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https://www.sciencedirect.com/topics/engineering/photocell
13.03.12 Light Sensing Materials 13.03.12 Light Sensing MaterialsA light sensor, as its name suggests, is a device that is used to detect light. Devices that include these sensors have many uses in scientific applications, but they are also found in items that people encounter each day. They are very simple and inexpensive, allowing their inclusion in a multitude of consumer products, including night lights, cell phones, burglar alarms, garage door openers, bar code readers, etc. There are many ways to detect light, and based on the working principle, light sensors can be of different types.13.03.12.1 Photocell or PhotoresistorA photocell or photoresistor is a sensor that changes its resistance when light shines on it. The resistance generated varies depending on the light striking at his surface. A high intensity of light incident on the surface will cause a lower resistance, whereas a lower intensity of light will cause higher resistance. Cadmium sulfoselenide (CdS) is a photoconductive material commonly used in photoresistors (226–229).13.03.12.2 Charge-Coupled DeviceA charge-coupled device (CCD) is a metal oxide semiconductor chip sensor that transports electrically charged signals. A CCD generally has an array of cells to capture a light image via the photoelectric effect. The packets of charge are not initially converted to an electrical signal, but rather moved from cell to cell by the coupling and decoupling of potential wells within the semiconductor that makes up the CCD. At the end of the line, the charges from all the different picture elements (pixels) can be converted to electrical signals (230–232).13.03.12.3 PhotomultipliersPhotomultipliers are vacuum tubes that are extremely sensitive to light. They detect light and multiply the current produced by the incident light by as much as 100 million times (160 dB), enabling individual photons to be detected when the incident flux of light is very low. Incident photons strike a photocathode material that is present as a thin deposit on the entry window of the device. Electrons are produced as a consequence of the photoelectric effect. These electrons are directed by a focusing electrode toward the electron multiplier, where electrons are multiplied by the process of secondary emission.Photocathodes can be made of a variety of materials based on the desired properties. Typically the materials should have a low work function and are therefore prone to thermionic emission. The thermionic emission causes noise and dark current, especially in the materials sensitive in infrared. However, cooling the photocathode lowers this thermal noise. Some of the common photocathode materials are as follows (76,145).1.Ag–O–Cs: sensitive from 300 to 1200 nm. High dark current; used mainly in near-infrared, with cooled photocathode.2.GaAs:Cs (cesium-activated gallium arsenide): flat response from 300 to 850 nm, fading toward ultraviolet and to 930 nm.3.InGaAs:Cs (cesium-activated indium gallium arsenide): Higher infrared sensitivity than GaAs:Cs. Between 900 and 1000 nm and much higher SNR than Ag–O–Cs.4.Sb–Cs (cesium-activated antimony): used for reflective mode photocathodes. The response range is from ultraviolet to visible.5.Bialkali; Sb–K–Cs, Sb–Rb–Cs (cesium-activated antimony-rubidium or antimony-potassium alloy): Similar to Sb:Cs, with higher sensitivity and lower noise. It can be used for transmission mode and has a favorable response to NaI:Tl scintillator flashes, which allows them to be widely used in gamma spectroscopy and radiation detection.6.High-temperature bialkali; (Na–K–Sb): can operate up to 175 °C, low dark current at room temperature.7.Multialkali (Na–K–Sb–Cs): wide spectral response from the ultraviolet to the near-infrared region. With special cathode processing, it can be extended to 930 nm.8.Solar-blind (Cs–Te, Cs–I): highly sensitive to vacuum-UV and ultraviolet. However, it is insensitive to visible light and infrared. CsTe has a cutoff at 320 nm whereas CsI has a cutoff at 200 nm.13.03.12.4 PhotodiodesPhotodiodes are semiconductors sensitive to visible or infrared light depending on their manufacturing equipment. When a photodiode is excited by light, there will be an increase in the current flow through the diode, Figure 23.Figure 23. Schematic of a photodiode.Yoshinobu et al. reported on a new class of light sensors called light