light sensors are typically used together with light sources; hence we will cover them (light emitting diode and laser diode) as well.
7.1
Light
Before we begin our discussion, we need to understand what light is. Light is a part of electromagnetic radiation that is visible to human eye. However, people also use the word light for some electromagnetic radiations that are not visible to human eye, for example, ultraviolet (UV) light or infrared (IR) light. Because of this, the light that can be seen by human eye is specifically called visible (Vis) light. Light exists in tiny “packets of energy” called photons, and it exhibits properties of both waves and particles (called wave-particle duality). As light (and all other electromagnetic radiations) is a wave, we can draw its wave form as shown in Fig. 7.1. The wave is sinusoidal and its peak-to-peak distance is called wavelength (l). In visible light, the wavelength determines its color. There may be a single wavelength (monochromatic), or a mixture of multiple wavelengths (polychromatic) that is more common. The speed of light (and all other electromagnetic radiations) under vacuum is a universal constant and does not vary with the wavelength. The speed of light in air is almost the same as that under vacuum. This value is 3.0 108 m/s. If we divide the speed of light (m/s unit) with the wavelength (m unit), we will get a unit of s1, indicating the number of wave oscillations per second. The SI unit of s1 is Hz (hertz), J.-Y. Yoon, Introduction to Biosensors: From Electric Circuits to Immunosensors, 103 DOI 10.1007/978-1-4419-6022-1_7, # Springer Science+Business Media New York 2013
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7 Light Sensors
Fig. 7.1 Wavelength (l) and amplitude (A) of light (electromagnetic wave)
Table 7.1 Electromagnetic radiations
Wavelength l
Type
Frequency n
g (gamma) ray 1 m
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