Surface reflectance. Reflection coefficient (in radio engineering) Dependence of reflection coefficient on surface color
Light when colliding with reflective surface.
It lies in the fact that falling, And reflected Ray placed in a single plane with a perpendicular to the surface, and this perpendicular divides the angle between the indicated rays into equal components.
More often it is simplistically formulated as follows: corner falls and angle light reflections the same:
α = β.
The law of reflection is based on features wave optics. It was experimentally substantiated by Euclid in the 3rd century BC. It can be considered a consequence of using Fermat's principle for mirror surface. Also, these laws can be formulated as a consequence of Huygens’ principle, according to which every point in the medium to which a disturbance has reached acts as a source secondary waves.
Any environment specifically reflects and absorbs light radiation. The parameter describing the reflectivity of the surface of a substance is denoted as reflection coefficient(ρ orR) . Quantitatively, the reflection coefficient is equal to the ratio radiation flux, reflected by the body, to the flow hitting the body:
The light is completely reflected from a thin film of silver or liquid mercury deposited on a sheet of glass.
Highlight diffuse And mirror reflection.
The distribution of currents and voltages in a long line is determined not only by the wave parameters, which characterize the line’s own properties and do not depend on the properties of the circuit sections external to the line, but also by the line reflection coefficient, which depends on the degree of matching of the line with the load.
Complex reflectance of a long line is the ratio of the complex effective values of voltages or currents of reflected and incident waves in an arbitrary section of the line:
For determining p(x) it is necessary to find constant integrations A And A 2, which can be expressed in terms of currents and voltages at the beginning (x = 0) or end (x =/) lines. Let at the end of the line (see Fig. 8.1) the line voltage
and 2 = u(l y t) = u(x, t) x =i, and its current i 2 = /(/, t) = i(x, t) x =[. Denoting the complex effective values of these quantities through U 2 = 0(1) = U(x) x =i = and 2 and /2 = /(/) = I(x) x= i = i 2 and putting in expressions (8.10), (8.11 ) x = I, we get
Substituting formulas (8.31) into relations (8.30), we express the reflection coefficient in terms of current and voltage at the end of the line:
Where x" = I - x - distance measured from the end of the line; p 2 = p(x)|, =/ = 0 neg (x)/0 pal (x) x =1 = 02 - Zj 2)/(U 2 + Zj 2) - reflection coefficient at the end of the line, the value of which is determined only by the relationship between the load resistance Z u = U 2 /i 2 and characteristic impedance of line Z B:
Like any complex number, the line reflectance can be represented in exponential form:
Analyzing expression (8.32), we establish that the modulus of the reflection coefficient
gradually increases with growth X and reaches its greatest value p max(x)= |р 2 | at the end of the line.
Expressing the reflection coefficient at the beginning of the line p ^ through the reflection coefficient at the end of the line p 2
we find that the modulus of the reflection coefficient at the beginning of the line is e 2a1 times less than the modulus of the reflection coefficient at its end. From expressions (8.34), (8.35) it follows that the modulus of the reflection coefficient of a homogeneous line without loss has the same value in all sections of the line.
Using formulas (8.31), (8.33), voltage and current in an arbitrary section of the line can be expressed in terms of voltage or current and the reflection coefficient at the end of the line:
Expressions (8.36) and (8.37) allow us to consider the distribution of voltages and currents in a homogeneous long line in some characteristic modes of its operation.
Traveling wave mode. Traveling wave mode is called the operating mode of a homogeneous line in which only the incident voltage and current wave propagates in it, i.e. the voltage and current amplitudes of the reflected wave in all sections of the line are equal to zero. It is obvious that in the traveling wave mode the reflection coefficient of the line p(r) = 0. From expression (8.32) it follows that the reflection coefficient p(.r) can be equal to zero either in a line of infinite length (at 1=oo the incident wave cannot reach the end of the line and be reflected from it), or in a line of finite length, the load resistance of which is chosen in such a way that the reflection coefficient at the end of the line p 2 = 0. Of these cases, only the second is of practical interest, for the implementation of which, as follows from expression (8.33), it is necessary that the line load resistance be equal to the characteristic impedance Z lt (such a load is called agreed upon).
