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Basics of Radiation Power Transfer

When an object (more correctly a surface) is at a temperature above 0K (Zero Kelvin) it emits radiation, called electromagnetic (em) radiation.  This radiation can comprise of many frequencies (i.e comprises of a spectrum of wavelengths) which is described by Planck's Law described further below. The radiation can also be thought of comprising of massless photons of a spectrum of energies emitted per second. 

The maximum rate of radiation emitted (Power) by a surface with an area A, integrated over all wavelengths of the radiation, is given by the Stefan-Boltzmann Law which is written as: Radiated Power Intensity, Pe (W) = esATe4  where e is the emissivity, Te is the Temperature is in Kelvin of the emitting surface and s is the Stefan-Boltzmann Constant = 5.6703 x 10-8 W/m2K4Emissivity (e) is a relative complex variable that encompasses several Laws of Physics (e ranges from 0-1) for materials.  The emissivity depends on temperature, wavelength and the angle of emission. It is the ratio of power radiated by a real material to power radiated by a black-body at the same temperature. For a black-body e = 1.  What is the total power radiated by a surface (across the spectrum of radiation that is emitted) ?  The plot of radiation power per unit area is shown below, along with a table that s how steeply the total radiation power increases with the surface temperature of the emission surface.

File:Stefan Boltzmann 001.svg

Centigrade

kW/m2

250C

~2.5

500C

~20

1000C

~10000

Approximate Radiation Power and Surface Temperature of the Emitting Surface.

When radiation encounters an object, it may be absorbed, reflected and refracted/transmitted.  For heat (infra red radiation) the transmission and refraction may only be through a few atomic layers where the energy is "absorbed".  One may ignore transmission through more than a few atomic layers for thermal (heat) radiation (i,e.  for a predominantly infrared IR spectrum comprising only of long wavelengths 1-100 micron) for most solid bodies as they are are opaque to this radiation. An important law called Kirchhoff's law states that for a material in thermal equilibrium the emissivity el, at any given wavelength and the absorptivity al, at the same wavelength are equal (at a fixed surface temperature).  A black-body is a perfect absorber as well as a perfect emitter.  An object is called a black body if the following formula holds at all frequencies : a=e=1. What is commonly known as heat radiation lies in the Infrared part (the longer than light wavelengths).   For solving engineering problems related to heating by radiation, it is a common assumption to consider the emissivity as independent of wavelength. Such an approximation is called the grey-body approximation.  Grey-body approximations are commonly made across the infra-red (thermal radiation) spectrum.   The net power transfer P, is described below in terms of the areas and temperatures of the absorbing and emitting surfaces.  The subscripts a and e stand for absorbing and emitting surface of the radiation.  For a grey-body in thermal equilibrium, Pe and Pa are equal and e = aThe sun is a black body, e=1 because it is very hot and dense at its core.  Although it is in equilibrium for all purposes (because of intense internal heating), it could have temperature variations albiet slowly changing when measured over the surface.  Most materials that are found on earth are at a lower temperature correspondingly with an absorptivity and emissivity below 1.  Note from the table above that radiation power is magnified with an increse in the temperature i.e. the radiative power follows a power law that is T4.  The net power transfer by radiation is given by:

P= [(aa)(ee)sATe4 - (ea)sATa4].  

The spectrum of radiation in the electromagnetic spectrum, is described in terms of its constituent frequency or wavelength of the radiation.   The total radiation emitted by a surface at a given temperature encomposes several wavelenghths.  The temperature (T), and peak wavelength (λ) and the frequency (v) relationship is shown below.   Note that the visible spectrum region is only a small portion of the entire em spectrum.   Radiation from a body comprises of multiple frequencies that may be considered to sum to a form of a resultant wave.   What is commonly known as heat radiation lies in the Infrared part (longer than light wavelengths).  Ultaviolet and X-Ray radiation are much higher frequency radiation (shorter wavelengths).  These are the high energy radiation and penetrate matter more than infrared radiation.   ote that the radiative power follows a power law that is T4.   Free radiating elements like Microheaters™ and Magnacoils™ are designed for maximizing the heat power output,   Furnaces on the other hand are optimized and designed for maximum temperature and low power loss (insulated).  When the transfer mechanism is convective the best power transfer is achieved by Airtorch.  When an extremely rapid power transfer is required, a new breakthrough device called the the Cascade e-ion may be employed.

