Emittance reflectance

The inclusion of radiative heat transfer effects can be accommodated by the stagnant layer model. However, this can only be done if a priori we can prescribe or calculate these effects. The complications of radiative heat transfer in flames is illustrated in Figure 9.12. This illustration is only schematic and does not represent the spectral and continuum effects fully. A more complete overview on radiative heat transfer in flame can be found in Tien, Lee and Stretton [12]. In Figure 9.12, the heat fluxes are presented as incident (to a sensor at T,, ) and absorbed (at TV) at the surface. Any attempt to discriminate further for the radiant heating would prove tedious and pedantic. It should be clear from heat transfer principles that we have effects of surface and gas phase radiative emittance, reflectance, absorptance and transmittance. These are complicated by the spectral character of the radiation, the soot and combustion product temperature and concentration distributions, and the decomposition of the surface. Reasonable approximations that serve to simplify are ... 

Procedures. The light reflectance instrument was turned on 30 min prior to initiating reflectance observations. The sensitivity switch was set in the low position. The combination visible light emitter-reflectance detector was positioned vertically the active end of the detector faced upward. The sample cup was a glass cylindrical cuvette with optically flat bottom. A constant mass of 165 g (brass slug) was placed on top... 

An extensive compilation of thermophysical data has been published by Plenum Press, Touloukian (1970-77). This multiple-volume work covers conductivity, specific heat, thermal expansion, viscosity and radiative properties (emittance, reflectance, absorptance and transmittance). 

A direct reflective photosensor has the emitter and receiver parallel to each other in a common housing (Fig. 7). Light from the emitter reflects off the target and back to the receiver to actuate the sensor. Types of direct reflective photosensors include optical proximity and diffuse reflective sensors. Direct reflective photo-... 

The emissivity, S, is the ratio of the radiant emittance of a body to that of a blackbody at the same temperature. Kirchhoff s law requires that a = e for aH bodies at thermal equHibrium. For a blackbody, a = e = 1. Near room temperature, most clean metals have emissivities below 0.1, and most nonmetals have emissivities above 0.9. This description is of the spectraHy integrated (or total) absorptivity, reflectivity, transmissivity, and emissivity. These terms can also be defined as spectral properties, functions of wavelength or wavenumber, and the relations hold for the spectral properties as weH (71,74—76). 

The emissivity of the surfaces. Dull matte surfaces are good emitters/receivers bright reflective surfaces are poor ones. 

Thus, we have two units of measurement of intensity. One is related to scattering from a surface, L, i.e.- in foot-lamberts and the other is related to emittance, H, i.e.- in lumens per square foot. Although we have assumed "white" light up to now, either of these two can be wavelength dependent. If either is wavelength dependent, then we have a pigment (reflective- but more properly called scattering) with intensity in foot-lamberts, or an emitter such as a lamp or phosphor (emittance) with intensity in lumens. 

If we have a certain color, a change in intensity has a major effect on what we see (in both reflectance and emittance). For example, if we have a blue, at low intensity we see a bluish-black, while at high intensity we see a bluish-white. Yet, the hue has not changed, only the intensity. This effect is particularly significant in reflectance since we can have a "light-blue" and a "dark-blue", without a change in chromaticity coordinates. 

Strictly, a black body is defined as something that absorbs photons of all energies, and does not reflect light. Furthermore, a black body is also a perfect emitter of light. A black body is a theoretical object since, in practice, nothing behaves as a perfect black body. The best approximations are hot objects such as red- or white-hot metals. 

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