addressable potentiometric sensors (LAPSs). They are semiconductor-based chemical sensors with an electrolyte–insulator-semiconductor structure (233), e.g., Si/SiO2/Si3N4, Si/SiO2/Al2O3. When a certain light pointer illuminates the LAPS chip, the semiconductor absorbs energy and leads to energy band transition, i.e., it produces electron–hole pairs. The electron and hole would compound soon, and current is unable to be detected by the peripheral circuit. If the LAPS is biased on depletion, the width of the depletion layer is a function of the local value of the surface potential. When the LAPS is biased in reverse voltage, the depletion layer is enlarged. The local value of the bias voltage can be read out with AC photocurrent that is generated when a modulated light pointer is illuminated at the bulk silicon. By illuminating parts of the surface of the device with a modulated light pointer, additional charge carriers are generated and AC photocurrent flows. This photocurrent is due to a rearrangement of charge carriers in the depletion layer of the semiconductor, while the illumination is turned on and off. The arrangement of charge carriers is voltage-dependent (234,235). By integrating polymer membranes and ionophores on the sensor, surface ions such as Li+, Ca2+, K+, and heavy metals can be detected (236–238).URL: https://www.sciencedirect.com/science/article/pii/B9780080965321013030Aldo da Rosa, in , 201314.2 Theoretical Efficiency 14.2 Theoretical EfficiencyIn this section, we derive the theoretical efficiency of photocells without direct reference to the exact mechanism of their implementation except that we assume that all cells have to perform the functions of carrier generation and carrier separation. These functions can be carried out either in a same region of the cell or in separate ones.In the general discussion of photocell efficiency, in this section, we assume that the carrier separation function is carried out without any losses and that one electron-hole pair is created for every incident photon that has an energy,hf⩾Wg.7We will callWgtheband gap energyeven though in some cells, the required energy is not associated with raising an electron from the valence to the conduction band.We also assume that the material is transparent to photons of energy less thanWg. These photons do not interact with the photo sensitive material and thus have no photoelectric effect. Finally, we assume that all photons with energy above the band gap contribute to the load an amount of electric energy exactly equal toWg. The excess energy,hf-Wg, is simply transformed into heat and constitutes a loss.An appropriate material—in general a semiconductor—will be transparent or not to a photon depending on the frequency of the latter. The exact boundary between transparency and opacity depends on the type of material considered.Table 14.2displays the data for some semiconductors. Diamonds, a form of carbon that crystallizes in the same manner as silicon and germanium, being highly resistant to heat and radiation, are a promising material for transistors that have to operate in hostile environments.Table 14.2. Light Absorption Limits for Some SemiconductorsMaterialν0 (THz)λ (nm)Wg (eV)Region in which transitionfrom transparent to opaqueoccursα-Sn19.315, 5000.08Far infraredGe16218500.67InfraredSi26511301.10InfraredGaAs3269201.35Near infraredGaP5405552.24VisibleC13002305.40UltravioletThe mechanism that leads to the creation of energy bands in solids is discussed, in a simplified fashion, in Section14.11.1.1“Band structure in inorganic semiconductors”.A structure that, exposed to light, generates electric energy constitutes a photovoltaic cell, or simply, a photocell. Photocells made of bulk semoconductors are refered to as photodiodes.Photovoltaic (PV) cells exposed to monochromatic light can, theoretically, achieve 100% efficiency converting radiation to electric energy. In the majority of cases, photocells are exposed to broad-band radiation—that is, to a stream of photons of different energies. Under such circumstances, the efficiency is limited by the two mechanisms discussed in the preceding page:1.Weaker photons (those with less than a given frequency), fail to interact with the material.2.More energetic photons will deliver to the load only a part of the energy, the rest being thermalized.In all cases, whether we are considering ideal or practical devices, their efficiency is defined as the ratio of the power, PL, delivered to the load to the power, Pin, of the incident radiation,(14.1)η≡PLPin.The characteristics of broad-band radiation can be described by specifying the power density, ΔP, of the radiation in a given frequency interval, Δf, as was done for solar radiation in Table 12.1 (Chapter 12). Alternatively, taking the ΔP/Δf ratio to the limit, one writes an equation expressing the dependence of ∂P/∂f on f. The total incident power density is, then,(14.2)Pin=∫0∞∂P∂fdf.In the case of black body, ∂P/∂f is given by Planck’s equation,(14.3)∂P∂f=Af3ehfkT-1where A is a constant having the units of W m-2Hz-4. Hence,(14.4)Pin=A∫0∞f3ehfkT-1df.Let x≡hfkT, then(14.5)df=kThdxandf3=kTh3x3.