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Metr 415/715
Wednesday February 27 2013
Using Spectracalc Program
• Be aware of terminology different from Petty’s
book
• Blackbody Calculator tab default value:
– Temperature 300K
– Emissivity 1
– Recession velocity 0 – (applicable for hot bodies like
star moving a great speed with respect to observer)
– Wavelength 10 micrometers
– Lower and upper limit 8 and 12 micrometers
respectively (defining width of spectral band to be
calculated)
Spectralcalc
Petty
Radiant emittance
Radiant flux (Wm-2) over all wavelengths
Radiance
Radiant intensity (Wm-2sr-1) over all wavelengths
Peak Spectral Radiance
Monochromatic intensity emitted at wavelength
of maximum emission (Wm-2sr-1 μm-1) Notice
that here we must divide by μm
Wavelength of peak
Monochromatic wavelength of maximum
emission (μm)
Spectral radiance (refers
Monochromatic intensity at the wavelength you
to the specific wavelength specified
you selected)
Band radiance
Radiant flux (Wm-2) over all wavelengths for the
specific “channel” you specified
Color temperature
• Temperature that corresponds to the
wavelength of maximum emission of a body
emitting radiation, regardless of whether the
object is a blackbody or not
Brightness temperature
• Described in page 125-126, and equation (6.13)
• If you know the temperature of a blackbody you know
the monochromatic intensity it emits at a particular
wavelength
• Whether an object is a blackbody or not, if you know
the monochromatic intensity it emits at a particular
wavelength, you know the temperature a black body
would have to have to emit monochromatic radiation if
equal intensity at that wavelength
• That is called the brightness temperature of the object
Candela
• The SI unit of luminous intensity. One candela
is the luminous intensity, in a given direction,
of a source that emits monochromatic
radiation of frequency 540 × 1012Hz and has a
radiant intensity in that direction of 1/683 watt
per steradian
Lumens
• The SI unit of luminous flux, equal to the
amount of light emitted per second in a unit
solid angle of one steradian from a uniform
source of one candela
Candelas, lumens and Watts
• http://www.rapidtables.com/calc/light/howlumen-to-watt.htm
Electric Stovetop Burner
Using Blackbody calculator from
Spectracalc – Step 1
• Black body emitting upward at 300K
• Use default settings of Spectracalc
(wavelength 10 microns, 8-12 micron band)
• What is band radiance (radiant flux)? In what
units is it expressed?
Using Blackbody calculator from
Spectracalc – Step 2
• Consider a medium through which that
blackbody radiation passes that has an
absorptivity of 0.3 and a temperature of 250 K
• What is band radiance (radiant flux)? In what
units is it expressed?
Using Blackbody calculator from
Spectracalc – Step 3
• What is total band radiance (radiant flux)
received at the top – radiation from black
body plus radiation form the medium with 0.3
absorptivity?
• What is the brightness temperature of the
radiation received?
Setup for next homework set
• Do problems: 6.4, 6.5, 6.6, 6.9, 6.10, 6.11.
6.12
• Let’s discuss the givens and what needs to be
found
• Hint: use Spectralcalc program – it will come
in handy