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Unformatted text preview: 08-1 08-1 Light Definition: wave or particle of electromagnetic energy. Consider photon character of electromagnetic energy. Photon energy, E = ch λ , where c = 2 . 99792458 × 10 9 m s , h = 6 . 6260755 × 10- 34 Js, and λ is the particle wavelength. Effect of Photon Type of effect depends upon photon energy, . . . . . . which is proportional to its frequency. Amount of effect depends upon number of photons. Our Concerns Need to measure photon energy and “amount.” Therefore, need to know effect of photons on materials. Useful Photons When a photon strikes an atom, . . . . . . it might cause an electron to change energy levels. Energy level changes to be considered: • Valence to conduction in a semiconductor. • Bound-to-an-atom to free-of-the-atom. 08-1 EE 4770 Lecture Transparency. Formatted 8:22, 5 February 1999 from lsli08. 08-1 08-2 08-2 Optical Units Two Types of Units • Radiometric units . Based on actual photon energies. • Photometric units . Based on human perception. Examples of When Used Radiometric units might be used . . . . . . find out how much an item would be warmed by light. Photometric units might be used . . . . . . for designing room lighting. 08-2 EE 4770 Lecture Transparency. Formatted 8:22, 5 February 1999 from lsli08. 08-2 08-3 08-3 Radiometric Units Consider a light source. Radiant Energy, Q , total energy. Could be found by totaling energy of each photon over a specified period of time. Popular units: erg, joule, kilowatt-hour. This unit is not usually used to measure light. Radiant Flux, Φ = dQ dt , total power. Total amount of light being radiated from a source per unit time, t . Popular units: erg/second, watt (sometimes called light-watt). Irradiance, E = d Φ dA , flux density. Amount of light per unit area, A . Popular unit: watt per square meter. Radiant Intensity, I = d Φ dω . Amount of light per unit solid angle, ω . Popular unit: watt per steradian. 08-3 EE 4770 Lecture Transparency. Formatted 8:22, 5 February 1999 from lsli08. 08-3 08-4 08-4 Steradian Not-very-familiar-unit: steradian, a unit of solid angle. Abbreviation for this pseudo dimension: sr. Consider a sphere of radius 1. Paint a circle on the sphere of area 1. The circle would subtend one steradian of solid angle. The entire surface of a sphere (of any radius) is 4 π steradians. Example A light source radiates uniformly in all directions. Its radiant flux is Φ = 1 W . Find the radiant intensity and irradiance at a distance of 1 m and 3 . 048 m . At 1 m: E = 1 W 4 π (1 m) 2 = 79 . 6 mW m 2 (irradiance). and I = 1 W 4 π sr = 79 . 6 mW sr (radiant intensity). At 3 . 048 m: E = 1 W 4 π (3 . 048 m) 2 = 8 . 56 mW m 2 (irradiance). and I = 1 W 4 π sr = 79 . 6 mW sr (radiant intensity)....
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This note was uploaded on 12/11/2011 for the course EE 4770 taught by Professor Staff during the Fall '99 term at LSU.
- Fall '99