(14.6)Pin=AkTh4∫0∞x3ex-1dxThe definite integral, ∫0∞x3ex-1dx has the value π4/15, therefore(14.7)Pin=AkTh4π415=aT4,where a(W m-2K-4) is also a constant.When the temperature of a black body radiator increases, not only does the total power, P, increase (Eq. 14.7), but, in addition, the peak radiation is shifted to higher frequencies as can be seen from Figure 14.3. There is a simple relationship between the frequency, fpeak, and the temperature, T.Figure 14.3. The peak of the p vs f curve of a black body moves toward higher frequencies as the temperature increases.The proportionality between the light power density and the fourth power of the temperature is related to the Stefan-Boltzmann law.From Eq. 14.3 we see that the shape of the distribution curve is determined by the factor, f3ehfkT-1. The peak occurs when(14.8)ddff3ehfkT-1=0.Making the x≡hfkT substitution and taking the derivative, we obtain,(14.9)(3-x)expx-3=0,whose numerical solution is x=2.821. From the definition of x,(14.10)fpeak=khxT=59.06×109T.For T=6000K,fpeak=354Thz.The relation between fpeak and T is the Wien’s displacement law.It is useful to relate the total flux, ϕ, of photons that, given a specified spectral distribution, corresponds to a power density, Pin. Consider a small frequency interval, Δf, centered on the frequency f. Since each photon has energy hf, the power density of radiation in this interval is(14.11)ΔP=ΔϕhfW/m2,where Δϕ is the photon flux (photons m-2s-1) in the interval under consideration. In the limit, when Δf→0 (and dividing both sides by df),(14.12)dϕdf=1hf∂P∂f,and(14.13)ϕ=1h∫0∞1f∂P∂fdf.Particularizing for the black body case and, once more, letting x≡hf/kT,(14.14)ϕ=Ah∫0∞1ff3ehfkT-1df=Ah∫0∞f2ehfkT-1df,(14.15)ϕ=AhkTh3∫0∞x2ex-1dx=2.404AhkTh3.because the definite integral, in this case, has the value 2.404.Still for black body radiation, we can find the ratio of the light power density to the corresponding photon flux. From Eqs. 14.7 and 14.15,(14.16)Pϕ=AkTh4π4152.404AhkTh3=37.28×10-24T.It should be noted that the formula above, is valid only if the full spectrum is considered. For a truncated spectrum, for instant one that has some regions removed by a filter, it is necessary to calculate separately the total power density, P, and the total flux of photons, ϕ, and form the ratio.Not surprisingly, the ratio of total power to total photon flux increases proportionally to the temperature because, as we saw when we derived Wien’s displacement law, the higher the temperature the more energy the average photon has.Example 14.1What is the photon flux when light radiated from a 6000 K black body has a power density of 1000W/m2? From Eq. 14.16,(14.17)Phi=P37.28×10-24T=100037.28×10-24×60004.47×1021m-2s-1.For the ideal case, the efficiency of the device is, of course,(14.18)ηideal=PLidealPin.We now need to know PLideal.If broad-band radiation falls on a semiconductor with a band-gap energy, Wg=hfg, the photons with frequency f<fg will not create carriers. A fraction(14.19)GL=1P∫0fg∂P∂fdf,of the total radiation power density, Pin, will be lost.Let ϕg be the total flux of photons with f>fg. Each photon creates a single electron-hole pair with energy hf. However, as stated, the energy in excess of Wg will be randomized and will appear as heat and each photon contributes only Wg joules to the electric output. The useful electric energy (the energy, PL, delivered to a load) will be,(14.20)PL=ϕgWgW/m2.The flux of photons with energy larger than hfg is (adapting Equation(14.21)ϕg=1h∫fg∞1f∂P∂fdf.The useful power is(14.22)PL=hfgϕg=fg∫fg∞1f∂P∂fdf,and the efficiency is(14.23)ηideal=PLPin=fg∫fg∞1f∂P∂fdf∫0∞∂P∂fdf.Observe that ηideal depends only on the spectral distribution and on the Wg of the semiconductor. It completely ignores the manner in which the device operates. Unlike the efficiency