Assuming p 2 = 0 in expressions (8.36), (8.37), we express the complex effective values of voltage and current in an arbitrary section of the line in the traveling wave mode through the complex effective values of voltage 0 2
and current / 2 at the end of the line: 
Using expression (8.38), we find the complex effective values of voltage and current at the beginning of the line:
Substituting equality (8.39) into relations (8.38), we express the voltage and current in an arbitrary section of the line in the traveling wave mode through the voltage and current at the beginning of the line:
Let us represent the voltage and current at the beginning of the line in exponential form: Ui = G/ 1 e;h D = Let's move from complex effective values of voltage and current to instantaneous ones:
As follows from expressions (8.41), in running mode, the amplitudes of voltage and current in a line with losses(a > 0) decrease exponentially with increasing x, and in a line without loss(a = 0) retain the same value in all sections of the line(Fig. 8.3).
The initial phases of voltage y (/) - р.г and current v|/ (| - р.г in the traveling wave mode change along the line according to a linear law, and the phase shift between voltage and current in all sections of the line has the same value i|/ M - y,y
The input impedance of the line in the traveling wave mode is equal to the characteristic impedance of the line and does not depend on its length:
In a lossless line, the wave impedance is purely resistive in nature (8.28), therefore, in the traveling wave mode, the phase shift between voltage and current in all sections of the line without loss is zero(y;
Instantaneous power consumed by a lossless line section located to the right of an arbitrary section X(see Fig. 8.1), equal to the product of the instantaneous values of voltage and current in the cross section X.
Rice. 83.
From expression (8.42) it follows that the instantaneous power consumed by an arbitrary section of the line without losses in the traveling wave mode cannot be negative, therefore, In the running mode, energy is transferred in the line in only one direction - from the energy source to the load.
There is no energy exchange between the source and the load in the traveling wave mode and all the energy transferred by the incident wave is consumed by the load.
Standing wave mode. If the load impedance of the line in question is not equal to the characteristic impedance, then only part of the energy transferred by the incident wave to the end of the line is consumed by the load. The remaining energy is reflected from the load and returns to the source as a reflected wave. If the modulus of the line reflection coefficient |p(.r)| = 1, i.e. amplitudes of the reflected and incident waves in all sections of the line are the same, then a specific regime is established in the line, called standing wave regime. According to expression (8.34), the modulus of the reflection coefficient | r(lg)| = 1 only if the modulus of the reflection coefficient at the end of the line |p 2 | = 1, and the line attenuation coefficient a = 0. Analyzing expression (8.33), we can verify that |p 2 | = 1 only in three cases: when the load resistance is either zero or infinity, or is purely reactive.
Hence, standing wave mode can only be established in a line without losses due to a short circuit or open circuit at the output, and, if the load resistance at the line output is purely reactive.
If there is a short circuit at the output of the line, the reflection coefficient at the end of the line is p 2 = -1. In this case, the voltages of the incident and reflected waves at the end of the line have the same amplitudes, but are shifted in phase by 180°, so the instantaneous value of the voltage at the output is identically equal to zero. Substituting p 2 = - 1, y = ur, Z B = /?„ into expressions (8.36), (8.37), we find the complex effective values of the line voltage and current:
Assuming that the initial phase of the current /? at the line output is zero, and moving from complex effective values of voltages and currents to instantaneous
We establish that during a short circuit at the output of the line, the amplitudes of voltage and current change along the line according to a periodic law
taking maximum values at individual points of the line Um check = V2 I m max = V2 /2 and vanishing at some other points (Fig. 8.4).
It is obvious that at those points of the line at which the amplitude of the voltage (current) is equal to zero, the instantaneous values of the voltage (current) are identically equal to zero. Such points are called voltage (current) nodes.
The characteristic points at which the voltage (current) amplitude takes its maximum value are called voltage (current) antinodes. As is obvious from Fig. 8.4, voltage nodes correspond to current antinodes and, conversely, current nodes correspond to voltage antinodes.
