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Radiometry is concerned with the measurement of optical radiation for a wider optical spectrum from UV to IF. The parameters of photometry only look into and evaluate the alterations for various wavelengths of the visible light band [ 68 ]. Stereotypical parameters of radiometry include radiant power flux, irradiance, radiant intensity, and radiance. VLC is a wireless communication technology that relies on the conversion of light energy into electricity; therefore, it is a necessity to convert between radiometric and photometric parameters.
This conversion takes into consideration the relative visibility of light for a specific wavelength. The relative visibility of light is given by the eye sensitivity curve which is interchangeable with the luminous efficiency curve [ 3 ]. This curve represents the ratio of all photometric parameters to their equivalent radiometric parameters. The correlation between photometric and radiometric parameters is given by where is the relative spectral sensitivity function at wavelength.
Typical aspects of photometry to be understood with regard to VLC systems are luminous flux, luminous intensity, illuminance, and luminance; each of these aspects is briefly described as follows.
The total quantitative amount of power emitted per second from an optical source and within the visual sensation of the human eye is referred to as luminous flux. SI unit for luminous flux is lumen or candela steradian. In general terms, luminous flux may mathematically be given as where is the light intensity in candela, and is the angular span in steradian over which the light is emitted. With respect to isotropic sources, equation 6 may be rewritten as to yield.
The sensitivity of human eyes due to various visible light wavelengths changes notably when adjusting the luminous flux. For sources that emit light through various wavelengths over the visible light spectrum, the luminous flux may be obtained using equation 8. Integrating the radiant flux in all directions gives the total optical power which is expressed by equation 9.
The intensity of the power radiated per unit time from an optical source is known as luminous intensity. In simpler terms, it is the luminous flux per solid angle in a given direction. The intensity may be given as [ 69 , 70 ].
The amount of irradiance on a given surface, i. Illuminance is measured in or lx [ 69 , 70 ], hence, given by where is the illuminated area. In an instance where the illuminated area is large, the illuminance is simply the ratio of the luminous flux to the total area. The amount of radiance on a given surface area and a given solid angle in a given direction, i. Luminance is measured in or. Luminance may therefore be expressed as the following equation [ 69 , 70 ]. Of all the three parameters that are based on the flux intensity, this is the most significant one.
This is on account of the fact that the light energy is radiated from an extended source instead of a point source. Moreover, it is the only parameter, which is conserved during the propagation of light [ 70 ]. An estimation of power transfer for a radiant source having a uniform radiance across its surface and uniformly emitting in an isotropic manner is very useful in optics.
Such a source is said to be a Lambertian source [ 70 , 71 ]. The law states that the luminous intensity viewed from an ideal diffuse radiator varies directly with the cosine of the angle between the surface normal and the incident light ray [ 3 , 72 ].
This is graphically depicted in Figure 5 and mathematically represented by equation In order to obtain worthwhile radiance measurements of an object that has a large or uncertain angular distribution, cosine receptors on detectors are a requirement.
As a consequence, a commonly used model for radiance estimation is the Lambert radiator [ 73 ]. Take luminous intensity with an arbitrary direction of a Lambertian radiator to be denoted as where, is the angle between the surface normal and the incident light ray, is the intensity of the light source in the normal direction, is the order of Lambertian emission, is the diffuse reflection coefficient, is the unit normal vector, and is the unit light vector.
The luminance of such a radiator is independent from the orientation angle despite different levels of the luminous intensity at different viewpoints. Therefore, with this condition, the luminance from any point of view is equal. The spatial distribution of LEDs is similar to a Lambert radiator. The International Commission on Illumination CIE states that different levels of illuminations are required depending on the type of environment.
Figure 6 depicts the illumination distribution of an LED with a 30 semiangle. One other element which is of significance to the realization of VLC systems is photodetection. Light detection is normally achieved by the use of photodetectors PDs , which are also known as nonimaging receivers.
A PD is a square-law optoelectronic transducer sensor that detects and responds to electromagnetic radiation EMR , specifically light, which is incident to its surface. They respond to the impinging EMR signals by converting the light photons into current which is proportional to the square of the received radiation.
Alternatively, photodetection can be done by the utilization of imaging sensors also referred to as camera sensors. An imaging sensor is a matrix of multiple PDs.