of real photocells, ηideal does not depend on the level of illumination. Again, for a black body,(14.24)ϕg=Ah∫fg∞f2ehfkT-1df=AhkTh3∫X∞x2ex-1dx,where X=hfg/kT=qVg/kT.It should be obvious that the ratio σ≡ϕg/ϕ depends only on the nature of the radiation considered, not on its intensity. The ratio is(14.25)σ≡ϕgϕ=∫X∞x2ex-1dx∫0∞x2ex-1dx=∫X∞x2ex-1dx2.404=0.416∫X∞x2ex-1dxFor 6000-K black body radiation, the ratio is a fixed 0.558 if Wg=1.1 eV, the band gap energy of silicon. The ideal efficiency of a photodiode is then(14.26)ηideal=15π4hk4fgT4∫fg∞f2ehfkT-1df.It is more convenient to work with the band-gap voltage, Vg, instead of the corresponding frequency, fg=qhVg,(14.27)ηideal=15π4hk4qhVgT4∫qVgh∞f2ehfkT-1df.Letting x≡hfkT as before,(14.28)ηideal=15π4hk4qhkTh3VgT4∫qVgkT∞x2ex-1dx=15π4qkVgT∫qVgkT∞x2ex-1dx=1780VgT∫qVgkT∞x2ex-1dx.The lower limit of the integral is that value of x corresponding to fg.There is no analytical solution to the preceding integral, but it can either be solved numerically or the table in Appendix A to this chapter can be used to determine the value of the definite integral (which is, of course, a simple number, function of the lower limit of the integral).Example 14.2What is the flux of photons that have more energy than that of the silicon band-gap (1.1 eV, i.e., Vg=1.1V) when light radiated from a 6000-K black body has a power density of 1000W/m2? Eq. 14.25 gives us the ratio, σ, of ϕg to ϕ. For the particular combination of this example (Vg=1.1 V and T=6000 K), the ratio is 0.558, and from example 14.2, ϕ=4.47×1021 photons m-2s-1. Consequently,(14.29)ϕg=σϕ=0.558×4.47×1021=2.49×1021photonsm-2s-1.Example 14.3What is the ideal efficiency of the photocell under the circumstance of the previous example? Using Eq. 14.28,(14.30)ηideal=17801.16000∫2.125∞x2ex-1dx.The lower limit of the integral is X=hfg/kT=qVg/kT=2.125. The value of the definite integral is 1.341 (by interpolation in the table in Appendix A to this chapter), hence,(14.31)ηideal=17801.160001.341=0.438.Figure 14.4 shows how the ideal efficiency of a photocell depends on the band gap energy when exposed to a black body at 6000 K (about the temperature of the sun). Our efficiency calculations, based on Eq. 14.28, use a very simple model that totally ignores the photocell itself which is assumed to be 100% efficient. Its results are identical to the ultimate efficiency of Shockley and Queiser (SQ).Figure 14.4. Dependence of the efficiency of a photodiode on its band-gap energy. Black body at 5800 K.Perhaps one of the earliest calculation of theoretical efficiency as a function of band gap is the work by Prince (1955). His model considers the best possible silicon cell made under the limitations of the, then, primitive technology. Specifically, it assumes values of lifetimes of minority carriers that have been vastly improved upon. Although the general shape of the efficiency versus band gap curve is roughly the same as that from Eq. 14.28, the absolute values of estimated efficiencies are much lower. He sets the maximum theoretical efficiency at 21.7% and proceeds to explain why this value is unattainable.Up to 1961, there was no clear agreement as to what band gap would yield (theoretically) the highest efficiency when exposed to sun light. See Loferski. In 1961, Shockley and Queiser published a much cited paper deriving the theoretical limits of solar cell efficiencies operating under certain assumptions some of which we used in our derivation. One assumption we did not make was that the photocell involved a p-n junction which implies irreducible radiative recombination of electron-hole pairs. For this reason, the SQ detailed balance model predicts somewhat lower efficiencies than the ultimate efficiency model in Fig. 14.4.Since the solar spectrum is not exactly that of a black body, the dependence is somewhat different from that shown in the figure. Also, the exact spectral distribution of sunlight in space differs from that on the ground owing to atmospheric absorption.Notwithstanding all these limitations, efficiencies greater than these black body spectrum efficiencies can be achieved. This is done by creating situations in which one or both of the efficiency limiting mechanisms discussed at the beginning of this section are circumvented. Three techniques are discussed in the next three sections.URL: https://www.sciencedirect.com/science/article/pii/B978012397219400014XAldo Vieira da Rosa, Juan Carlos Ordóñez, in , 20228.7.1 Analogue Sensor 