iam11_lecture04

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Unformatted text preview: le which cuts out 1 m of circumference from a circle with radius 1 m). 56 78 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Measuring Light (2) 12 Radiometric Quantities 34 Quantity Symbol Unit Definition 56 Radiant energy Q Ws Total energy emitted/received 78 Radiant flux Φ W Total power (energy per time) emitted/received 9 10 Radiant exitance M W/m2 Power emitted per surface area 11 12 Irradiance E W/m2 Power received per surface area 13 14 Radiant intensity I W/sr Power leaving a surface point per solid angle 15 16 Radiance L W/(m2 sr) Power leaving a surface per area per solid angle (Table adapted from Haußecker 1999) 17 18 19 20 21 22 23 24 Remark: The inverse square law (Lecture 3) describes the behaviour of irradiance. 25 26 27 28 Measuring Light (3) 12 Sensitivity of Human Eye 34 Human Perception: The radiometric power needed to create a brightness perception in the human eye varies with wavelength, and also differs between: • Photopic Vision: vision under high illumination, using the retina cones • Scotopic Vision: vision under poor illumination, using the retina rods From Radiometry to Photometry: For each radiometric quantity Xr, a corresponding spectrally weighted quantity Xν measuring visual impression can be derived using the spectral response V (λ) and a normalisation factor N via λ2 Xν = N λ1 λ1 = 380 nm λ2 = 780 nm N = 683 lm/W N = 1754 lm/W V (λ) dXr V (λ) dλ . dλ for visible light for photopic vision for scotopic vision luminous efficiency 56 78 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Luminous efficiency V (λ) for human visual system (Haußecker 1999) 25 26 27 28 Measuring Light (4) 12 Photometric Quantities 34 Quantity Symbol Unit Definition 56 Luminous energy Qν lm s Total spectrally weighted energy emitted/received 78 Luminous flux Φν lm (lumen) Total luminous power (energy per time) emitted/received 9 10 2 Luminous exitance Mν lm/m Luminous power emitted per surface area Illuminance Eν lm/m2 = lx (lux) Luminous power received per surface area Luminous intensity Iν lm/sr = cd (candela) Luminous power leaving a surface point per solid angle Luminance Lν W/(m2 sr) = cd/m2 Luminous power leaving a surface per area per solid angle (Table adapted from Haußecker 1999) 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Measuring Light (5) 12 Subjective Impression of Brightness 34 Fechner’s Law 56 • The subjective impression of brightness given a luminous power received follows the logarithm of the luminous power, i.e. it is a multiple of ln Φν Φν min , Φν min ≤ Φν ≤ Φν max 78 9 10 11 12 where Φν min and Φν max represent thresholds of visibility and saturation. 13 14 • Since illumination laws tie the radiance emitted by a surface multiplicatively to the received irradiance, perceived brightness differences according to Fechner’s law are illumination independent. 15 16 17 18 19 20 • Logarithms of ratios are often measured in decibels (dB). For given energies or energy-derived...
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This note was uploaded on 07/09/2013 for the course SC 111 taught by Professor S during the Winter '12 term at Uni Saarland.

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