Such sensors are found in many of the existing mobile devices including smartphones. This makes it possible for the mobile devices with imaging sensors to be easily converted and used as VLC receivers. The need for the conversion is motivated by a very high number of PDs which facilitates high-resolution imaging.
Having numerous PDs notably decrease the rate at which frames are being captured by the sensor. This reduction consequently leads to the allocation of limited data rates in the range of merely a few kpbs for VLC signals due to low sampling rates of the sensor.
In contrast to imaging sensors, detached nonimaging sensors are able to offer a provision of higher data rates in the range of hundred mbps. In an optical communication channel, the PD must have precise and uncompromising performance requirements because the received optical signal is conventionally weak.
The said performance requirements include a sufficient bandwidth to satisfy the desired data transfer rate, a very low noise level, and a very high sensitivity in the range of wavelengths that the PD operates within. The quantum efficiency , which is defined as the ratio of electron-hole e-h pairs produced by the PD to the incident photons within a stipulated time, is given by [ 74 ].
The most critical compatibility and performance requirements for detectors have a number of properties in which they are defined. Firstly, a large surface area to allow for a wider detection field of view FOV and a large collection aperture is required. In applications which require high-speed, it is more appropriate and best to have PD arrays that bear a small surface area.
Lastly, the PD must have a fast response time as well as high reliability, small size, and low cost. Performance characteristics for various PDs are given in Table 2 as adapted from [ 3 ]. A PIN PD is made up of two semiconductor materials with - type and - type regions which are detached by an intrinsic region which is sparingly - doped. For it to operate, an adequately large reverse bias voltage is applied across the device.
The schematic is depicted in Figure 7. For the conversion of an impinging photon into an electric current, the energy of the incident photon has to be equal to or greater than the band-gap energy. An electron from the valance band is excited by the energy of the incident photon to the conduction band; during this excitation process, an electron-hole pair is generated.
Concentration of impinging light is under conventional conditions directed to the depleted intrinsic region. After the separation of the generated charge carriers because of the high electric field in the depleted region, the charges accumulate across the reverse-biased junction.
In turn, this process leads to the flow of current along the load resistor as shown in Figure 7. There is a single electron flow for each of the generated carrier pair.
The upper cut-off wavelength, which is determined by the type of semiconductor material used, may generally be computed by equation 3. This leads to an elevated sensitivity of the device because of photocurrent multiplication prior to its encounter with the thermal noise of the receiver circuitry.
Note that the value of is equal to that of in an instance where is unity and is measured in. Responsivity, is given by. However, it is worth mentioning that there is an omnipresent multiplication noise when it comes to the APD following the ionization process which is of a statistical nature [ 82 ]. In addition, the ionization process is highly sensitive to temperature.
When an APD is used in a VLC system, it is advisable to take into account these factors as they are very significant to the performance of the system. The process of photodetection is very crucial in OWC systems. It is used to transform an optically radiating signal which is carrying information into a corresponding electrical signal.
Prior to transmission, the signal carrying information is encoded on the radiation intensity or the frequency of the optical source. On the front end of the receiver, after the encoded signal has been sent via free-space or optical fiber, the front-end equipment focuses the filtered signal radiation on the surface of the PD. The said method involves the use of the radiated emissions from the light source. A local oscillator is not required for a direct detection scheme since it does not contribute to any of the processes during detection.
When employing direct detection to retrieve the encoded information, it is imperative for the transmitted information to have certain attributes. A receiver based on DD is depicted in Figure 8. The instantaneous current of the PD, for an instantaneous power , is defined by. Like any other communication systems, noise source identification at the front-end of the receiver is crucial.
The performance of an OWC channel is influenced by noise sources together with the frequency and distortions induced on the link. In OWC, the noise from the electronic equipment of the receiver and the shot noise induced on the received photocurrent are the most predominant noise sources at the receiver input. Depicted in Figure 9 is the diagram of the optical receiver front-end. This front-end is normally made up of a preamplifier for amplification of the electrical current produced in the PD.
There exists three classes of amplifiers used for optical receivers, and they are transimpedance amplifier TIA , low impedance amplifier, and high impedance amplifier. Speed and sensitivity are the two factors that determine the design of the front-end on account of the balance required between these two factors [ 83 ]. In the event that a high impedance amplifier is utilized, there is a great reduction of thermal noise, thus, enhancing the sensitivity, but this also reduces the bandwidth.