8.7.1 Analogue SensorThe basic sensor is a single photocell (known as “cosine sensor”) whose output current is proportional to the cosine of the Sun aspect angle α (a generic name to be used below is “incidence angle”) between the Sun direction s→ and the photocell normal n→s=s→3, and to the solar energy flux (irradiance) Φs [W/m2]. The solar power delivered to a photocell area A holds(8.146)Ws=AP→s·n→s=AΦss→·n→s=AΦscosα,|α|<π/2,where P→s is the Poynting vector (the energy flux vector of an electromagnetic field) of the solar electromagnetic radiation. Supplying the cell with a constant voltage and assuming a constant solar flux, the cell current I becomes proportional to cosα as follows:(8.147)I(α)=I(0)cosα,where I(0) ≈ 0.1 mA is the peak current depending on the cell area A, the solar irradiance Φs, and other sensor parameters. The sensor field-of-view (FoV) denoted by 2αmax is lower than π due to the dead zone δ ≤ 0.2 rad of the photocell, and is written as(8.148)2αmax=π−2δ.When the incident radiation is normal to the cell surface, the circulating current reaches the maximum value, but the current variation around this condition, namely α ≅ 0, results to be very low due to the cosine law in Eq. (8.147). This is the same problem encountered by FOG sensors in Section 8.5.4. As a result, the sensor scale factor and the relevant accuracy are greatly reduced because the current variation holds(8.149)ΔI(α)=−I(0)αsinα≅−I(0)α2.This configuration can only be used for detecting the cone (axis and aperture) where the Sun is visible from the sensor. In this way, the sensor is used as a Sun detector and not as an accurate direction sensor. In addition, the signal (8.147) is affected by the sign ambiguity.To make available an accurate measurement and to eliminate the sign ambiguity, multiple cells must be adopted. Here we limit to a single-axis sensor, made by a pair of photocells, k = 0, 1, which are symmetrically inclined with respect to the sensor normal n→s as in Fig. 8.12, left. By denoting the tilt of the kth photocell normal direction n→sk relative to n→s with skα0, where s0 = 1, s1 = −1, the photocell currents becomeFigure 8.12. Left: layout of a single-axis sun sensor. Right: output signals of a cosine sensor and a single-axis sensor together with the single photocell signals.(8.150)Ikα=Ik0cosα0+skα,k=1,2.Taking the difference and assuming equal zero-angle current, the differential current(8.151)ΔI=I1−I0=I0cosα0−α−cosα0+α=2I0sinα0sinαbecomes proportional to the sine of the Sun aspect angle. The FoV reduces to(8.152)2αmax=π−2(α0+δ).Outside the FoV, the sensor behaves as a single photocell as in Eq. (8.147), thus affected by the sign ambiguity. Fig. 8.12, right, shows the voltage response V(α) = RI(α) versus the incidence angle of a cosine sensor as in Eq. (8.147), and the single-axis response accompanied by the positive and negative components. The parameters of the sensor responses in Fig. 8.12, right, are as follows:(8.153)α0=π/4,I(0)=0.1mA,R=50kΩ,δ≅0.1rad.The single-axis response in Fig. 8.12, right, is rather linear within the FoV equal to 2αmax ≅ 1.4 rad. The largest fractional deviation ∂α from linearity (the linearity error) occurs at α = ±αmax and holds(8.154)∂α=αmax−sin(αmax)sin(αmax)≅0.087.Linearity can be compensated by employing the nonlinear response in Eq. (8.151).To retrieve the Sun direction, two incidence angles α and β must be measured, which is achieved by mounting a pair of single-axis sensors 90 degrees apart (one along s→1 and another along s→2) and with the same optical axis n→s. Then, the Sun unit vector in the instrument frame S={S,s→1,s→2,s→3} holds(8.155)ss=11+tan2α+tan2β[tanαtanβ1].Recent analogue Sun sensors, such as those mounted on the European GAIA and Bepi Colombo satellites [8], use four-quadrant semiconductor photodiodes, where the quadrants k = 1, 2, 3, 4 in the detector plane (x, y) are numbered clockwise, starting from the positive quadrant. Closely above the detector surface, at a distance h, a mask of radius r is the entrance aperture of the sunlight. The distribution of the incident light across the quadrants is proportional to the illuminated area, which in turn depends on the incidence angles α and β in Eq. (8.155). The relation between quadrant currents Ik,k = 1, 2, 3, 4 and incidence angles is very simple:(8.156)tanαtanαmax=I2+I3−I1−I4∑k=14Ik,tanβtanβmax=I1+I2−I3−I4∑k=14Ik,where the identities tan(αmax) = tan(βmax) = r/h define the sensor FoV. Typical FoV values are ±1.1 