Radiation can also be considered as comprising of photons with a spectrum of energies (this is the wave and particle duality).  A photon is a fundamental particle which represents a quantum of electromagnetic radiation. The energy of an individual photon is higher for photons of a higher frequency.   It is only particles bound in potentials that get quantized energies, thus the photons that mediate transitions between these quantized energy states are quantized.  This explains why even though the Planck radiation equation is expressed as a continuous variable of wavelength, one notes that the emission and absorption spectra of atoms only yield discrete line-signatures. 

The relationship between the energy and frequency is given by Planck's energy law for a Photon equation E = , where E is the energy per photon, ν is the frequency of the photon, and h is Planck's constant equal to 6.626×10−34 J.s.  When considering the wave-particle duality, the frequency of a wave is its rate of oscillation and is measured in Hertz, the SI unit of frequency, where one Hertz is equal to one oscillation per second.  A photon carries energy that is proportional to the frequency of the radiation (or inversely with wavelength as the velocity is constant).   Photons belong to a  class of fundamental particles called bosons. Photons are the force carriers of the electromagnetic field.   Photons are not conserved.  Photons have a zero rest mass.  The number of photons per second is related inversely to the electromagnetic or light frequency.   Photons/second =Pλ/hc = P/hν where P is the power in Watts and c is the speed of light, h is Planck's constant, and ν the frequency.  The energy of a photon is given in electron volts.  Red light photons are about 1.7 eV and blue light photons about 3.2eV (alternately we could have said that the red ligh wavelength is about 0.8 micron and blue light is 0.3 microns).  Note that  1eV=1.60218e-19 Joules. Note that the energy of a photon in eV (electron volts) is equal to 1.239/ (Wavelength in microns). 

The radiation power density as a function of wavelength and temperature is given by the Planck's Radiation Law for Black Bodies (shown in the plot below).   S represents Radiation Power Density, W/m3 or Spectral Irradiance (per steradian obtained from Planck's  Radiation Law),  a function of the wavelenght.  The units for the three graphs below should not to be confused with the units of P.   The Planck's constant h= 6.626×10−34 J.s.

Several plots of the radiated power density as a function of wavelength are shown below for different temperatures. Note that the power increases with the temperature for any given wavelength. A peak is also noted in the plot for any given temperature (with a λmax).  This peak shifts to a lower wavelength (higher frequency) with an increase in the temperature. The peak wavelength and temperature are related by the Wein's radiation constant.  The mathematical relationship is λmax. T = W, where W is a constant where the λmax is the peak wavelength, and T is the absolute temperature.  The radiation constant. W  is the Wien's displacement constant equal to 2.8985×10−3 m·K.  The Planck equation (shown alongside the plots below) can be integrated over all wavelengths to yield the Stefan-Boltzmann law discussed above.

Stefan Boltzmann Law    

In the visible region the range of photon energy with color is shown below.

Energy of photon from near IR to UV.  Note energy is inversely proportional to wavelength

These graphs are reproduced from the open source "Physics".  

Sun/Greenhouse effect:  The sun is a black-body.  The greenhouse effect pertains to the earth receiving radiative energy with the distribution of wavelengths in the solar spectrum (from the sun which has a surface temperature of ~5778K), but then emitting and reflecting a spectrum associated with the much lower temperature of earth (~ 300K).  The emission is mostly in the IR (infrared wavelengths).  Molecules absorb characteristic wavelengths.  Some of the emitted and reflected (polarized) radiation from earth falls in the absorption band-characteristics of CO2, water vapor, fluorocarbon, and other gasses in air, thus trapping a considerable part of the emitted and reflected IR radiation (heat). The interaction whether elastic type or inelastic type influences reflection and radiation. The sun may be considered to be a black-body (e=1) while the earth may be considered to be a grey-body (with e=0.8) for making of any approximate calculations pertaining to the energy distribution. Note that only a grey body can reflect. It should be noted that the emissivity of the integrated emmisivity of an object is a function of temperature.  For approximate energy calculations, it may inferred that the Sun radiates 6.3 x 10^7 W/m2.  The earth (over the atmospheric area of the planet radius) receives ~1.3 x 10^3 W/m2 sunlight and radiates (from its actual surface) ~(250-350) W/m2, which is considerably absorbed by gasses in the atmosphere. The units of P in the equations above for power transfer between objects at two temperatures are W or W/m2, from the Stefan-Boltzmann law.  The units of power density above S are W/m3 or W/m3 per steradian (y axis of the the graphs above) i.e. from Planck's Radiation Law formulation above.  The conversion between P and S at equilibrium involves the Wein's radiation constant or integration over the spectrum (all wavelengths) as described by Planck's Law for the Energy of a Photon namely that E = , where E is the energy per photon, ν is the frequency of the photon, and h is Planck's constant equal to 6.626×10−34 J.s. 

