|
Laws of Radiation
|
|
|
Fig. 1 The spectrum of radiation from the sun is similar to that from a 5800K blackbody.
|
|
Planck's Law
|
This law gives the spectral distribution of radiant energy inside a blackbody.
|
|
Where:
T = Absolute temperature of the blackbody
h = Planck’s constant (6.626 x 10-34 Js)
c = Speed of light (2.998 x 108 m s-1)
k = Boltzmann's constant (1.381 x 10-23 JK-1)
l = Wavelength in m
The spectral radiant exitance from a non perturbing aperture in the blackbody cavity, Mel (l,T), is given by:
|
|
Lel (l,T), the spectral radiance at the aperture is given by:
|
|
The curves in Fig. 3 show MBl plotted for blackbodies at various temperatures. The output increases and the peak shifts to shorter wavelengths as the temperature, T, increases.
|
|
Stefan-Boltzman Law
|
Integrating the spectral radiant exitance over all wavelengths gives:
|
|
s is called the Stefan-Boltzmann constant
This is the Stefan-Boltzmann law relating the total output to temperature.
If Me(T) is in W m-2, and T in kelvins, then s is 5.67 x 10-8 Wm-2 K-4.
At room temperature a 1 mm2 blackbody emits about 0.5 mW into a hemisphere. At 3200 K, the temperature of the hottest tungsten filaments, the 1 mm2, emits 6 W.
|
|
Wien Displacement Law
|
This law relates the wavelength of peak exitance, lm, and blackbody temperature, T:
lmT = 2898 where T is in kelvins and lm is in micrometers.
The peak of the spectral distribution curve is at 9.8 mm for a blackbody at room temperature. As the source temperature gets higher, the wavelength of peak exitance moves towards shorter wavelengths. The temperature of the sun’s surface is around 5800K. The peak of a 6000 blackbody curve is at 0.48 mm, as shown in Fig. 3.
|
|
Emissivity
|
The radiation from real sources is always less than that from a blackbody. Emissivity (e) is a measure of how a real source compares with a blackbody. It is defined as the ratio of the radiant power emitted per area to the radiant power emitted by a blackbody per area. (A more rigorous definition defines directional spectral emissivity e(θ,f,l,T). Emissivity can be wavelength and temperature dependent (Fig. 2). As the emissivity of tungsten is less than 0.4 where a 3200 K blackbody curve peaks, the 1 mm2 tungsten surface at 3200 K will only emit 2.5 W into the hemisphere.
If the emissivity does not vary with wavelength then the source is a “graybody”.
|
Fig. 2 Emissivity (spectral radiation factor) of tungsten.
|
Fig. 3 Spectral exitance for various blackbodies
|
|
Tech Note
|
Sometimes you prefer to have low emissivity over a part of the spectrum. This can reduce out of band interference. Our Ceramic Elements have low emissivity in the near infrared; this makes them more suitable for work in the mid IR. Normally one wants a high blackbody temperature for high output, but the combination of higher short wavelength detector responsivity and high near IR blackbody output complicates mid infrared spectroscopy. Because of the emissivity variation, the Ceramic Elements provide lower near IR than one would expect from their mid IR output.
|
|
Kirchoff's Law
|
Kirchoff’s Law states that the emissivity of a surface is equal to its absorptance, where the absorptance (a) of a surface is the ratio of the radiant power absorbed to the radiant power incident on the surface.
|
|
|
Lambert's Law
|
Lambert’s Cosine Law holds that the radiation per unit solid angle (the radiant intensity) from a flat surface varies with the cosine of the angle to the surface normal (Fig. 4). Some Oriel Sources, such as arcs, are basically spherical. These appear like a uniform flat disk as a result of the cosine law. Another consequence of this law is that flat sources, such as some of our low power quartz tungsten halogen filaments, must be properly oriented for maximum irradiance of a target. Flat diffusing surfaces are said to be ideal diffusers or Lambertian if the geometrical distribution of radiation from the surfaces obeys Lambert’s Law. Lambert’s Law has important consequences in the measurement of light. Cosine receptors on detectors are needed to make meaningful measurements of radiation with large or uncertain angular distribution.
|
Fig. 4 Lamberts cosine law indicates how the intensity, I, depends on angle.
|