Rice. 8.4. Voltage amplitude distribution(A) and current(b) along the line in short circuit mode
Rice. 8.5. Distribution of instantaneous voltage values (A) and current (b) along the line in short circuit mode
The distribution of instantaneous voltage and current values along the line (Fig. 8.5) obeys a sinusoidal or cosine law, however, over time, the coordinates of points that have the same phase remain unchanged, i.e. the waves of voltage and current seem to “stand still.” That is why this mode of line operation was called standing wave regime.
The coordinates of the voltage nodes are determined from the condition sin рх/, = 0, from which
Where To= 0, 1,2,..., and the coordinates of the voltage antinodes are from the condition cos р.г" (= 0, whence
Where P = 0, 1,2,...
In practice, it is convenient to count the coordinates of nodes and antinodes from the end of the line in fractions of the wavelength X. Substituting relation (8.21) into expressions (8.43), (8.44), we obtain x"k = kX/ 2, x"„ = (2 n + 1)X/4.
Thus, the nodes of voltage (current) and voltage (current) antinodes alternate with an interval X/4, and the distance between adjacent nodes (or antinodes) is X/2.
Analyzing the expressions for the voltage and current of the incident and reflected waves, it is easy to verify that voltage antinodes arise in those sections of the line in which the voltages of the incident and reflected waves coincide in phase and, therefore, are summed, and the nodes are located in sections where the voltages of the incident and reflected waves the waves are out of phase and therefore subtracted. The instantaneous power consumed by an arbitrary section of the line varies over time according to the harmonic law
therefore, the active power consumed by this section of the line is zero.
Thus, in the standing will mode, energy is not transferred along the line and at each section of the line there is only an exchange of energy between the electric and magnetic fields.
Similarly, we find that in no-load mode (p2 = 1) the distribution of voltage (current) amplitudes along the line without losses (Fig. 8.6)
has the same character as the distribution of current (voltage) amplitudes in short circuit mode (see Fig. 8.4).
Consider a lossless line whose output load resistance is purely reactive:
Rice. 8.6. Voltage amplitude distribution (A) and current (b) along the line at idle
Substituting formula (8.45) into expression (8.33), we obtain
From expression (8.46) it follows that with a purely reactive load, the modulus of the reflection coefficient at the output of the line |p 2 | = 1, and the values of the argument p p2 at finite values x n lie between 0 and ±l.
Using expressions (8.36), (8.37) and (8.46), we find the complex effective values of the line voltage and current:
where φ = arctan(/? B /x„). From expression (8.47) it follows that the amplitudes of voltage and current vary along the line according to a periodic law:
and the coordinates of the voltage nodes (current antinodes) x"k = (2k + 1)7/4 + 1у Where 1 = f7/(2tg); k= 0, 1, 2, 3,..., and the coordinates of the voltage antinodes (current nodes) X"" = PC/2 + 1, Where P = 0, 1,2,3,...
The distribution of voltage and current amplitudes with a purely reactive load generally has the same character as in idle or short circuit modes at the output (Fig. 8.7), and all nodes and all antinodes are shifted by the amount 1 L so that at the end of the line there is neither a node nor an antinode of current or voltage.
With capacitive load -k/A 0, so the first voltage node will be located at a distance less k/A from the end of the line (Fig. 8.7, A); with inductive load 0 t k/A the first node will be located at a distance greater than 7/4, but less To/2 from the end of the line (Fig. 8.7, b).
Mixed wave mode. The traveling and standing wave regimes represent two limiting cases, in one of which the amplitude of the reflected wave in all sections of the line is equal to zero, and in the other, the amplitudes of the incident and reflected waves in all sections of the line are the same. In os-
Rice. 8.7. Distribution of voltage amplitudes along a line with capacitive(A) and inductive
In other cases, a mixed wave regime occurs in the line, which can be considered as a superposition of the traveling and standing wave regimes. In the mixed wave mode, the energy transmitted by the incident wave to the end of the line is partially absorbed by the load and partially reflected from it, so the amplitude of the reflected wave is greater than zero, but less than the amplitude of the incident wave.