and ±0.5 rad. The incidence angles are computed from 16-bit digitized currents. The response nonlinearity is compensated via on-board look-up tables. The main error component is the bias due to mounting misalignments between sensor axes and body axes. A typical upper bound is 5 mrad, but it may increase because of the Earth albedo. The standard deviation of a typical random error (the noise equivalent angle [NEA]) is 0.3 mrad.URL: https://www.sciencedirect.com/science/article/pii/B9780081007006000088S. Wijewardane, in , 201510.9 TPV power generation 10.9 TPV power generationTPV is a system that combines a photocell and an appropriate thermal emitter (Figure 10.10), which can emit most of its radiation at frequencies just above the cutoff frequency of the photocell. TPV systems have been researched for decades. However, the lack of a photocell that can convert thermal radiation efficiently to justify the cost of a thermal emitter delayed the progress of this field. With the availability of high-performance photocells based on semiconductor materials from the III–V family and with the development of spectrally selective thermal emitters that can emit radiation within a narrow frequency band, there has been a renewal of interest on TPV systems (Wijewardane and Goswami, 2014).Figure 10.10. Components of a possible thermophotovoltaic system.The current trend in TPV is to produce electricity from the waste heat of industrial furnaces in which the temperatures could go up to 1500 °C, and the prospective frequencies are within the near-infrared region. Although at the moment, the unit price of the electricity produced by conventional thermal plants using the industrial waste heat is slightly lower than the unit price of TPV generated electricity, in general, TPV systems exhibit several key advantages over conventional thermal plants. TPV systems produce no noise and occupy a comparatively small space. Also, these systems are portable and have little or no routine maintenance.Rare earth oxides such as erbium (Er2O3), samarium (Sm2O3), neodymium (Ne2O3), and ytterbium (Yb2O3) (Adair and Rose, 1994; Panitz et al., 2000; Good and Chubb, 2002) have the ability to emit radiation within narrow bands in contrast to most natural substances that emit radiation in wider and continuous bands. These rare earth oxides therefore are suitable for TPV applications. However, the high thermal expansion coefficients, combined with low thermal conductivities and high cost, make the production in bulk of rare earth oxide emitters for TPV applications impractical. Therefore, plasma spray techniques have been used to coat rare earth oxides on cheaper metal substrates mainly due to its cost-effectiveness and the ability to apply them on complicated geometries. Also, the ideal thickness range of rare earth oxides coatings for TPV applications (100–1000 μm) can be achieved easily with thermal sprays. Commercially available Yb2O3 and Er2O3 powders have been used recently (Tobler and Durisch, 2008a,b) to prepare the samples for thermal spray coatings. These coatings have shown good selective emitting properties and can be used in temperatures as high as 1600 °C.URL: https://www.sciencedirect.com/science/article/pii/B9780857097699000105Ashok Kumar L., ... Uma Maheswari Y., in , 20206.3.10.3 Optical proximity sensors 6.3.10.3 Optical proximity sensorsLight sensors have been used almost a century-photocells were first used on motion pictures for applications such as reading audio tracks. Yet current optical sensors are considerably more powerful. The optical sensors need a light source (emiter) as well as a detector. Emitters can use LEDs and laser diodes to create light beams in the visible and invisible spectrums. Detectors usually are built with photodiodes or phototransistors. The emitter and detector are designed to block or reflect a beam from an object while it is active. Fig. 6.39 displays a reference optical sensor.Figure 6.39. A basic optical sensor.The light beam is produced on the left in the figure, directed through a lens. On the detector side the beam with a second lens is centered on the detector. If the beam is interrupted the detector will show that there is an obstacle. The oscillating light wave is used to allow the sensor to filter out normal in-room light. At a fixed frequency the light from the emitter is switched ON and OFF. When the light is detected by the detector it scans to ensure it is at the same frequency. If light is emitted