This yields λmax = hc/zkTwhere z = 4.96511 (integration constant), which is Wein's law.   Here h is the Planck's constant and k is the Boltzmann constant. Note also that pressure has units of J/m^3 and so this radiation law also describes radiation pressure as a function of temperature and length scale, i.e., can thus be thought to be analogous to an 'ideal' equation of state for radiation.  

Shown below is the color spectrum of visible radiation and the energy associated with a particular wavelength of light. The solar spectrum credit is to https://en.wikipedia.org/wiki/File:Solar_Spectrum.png.  Note that absorption spectra for O2, O3 and H2O are captured in the center graph but it does not call out methane absorption which is minor constituent (because of low atmospheric concentration, 20-40 ppm level) but important when methane gas leaks are encountered (methane and other such gas lines (bands) absorb in the 1.62, 3.2 and 8.8-9.2 micron range).  Methane is a much smaller component than carbon dioxide but a more potent driver of the greenhouse effect.  Natural gas, has considerable methane content as do biogenic sources.

 Solar spectrum.

The units of P in the equations above for power transfer between objects at two temperatures are W or W/m2, from the Stefan-Boltzmann law.  The units of power density above S are W/m3 per steridian (y axis of the the two graphs above) i.e. from Planck's Law.  The conversion between P and S at equilibrium involves the Wein's radiation constant or integration over the spectrum (all wavelengths) as described by Plancks Law.

IR Spectrum Devices

Somewhere in the range of 600C to 800C, the blackbody spectrum encroaches enough in the the visible to be seen as a dull red glow.  At temperatures in the 50oC range (body to normal room temperatures) almost all of the radiated energy from the body or wall is in the infrared part of the spectrum. The wavelengths predominantly in this part of the spectrum is of the order of (1000nm = 1 micron) and above. The lower the temperature, the peak shifts to higher wavelength and as shown above could be mostly off the visible scale below 600oC.  Thus the human body mostly loses radiation (heat) by emitting in the infra-red (IR) region, invisible to the eye.   A typical human body controls its body temperature with a natural automatic regulation process and attempts to keep t around 37°C.   An average human-body emits about 100W of radiation at 37oC.  This number increases with vigorous exercise.   Detecting human body movement requires sensors that are particularly sensitive in the IR regions.   Night vision goggle have this type of feature that accentuates IR and converts it to a visible frequency which the eye can detect.  Almost all surface heating involves IR. Each quanta of IR has a lower frequency compared to UV.  As the frequency increases towards UV radiation the waves penetrate more as they have more energy.  The energy of a photon depends on the wavelength.  This is why UV shades are recommended  because UV has a shorter wavelength than IR.  Note that the radiation power of a surface increases with temperature and with the emissivity in a non-linear manner.  Note also the general trend of emissivity of a materials is to increase as a function of temperature.  However emissivity and reflectance are not simple properties.  Radiation is absorbed via mechanisms that involve allowed quantum states.  The dielectric constant has a real and imaginary component and directly impacts reflectance. Therefore oddities are noted in reflectance measurements e.g. reflectance for a shiny metal can often decreases with peak temperature or increase because of plasmonic influence.  Emissivity is associated with high temperature measurements (>300K), reflectance with low (<300K) temperature measurements.

Visible Spectrum Devices

Solar and UV panels can be of two types (1) those that absorb all radiation (black body) in the solar spectrum, generally used for heating  and (2) that selectively absorb in order to stimulate electron activity in semiconductors.  These are classified as visible spectrum devices.  The cascade e-ion devices may be used to produce a combination of the two types,

The cascade e-ion is a device that is able to make oxynitride, carbonitride, nitride and carbide surfaces/coatings of transition metals.Surfaces can be manipulated to absorb some wavelengths and reflect the others. Such surfaces made by Cascade e-ion or D-e-ion devices also often impart antimicrobial propertes.   MHI's patented high emissivity and controlled boson, fermion, photon stimulators are the key to the next generation of power producing surfaces.  See also duburr page for Golden/Blue surfaces/ microstructures for iron, zirconium and titanium alloys.   As noted above, Kirchoffs law applies, however the amount transmitted or reflected at various wavelengths can be manipulated with transparent coatings e.g. of glass or gold plating or the more recently rediscovered copper-tin mirror materials.  Variations is the surfaces made by the cascade e-ion are are being discovered e.g. a black body in the visible spectrum range can be covered with a layer (glass) which is trasparent in the visble range but opaque in the UV or IR range. Such layering techniques may be used for example for hot water solar heaters.

