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Lecture2_PropertiesOfSunlight

# Lecture2_PropertiesOfSunlight - Dr.AlanDoolittle,GaTech...

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Dr. Alan Doolittle, Ga Tech Lecture 2: The Nature of Light Reading Assignment – Chapter 2 of PVCDROM Reading Assignment Chapter 2 of PVCDROM Dr. Alan Doolittle* *Most of the figures and text are from the online text for this class, PVCDROM.PVEDUCATION.org. Original references are given therein.

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Dr. Alan Doolittle, Ga Tech The Nature of Light The photon flux is defined as the number of photons per second per unit area: power density in units of W/m²,
Dr. Alan Doolittle, Ga Tech The Nature of Light The spectral irradiance can be determined from the photon flux by converting the photon flux at a given wavelength to W/m 2 as shown in the section on Photon Flux . The result is then divided by the given wavelength as shown in the equation below The result is then divided by the given wavelength, as shown in the equation below. where: F is the spectral irradiance in Wm -2 μm -1 ; Φ is the photon flux in # photons m -2 sec -1 ; 2 1 E and λ are the energy and wavelength of the photon in eV and μm respectively; and q, h and c are constants .

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Dr. Alan Doolittle, Ga Tech The Nature of Light Modified Xenon lamps with decreased IR content are often used to simulate the solar spectrum. One popular method of removing some IR is to use a “Dichroic filter”. The spectral irradiance of xenon (green), halogen (blue) and mercury (red) light bulbs (left axis) are compared to the spectral irradiance from the sun (purple, which corresponds to the right axis).
Dr. Alan Doolittle, Ga Tech The Nature of Light The total power density emitted from a light source can be calculated by integrating the spectral irradiance over all wavelengths or energies of interest. However, a closed form equation for the spectral irradiance for a light source often does not exist. Instead the measured spectral irradiance must be multiplied by a wavelength range over which it was measured, and then calculated over all wavelengths. The following equation can be used to calculate the total power density emitted from a light source. where: H is the total power density emitted from the light source in W m 2 ; F(l) is the spectral irradiance in units of Wm 2 m 1 ; and d or  is the wavelength.

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Dr. Alan Doolittle, Ga Tech Bl kb d R di ti The Nature of Light Blackbody Radiation where: l is the wavelength of light (μm); T is the temperature of the blackbody (K); F is the spectral irradiance in Wm 2 μm 1 ; and h,c and k are constants .
Dr. Alan Doolittle, Ga Tech The Nature of Sunlight The sun’s internal temperature is ~20 million degrees kelvin due to nuclear fusion reactions. The inner radiation is strongly absorbed by a layer of hydrogen atoms closer to the sun's surface. The surface of the sun, called the photosphere, is at a temperature of 5762 ± 50 K and closely approximates a blackbody radiator.

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