at the correct frequency then the beam will not be damaged. The oscillation frequency is within the KHz range, and is too quick to note. A side effect of the frequency approach is the possibility to use the sensors at greater distances with low power.You can set an emitter to point directly to a detector, this is known as opposite mode. The component will be detected when the beam is split. As seen in Fig. 6.40, this sensor needs two separate parts. This design works well with opaque and transparent objects with the emitter and the detector segregated by distances of up to hundreds of feet.Figure 6.40. Opposed mode optical sensor.Using the separate emitter and detector raises maintenance issues and requires synchronization. One alternative solution is to house the detector and the emitter in one device. But, as seen in Fig. 6.41, this demands that light be reflecting back. Such sensors are ideal for bigger items up to a few feet away.Figure 6.41. Emitter and detector in one unit.The reflector is built with 90° focused, polarizing screens. Unless the light is actually reflected back the light is not going through the mirror in front of detector. The reflector is equipped to rotate the light process by 90°, so it is now going through the screen in front of the detector.The emitter sets out a beam of light in the figure. When the light returns much of the light beam is transferred to the detector from the reflector. The beam is no longer reflected back to the detector when an object blocks the beam between the emitter and the reflector, and the sensor remains operational. One possible issue with this sensor is that a good beam might return reflective artifacts. This problem is solved by polarizing the light at the emitter (with a mirror), then using the detector’s polarized mirror. The reflector uses small cubic reflectors, and the polarity rotates by 90° as the light is reflected. The light will not rotate by 90° if the light is reflected off the surface. As seen in Fig. 6.42, then the polarizing filters on the emitter and detector are rotated by 90°.Figure 6.42. Polarized light in retro reflective sensors.The reflectors are relatively easy to align for retro reflectors but this approach still requires two installed components. A diffuse sensor is a single device not using a reflector, but using focused light as shown in Fig. 6.43. The light is dispersed with diffuse reflection. This reduces the amount of light given back. As a consequence the lenses need to amplify the sun.Figure 6.43. Diffuse sensor.URL: https://www.sciencedirect.com/science/article/pii/B9780128194164000065J. Hayavadana, in , 2012
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http://learn.adafruit.com/photocells
Photocells are sensors that allow you to detect light. They are small, inexpensive, low-power, easy to use and don't wear out. For that reason they often appear in toys, gadgets and appliances. This guide will show you how they work, how to wire them, and give you some project ideas.
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https://www.reecesupply.com/125960/Category/Photocells
Photocells Found 11 Item(s) Bulk Actions Bulk Actions Add Item To Group Add Item To Cart ... Register or login for Pricing View All Branch Availability Branch Name Available Select Item Compare Intermatic EK4436SM NightFox™ Metal Stem Mount ...
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https://www.elprocus.com/photocell-working-and-its-applications/
Photocell Circuit Diagram. The photocell used in the circuit is named as dark sensing circuit otherwise transistor switched circuit.The required components to build the circuit mainly include breadboard, jumper wires, battery-9V, transistor 2N222A, photocell, resistors-22 kilo-ohm, 47 ohms, and LED.. The above photocell circuit works in two conditions like when there is light and when it is dark.
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https://www.envisionledlighting.com/led-controls/photocells.php
20 40 60 80 100 120. per page. Button Photocell. B-PC-2W. Twist Lock Button Photocell. TW-PC-2W. 2 Items. Sort By Position Product Name Price Dimmable Set Descending Direction. Show.
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https://www.homedepot.com/b/Lighting-Outdoor-Lighting-Outdoor-Lighting-Accessories/Photocells/N-5yc1vZc7qzZ1z0u58q
Compare. 1250 CFM Black Wi-Fi Power Roof Mount Attic Fan. by Master Flow. Shop this Collection. (45) $15158. Pickup. Free ship to store.