Low bandwidth on high impedance front-end can be mitigated by the use of an equalizer. Low impedance front-end is not a practical technique for VLC systems due to the prevalence of the thermal noise. A balance between a desirable bandwidth and sensitivity is available for systems with a front-end that employs TIAs.
The resistive component of the front-end produces thermal noise. This noise is not dependent on the received signal, and it has a normal Gaussian distribution. Different types of noise sources in OWC are discussed as follows. For a coherent light source, the aggregate of the radiated photons is never constant for a given time.
This is caused by the discrete nature of light impinging the receiver. Thus, the performance of an ideal PD is affected by nothing besides the noise. With regard to optical sources that produce continuous power, the number of actual photons produced per unit time has a Poisson distribution despite the source producing a constant mean number of photons per unit time.
This phenomena lead to quantum noise which is also termed as photon noise. This type of noise is prevalent in every photon detector.
The shot noise generated by both the incoming optical signal and the OLO with reference to a coherent receiver is, respectively, given by where is the bandwidth of the coherent receiver. There is always a small amount of current at the output of the PD even when there is no incident light.
This current is termed as dark current, and it conventionally bears no valuable data. Dark current is a composite current made up of the bulk and surface leakage currents. It is generated by the movement of electrons into the conduction band from the valence band. Dark current goes hand in hand with the energy band-gap of the material used to manufacture the PD.
Materials which possess a large band-gap exhibit values which are tremendously low for the dark current. Conversely, materials with a small band-gap may be significant when the device operates in ambient temperature. All conducting materials are susceptible to thermal noise which is also referred to as Johnson noise. This type of noise is caused by the thermal instability of the electrons in the receiver circuitry. The load resistance and temperature , in Kelvin of the PD, contribute to thermal noise.
Thermal noise has a Gaussian distribution; normally, it is assumed to have a zero mean. Relative intensity noise RIN is normally used to express intensity noise, and it is given by where is defined as the common-mode rejection ratio for a balanced receiver.
Mitigation of RIN effects with regard to heterodyne receivers can be done by using a configuration in which a balanced PD is employed. This configuration is depicted in Figure This is very beneficial for systems where there is a requirement specification for a higher SNR. Moreover, it is a very competent technique for common-mode noise compensation for LDs.
The signals from the first PD and the second PD are respectively given by equations 24 and 25 , whereas the resulting signal after subtraction is defined by equation Typically, different types of noise have different weights; in most cases, thermal noise and quantum noise are the most significant.
In an occurrence where the power of the optical signal is quite high, the thermal noise is far much less than the shot noise. Thus, the SNR is referred to as quantum noise limited.
Under the conditions where the power of the optical signal is low, the shot noise is dominated by the thermal noise, and the SNR is now termed to be thermal noise limited. The interaction of the optical radiation with the atomic structure of the detector surface is defined using semiclassical approach.
The probability of a PD with aperture area emitting number of electrons from the emitted photons over a time period has a Poisson distribution defined by.
The relationship of the mean count to the aperture area and the impinging irradiance is given by. The variance of the mean current produced by electrons is statistically denoted by where is the period of the irradiance.
Since the mean and variance for a Poisson distribution are equal, and with a bandwidth of the postdetection filter as Nyquist bandwidth , equation 21 can now be defined using. Now, we look at the system architecture of a VLC system. The consortium presented two standards being visible light communication system standard and visible light ID system standard. This standard comprises of the design specifications for both the physical layer and link layer.
The standard particularly defines three aspects for a high-speed indoor OWC transceiver based on the use of visible light. The said aspects are, system architecture, physical layer and data link layer. More details and the specifications on the G.
The VLC channel structure may be categorized in two main categories depending on the way in which the link configuration between the transmitter and the receiver is set up [ 3 , 44 , 71 , 73 , 92 ]. Figure 11 depicts the classes of link configurations and they are explained below.
For this category, the link may be established by means of having a directed or nondirected transmitter-receiver pair. Extending this category, there are three different feasible classes regarding directionality. In this class, the transmitter and the receiver are directed and aimed towards each other. With this class, power efficiency and immunity to distortion due to environmental culminations such as ambient light are substantially enhanced.