As in the standing wave mode, the distribution of voltage and current amplitudes in the mixed wave mode (Fig. 8.8)
Rice. 8.8. Voltage amplitude distribution (A ) and current(b) along the line in mixed wave mode with a purely resistive load(R„ > RH)
has clearly defined maxima and minima, repeating through X/2. However, the amplitudes of current and voltage at minima are not zero.
The less energy is reflected from the load, i.e. the higher the degree of line matching with the load, the less pronounced the maximum and minimum voltage and current are, therefore, the ratios between the minimum and maximum values of the voltage and current amplitudes can be used to assess the degree of line matching with the load. The value equal to the ratio of the minimum and maximum values of the voltage or current amplitude is called traveling wave coefficient(KBV)
The BPV can vary from 0 to 1, and, The more K()U, the closer the line’s operating mode is to the running will mode.
It is obvious that at points on the line at which the voltage (current) amplitude reaches its maximum value, the voltages (currents) of the incident and reflected waves are in phase, and where the voltage (current) amplitude has a minimum value, the voltages (currents) of the incident and reflected waves the waves are in antiphase. Hence,
Substituting expression (8.49) into relations (8.48) and taking into account that the ratio of the voltage amplitude of the reflected wave to the voltage amplitude of the incident wave is the modulus of the line reflection coefficient | p(lr)|, we establish a connection between the traveling wave coefficient and the reflection coefficient:
In a lossless line, the modulus of the reflection coefficient in any section of the line is equal to the modulus of the reflection coefficient at the end of the line, therefore the traveling wave coefficient in all sections of the line has the same value: Ks>=
= (1-ыУО+ы).
In a line with losses, the modulus of the reflection coefficient changes along the line, reaching its greatest value at the reflection point (at X= /). In this regard, in a line with losses, the coefficient of the traveling wave changes along the line, taking on a minimum value at its end.
Along with the KBV, to assess the degree of coordination of the line with the load, its reciprocal quantity is widely used - standing wave ratio(SWR):
In traveling wave mode K c = 1, and in the standing wave mode K c-? oo.
Low-emissivity coating: A coating, when applied to glass, the thermal characteristics of the glass are significantly improved (the heat transfer resistance of glazing using glass with a low-emissivity coating increases, and the heat transfer coefficient decreases).
Sun protection coating
Solar control coating: A coating that, when applied to glass, improves the protection of a room from the penetration of excess solar radiation.
Emission factor
Emissivity (corrected emissivity): The ratio of the emissive power of a glass surface to the emissive power of a black body.
Normal emission factor
Normal emissivity (normal emissivity): The ability of glass to reflect normally incident radiation; is calculated as the difference between unity and the reflectance in the direction normal to the glass surface.
Solar factor
Solar factor (total solar energy transmittance coefficient): The ratio of the total solar energy entering the room through a translucent structure to the energy of incident solar radiation. The total solar energy entering the room through a translucent structure is the sum of the energy directly passing through the translucent structure and that part of the energy absorbed by the translucent structure that is transferred into the room.
Directional light transmittance
The coefficient of directional light transmission (equivalent terms: light transmittance, light transmission coefficient), is denoted as τv (LT) - the ratio of the value of the light flux normally passing through the sample to the value of the light flux normally incident on the sample (in the wavelength range of visible light) .
Light reflectance
Light reflection coefficient (equivalent term: coefficient of normal light reflection, light reflectance coefficient) is denoted as ρv (LR) - the ratio of the value of the luminous flux normally reflected from the sample to the value of the luminous flux normally incident on the sample (in the wavelength range of visible light).
Light absorption coefficient
The light absorption coefficient (equivalent term: light absorption coefficient) is denoted as av (LA) - the ratio of the value of the light flux absorbed by the sample to the value of the light flux normally incident on the sample (in the wavelength range of the visible spectrum).
Solar transmittance
The solar energy transmittance coefficient (equivalent term: direct solar energy transmittance coefficient) is denoted as τе (DET) - the ratio of the value of the solar radiation flux normally passing through the sample to the value of the solar radiation flux normally incident on the sample.