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https://www.leviton.com/en/products/commercial/lighting-controls/photocells
Use the search filters below to refine results for photocells used in commercial applications. Additional photocell product documentation and resources can be found under each part number. Products. Filter by: Filter by: Product Type. Voltage Type. Product Line. Result links Always open results in new window.
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https://www.intermatic.com/
Get the most out of your ARISTA Advanced Lighting Control System with these helpful resources and video tutorials. From initial setup to an easy handoff, we'll show you how to tailor ARISTA to meet your needs. In this new partnership, Intermatic will serve as the sole representative of PEDOC product sales within the United States.
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https://www.dencodoorstuff.com/product/raynor-replacement-photocells-cr-2154/
Description. Manufactured to replace the beam sensors for Raynor Opener models R-160 & R-170. Beams have a self programming feature which allows you to simply hook up to the existing wiring. Mounting brackets and hardware included.
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https://www.sick.com/us/en
75 years of Pioneering Superpowers. Dr. Sick would be 112 years old now, and still ahead of his time – with his aspiration to do good for the world and all people by means of technical progress.
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https://bft-automation-uk.mybigcommerce.com/range-of-accessories/photocells/
FL130B Hardwired Photocells. SEAV 2241 180° wireless photocells. The THEA has been designed and patented by BFT to provide an adjustable photocell with an integrated flashing light where the lens is adjustable both horizontally and vertically. Beam directs 40 ° horizontally and 10 ° vertically.
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https://wirelesstelematics.com/
Our patented Lighting Control and Monitoring Service (LCMS) is the most cost-effective, simple, and energy saving alternative to timers and photocells. It easily replaces outdated and unreliable timers and photocells and saves you time, money, and hassles. Our LCMS can control an entire parking lot–it is not a fixture by fixture device.
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https://www.dencodoorstuff.com/product/universal-photocells-digi-code/
Will work with all major brands of residential openers including: Chamberlain, Liftmaster, Craftsman, Overhead, Stanley, Linear, Raynor and Morre-O-Matic The self programming feature allows you to simply hook up to the exitsting wiring
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https://www.thomasnet.com/products/photocells-57800674-1.html
Welcome to the premier industrial source for Photocells. The companies featured in the following listing offer a comprehensive range of Photocells, as well as a variety of related products and services. ThomasNet.com provides numerous search tools, including location, certification and keyword filters, to help you refine your results. For additional company and contact information, simply use ...
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https://www.prolighting.com/accessories1/photocontrols.html
Photocells are extremely helpful with nightlights, including security fixtures, floodlights and other evening safety features. Install a Precision photocell adapter with your floodlights! ... Two features I like about PROLIGHTING is the excellent chat support, and being able to login and pay using my Amazon account. Karen T. 10-21-2020.
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https://www.sansiled.com/blogs/learn/what-is-dusk-to-dawn-mode-how-do-d2d-photocells-work
Oct 16, 2021 . There is a wide range of photocells, however the way they all work the same. Photocells use semiconductors to control the electrical current of the light. When the semiconductor is exposed to a certain level of brightness, usually 150 lux or more, the light will be switched off. As the semiconductor will have stopped the current.
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https://bft-automation-uk.mybigcommerce.com/range-of-accessories/photocell-accessories/
Photocell Accessories. Photocells undergo extended exposure to the elements as they are often embedded in walls or placed in columns. At BFT We offer numerous photocell accessories for both internal pedestrian use and outside vehicle use. Scroll below to browse our collection of photocell posts, adaptors, individual mounts and protective covers.
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https://www.youtube.com/watch?v=cSWHGyfprgI
VIDEO: HOW TO WIRE A PHOTOCELL-https://www.youtube.com/watch?v=3CFZ1GHqlPw&t=1sSHOP PHOTOCELL: US STORE …
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https://rabdesign.ca/product-category/fixtures/outdoor-lighting/photocells/
For use with ALR photocells. The RSP-15 is a photocell with polycarbonate housing featuring an adjustable swivel base. Available in 120v. The RP-15 is a photocell with polycarbonate housing and a fixed base. Available in 120v. The RB-120 is a RAB Design photocell with twist and lock photo control. Ideal for illuminating large area spaces.
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