How is light measured? What do Lumens and Lux signify?  What is Brilliance?

Brilliance in the context of light, is related to the photons per second that our eye senses.  In the visible spectrum, the units of lumens (amount of light in the visible range) and lux are used to calculate/infer the illumination which normally pertains only to the visible spectrum objectives. Lumens is calculated by knowing the spectrum or number of photons per second in the "light" frequencies and converting it to an absolute number in a slightly round about manner.  The efficiency of the device for the objective of providing illumination is defined by the luminous efficacy η in lumens per watt of the device (lm/W).  The illuminance Ev in lux (lx) is equal to the power P in watts (W), multiplied the luminous efficacy η, divided by the surface area A (m2):  Ev(lx) = P(W) × η(lm/W) / A(m2) .   Note that red is a higher wavelength radiation compared to violet.  In terms of wavelength, the graphics above and below for the energy of photon (shown in eV) would range from 720 microns to 390 microns (left to right).

The power of radiation given in photons per second and the lumens are related to brightness by number pf photon per second in the visible range.  Different light emitters can now be compared - LED source gives 90 lumens per Watt  compared to incandascent which is about 30 lumers per Watt.  Lux as described above is a measure of the flux of lumens per unit area.  Remember we have used the word brightness above a bit loosely as it depends on the eye and other spects of roughness and color i.e on the numner of photons per second that we can observe.

 

The color of a surface?  Thin Film Interference.

The reflection color from an object for sunlight depends on the intensity (including angle), polarization, photonic band gap, refractive index (real and imaginary parts) and other properties. The reflection or refraction of individual wavelength may not be the same for all surfaces or thin films. Color seperation often occurs just like in a prism but for different reasons.  Such separation properties are also utilized for thin film devices. Thin film interference is a phenomena associated with films which determines colors. Thin films have many commercial applications including as partially transparent or anti-reflection coatings, mirrors, and on optical filters.  For graded and compositionally variant films, the seperation colors yield considerable important information about a coating or thin layer.   The thin film interference phenomena partially explains the colors of oxynitrided and nitrideded surfaces, tempered colors, and also the color of the wings of butterflies!  The reflected, partially reflected, or refracted wave (depending on polarization) may interact and thus reveal information about the surface(s) from which individual frequency components are reflected, e.g. information about the thickness of the film, or the effective refractive index of the film medium. 

Energy of photon from near IR to UV.  Note energy is inversely proportional to wavelength
Span of Visible Photon Energy with Color


Color Grating

Various colors of oxynitrides are often noted on a steel surface a shown below.    These are called colors of tempering.   The color varies with the type of steel, time of film formation, phases, atmosphere/thickness and incident light distribution.  Note specular and matte finishes below are both possible for the common variations that are noted on iron and steel surfaces.  

Why do these colors form?  

Thin film interference, absorption coefficients and internal scattering by small-scale phases like Fe2(CN) play a role in the color formation. There is some analogy with the prism light grating shown above, however the closest scientific analogy to the tempering colors of oxides and nitrides is noted in water-droplet residual drying colors on a surrcace, the colorful wings of an attractive butterfly,  the spectacular color seperation obtained from a prism-light grating, the red-shift explanation from moving objects, and the commonly observed variations in the color of the sky and across moonbows and rainbows.  However there are very few clear explanations that comprehensively explain all oxynitride colors.  The colors of titanium, zirconium and alloy oxynitrides vary from red-yellow to blue/violetr indicating also some variations in the band gap of the material that are enabled by changes in the the oxygen/nitrogen ratio. 

What is GoldenBlue® and what does it have to do with solar devices?  Did you know that the Cascade e-ion could possibly create photo catalytic or solar conversion surfaces?   Please contact MHI to learn more about surface quantum dots.

Product Questions? Please Contact Us.

What is Deburr?  How does the Cascade e-ion change the surface roughness?  Does it impact the color of the surface?

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