Solar reflectance
The solar energy reflectance coefficient is denoted as ρе (ER) - the ratio of the value of the solar radiation flux normally reflected from the sample to the value of the solar radiation flux normally incident on the sample.
Solar absorption coefficient
The solar energy absorption coefficient (equivalent term: energy absorption coefficient) is denoted as ae (EA) - the ratio of the value of the solar radiation flux absorbed by the sample to the value of the solar radiation flux normally incident on the sample.
Shading coefficient
The shading coefficient is designated as SC or G - the shading coefficient is defined as the ratio of the flux of solar radiation passing through a given glass in the wave range from 300 to 2500 nm (2.5 microns) to the flux of solar energy passing through glass 3 mm thick. The shading coefficient shows the fraction of the passage of not only the direct flow of solar energy (near infrared radiation), but also the radiation due to energy absorbed in the glass (far infrared radiation).
Heat transfer coefficient
Heat transfer coefficient - denoted as U, characterizes the amount of heat in watts (W) that passes through 1 m2 of structure with a temperature difference on both sides of one degree on the Kelvin scale (K), unit of measurement W/(m2 K).
Heat transfer resistance
Heat transfer resistance is designated as R - the reciprocal of the heat transfer coefficient.
Transmittance
reflection coefficient
And absorption coefficient
The coefficients t, r and a depend on the properties of the body itself and the wavelength of the incident radiation. Spectral dependence, i.e. the dependence of the coefficients on the wavelength determines the color of both transparent and opaque (t = 0) bodies.
According to the law of conservation of energy
F neg + F absorb + F pr = . (8)
Dividing both sides of the equality by , we get:
r + a +t = 1. (9)
A body for which r=0, t=0, a=1 is called absolutely black .
A completely black body at any temperature completely absorbs all the energy of radiation of any wavelength incident on it. All real bodies are not completely black. However, some of them in certain wavelength intervals are close in their properties to an absolutely black body. For example, in the wavelength region of visible light, the absorption coefficients of soot, platinum black and black velvet differ little from unity. The most perfect model of an absolutely black body can be a small hole in a closed cavity. Obviously, this model is closer in characteristics to a black body, the greater the ratio of the surface area of the cavity to the area of the hole (Fig. 1).
The spectral characteristic of absorption of electromagnetic waves by a body is spectral absorption coefficient a l is a quantity determined by the ratio of the radiation flux absorbed by the body in a small spectral range (from l to l + d l) to the flux of radiation incident on it in the same spectral range:
.
(10)
The emissive and absorptive abilities of an opaque body are interrelated. The ratio of the spectral density of the energy luminosity of the equilibrium radiation of a body to its spectral absorption coefficient does not depend on the nature of the body; for all bodies it is a universal function of wavelength and temperature ( Kirchhoff's law ):
. (11)
For an absolutely black body a l = 1. Therefore, from Kirchhoff’s law it follows that M e, l = , i.e. The universal Kirchhoff function represents the spectral density of the energy luminosity of an absolutely black body.
Thus, according to Kirchhoff’s law, for all bodies the ratio of the spectral density of energy luminosity to the spectral absorption coefficient is equal to the spectral density of energy luminosity of an absolutely black body at the same values T and l.
It follows from Kirchhoff’s law that the spectral density of the energy luminosity of any body in any region of the spectrum is always less than the spectral density of the energy luminosity of an absolutely black body (at the same values of wavelength and temperature). In addition, it follows from this law that if a body at a certain temperature does not absorb electromagnetic waves in the range from l to l + d l, then it does not emit them in this length range at a given temperature.
Analytical form of the function for an absolutely black body
was established by Planck on the basis of quantum concepts about the nature of radiation:
(12)
The emission spectrum of a completely black body has a characteristic maximum (Fig. 2), which shifts to the shorter wavelength region with increasing temperature (Fig. 3). The position of the maximum spectral density of energy luminosity can be determined from expression (12) in the usual way, by equating the first derivative to zero:
. (13)
Denoting , we get:
X – 5 ( – 1) = 0. (14)
Rice. 2 Fig. 3
Solving this transcendental equation numerically gives
X = 4, 965.