This is suitable for low data transfer point-to-point communication applications with no obstacles between transmitter-receiver pair. Another class of this category is defined by a scenario whereby the transmitter and the receiver are not specifically directed to a specific point. Transmitters with wide beams and receivers with a wide FOV are a fundamental requirement for the transmission of signals for this class. The requirement of high power levels in order to strive for the reduction of elevated optical path loss and multipath-induced malformation is the major pitfalls for this second class owing to a wider beam divergence [ 3 , 92 ].
On the other hand, it allows user mobility, and it also decreases the effects of shadowing as it is suited for broadcast applications. A case whereby the transmitter and the receiver may assume divergent levels of directionality leads us to another class under this category.
This is commonly known as a hybrid class. As an example for this class, consider a scenario whereby the transmitter with a narrow beam is directed towards a certain point and the receiver with a wide FOV is not arranged to align towards a specific direction or vice versa.
Systems that are based on this class suffer from multipath propagation [ 3 , 92 ]. The availability of a line-of-sight LOS pathway between the transmitter and the receiver yields the second category. Just like the previous category, this category may also be extended into three classes. This configuration simplifies the path loss calculation owing to the fact that there is no consideration of the reflections.
The power efficiency of this class is very high. The non-LOS systems with a nondirected transmitter and receiver are also referred to as diffuse systems. For this class, the transmitter-receiver pair do not have to be directed, neither do they need to have a clear path to establish a communication link.
Signals arrive at the receiver indirectly from the transmitter due to the nearly uniform distribution of the optical signal by the reflecting panels such as walls and ceilings. Diffuse systems are considered to be the most robust, easy to design and implement, and predominantly for mobile communication systems [ 3 , 44 , 71 ]. This class paves way for effortless use as they allow the transmitter and the receiver to operate even when there are obstacles in between them.
In this class of configuration, the transmitted signals are reflected from various objects and are received at different time intervals. The hybrid non-LOS architecture leads to multipath distortions which creates a severe difficulty for path loss estimation which makes them complex and very costly to implement.
It is worth noting, with regard to the transmission speed only, that VLC channels may be categorized into two categories. One category is a case where the use is for a high data rate and the other category is a case where the requirement is for low data rate [ 50 ]. For utilization where there is a need for a high data rate, high-speed photodiode receivers are required. As for low data rate cases, the existing hardware found in our day-to-day lives such as mobile devices may form part of the system.
A downlink communication to mobile devices in an office or flight cabin from the general omnipresent illumination lamps is one of the most promising high data rate VLC applications [ 93 ].
Other applications for high data rates include cases where files are being transferred between devices and streaming from a device to a display. Augmented reality AR , vehicle-to-vehicle V2V , vehicle-to-infrastructure V2I communication, underwater communication, and localization of indoor communication systems, just to mention a few, are examples of low data rate applications [ 94 ].
Due to the intrinsic nature of the LEDs producing incoherent light intensity, the most prominent, convenient, and favorable modulation technology employed for indoor VLC systems is IM [ 4 , 44 , 71 , 95 , 96 ]. Moreover, this technique is easy to implement and highly cost-effective. With the IM technique, the optical power emitted by the LEDs is changed with respect to a certain attribute of the baseband signal. For demodulation, DD is employed as apposed to heterodyning as in RF and optical laser communication where coherent detection is possible [ 4 , 44 , 71 , 95 — 97 ].
DD produces a photocurrent which is directly proportional to the instantaneous optical power which is incident to the photodetector. Nonetheless, flicker is intercepted by keeping the modulation frequencies far higher than the frequency at which the human eye blinks. This is achieved by switching LEDs on and off at a higher rate making flicker unrecognizable to the human eye [ 98 , 99 ].
The model is illustrated in Figure Moreover, it preserved all the chapters, scenes, audio tracks and subtitles in Blu-ray disc for your better enjoyment. Choose the right version for your operating system to download and then install VideoSolo Blu-ray Player on your computer. It will take seconds to finish this process. Then, launch it. Connect the optical drive with your computer and then insert the Blu-ray disc you want to play back. Then click "Open Disc" on the main interface.
The program will detect the Blu-ray disc automatically. You need to be connected to the Internet so that the software can decode the copy-protection used on the commercial Blu-ray disc. After loading, you can see the cover of your Blu-ray movie. Here you can choose the title, chapter, audio track, or subtitle track.
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