Hence,
, (15)
= = b 1 = 2.898 m K, (16)
Thus, the function reaches a maximum at a wavelength inversely proportional to the thermodynamic temperature of a black body ( Wien's first law ).
From Wien's law it follows that at low temperatures predominantly long (infrared) electromagnetic waves are emitted. As the temperature increases, the proportion of radiation in the visible region of the spectrum increases, and the body begins to glow. With a further increase in temperature, the brightness of its glow increases and the color changes. Therefore, the color of the radiation can serve as a characteristic of the temperature of the radiation. The approximate dependence of the color of a body’s glow on its temperature is given in Table. 1.
Table 1
Wien's first law is also called displacement law , thereby emphasizing that with increasing temperature the maximum spectral density of energetic luminosity shifts towards shorter wavelengths.
Substituting formula (17) into expression (12), it is easy to show that the maximum value of the function is proportional to the fifth power of the thermodynamic body temperature ( Wien's second law ):
The energetic luminosity of an absolutely black body can be found from expression (12) by simple integration over the wavelength
(18)
where is the reduced Planck constant,
The energetic luminosity of an absolutely black body is proportional to the fourth power of its thermodynamic temperature. This provision is called Stefan–Boltzmann law , and proportionality coefficient s = 5.67×10 -8 – Stefan–Boltzmann constant.
A completely black body is an idealization of real bodies. Real bodies emit radiation whose spectrum is not described by Planck's formula. Their energetic luminosity, in addition to temperature, depends on the nature of the body and the state of its surface. These factors can be taken into account if a coefficient is introduced into formula (19), showing how many times the energy luminosity of an absolutely black body at a given temperature is greater than the energy luminosity of a real body at the same temperature
from where , or (21)
For all real bodies<1 и зависит как от природы тела и состояния его поверхности, так и от температуры. В частности, для вольфрамовых нитей электроламп накаливания зависимость от T has the form shown in Fig. 4.
The measurement of radiation energy and temperature of an electric furnace is based on Seebeck effect, which consists in the occurrence of an electromotive force in an electrical circuit consisting of several dissimilar conductors, the contacts of which have different temperatures.
Two dissimilar conductors form thermocouple , and series-connected thermocouples are a thermocouple. If the contacts (usually junctions) of the conductors are at different temperatures, then in a closed circuit including thermocouples, a thermoEMF arises, the magnitude of which is uniquely determined by the temperature difference between the hot and cold contacts, the number of thermocouples connected in series and the nature of the conductor materials.
The magnitude of thermoEMF arising in the circuit due to the energy of radiation incident on the junctions of the thermal column is measured by a millivoltmeter placed on the front panel of the measuring device. The scale of this device is graduated in millivolts.
The temperature of a blackbody (furnace) is measured using a thermoelectric thermometer consisting of a single thermocouple. Its EMF is measured by a millivoltmeter, also located on the front panel of the measuring device and calibrated in °C.
Note. The millivoltmeter records the temperature difference between the hot and cold junctions of the thermocouple, so to obtain the furnace temperature, you need to add the room temperature to the reading of the device.
In this work, the thermoEMF of a thermocouple is measured, the value of which is proportional to the energy spent on heating one of the contacts of each thermocouple of the column, and, consequently, the energy luminosity (at equal time intervals between measurements and a constant emitter area):
Where b– proportionality coefficient.
Equating the right-hand sides of equalities (19) and (22), we obtain:
s× T 4 =b×e,
Where With– constant value.
Simultaneously with measuring the thermoEMF of the thermocolumn, the temperature difference Δ is measured t hot and cold junctions of a thermocouple placed in an electric furnace and determine the temperature of the furnace.
Using experimentally obtained values of the temperature of a completely black body (furnace) and the corresponding thermoEMF values of the thermocolumn, determine the value of the coefficient proportional to
sti With, which should be the same in all experiments. Then plot the dependence c= f(T), which should look like a straight line parallel to the temperature axis.
Thus, in laboratory work the nature of the dependence of the energetic luminosity of an absolutely black body on its temperature is established, i.e. The Stefan–Boltzmann law is verified.
