Physical and Luminous Units of Light Intensity

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Physical and Luminous Units of Light Intensity Energy vs Brightness Paul Avery Dept. of Physics University of Florida PHY 3400 Light, Color and Holography http://www.phys.ufl.edu/~avery/course/phy3400_f99/ Paul Avery (PHY 3400) 1 Physical and Luminous Units Physical ⇒ energy flow • Measured by instruments ⇒ radiant flux • Basic unit: watt = joule/sec Luminous ⇒ subjective brightness • Measured by observer ⇒ luminous flux • Same energy flow at different wavelengths gives different visual response and thus different brightness • Basic unit: lumen Paul Avery (PHY 3400) 2 Physical and Luminous Units Visual Sensitivity vs Wavelength 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 Relative Sensitivity 0.2 0.1 0.0 350 400 450 500 550 600 650 700 750 Wavelength in nm Relative Sensitivity of Eye vs. Wavelength Peak at λ = 555 nm Paul Avery (PHY 3400) 3 Physical and Luminous Units Define “Luminous efficiency” luminous flux in lumens LE= radiant flux in watts • LE is maximum for a light source emitting at λ = 555 nm. • To achieve desired lumens for lowest $ ⇒ maximize LE • Projectors, indoor/outdoor lighting, displays, floodlights Paul Avery (PHY 3400) 4 Physical and Luminous Units Luminous efficiencies for different sources Light Source LE (lumens/watt) Candle 0.1 Kerosene lamp 0.3 Tungsten-filament lamp (100-watt) 15 High-intensity tungsten-filament lamp 30 “Photoflood” lamp 36 Mercury-vapor lamp, low pressure 25–40 Mercury-vapor lamp, high pressure 40–65 Sodium-vapor lamp 50 LEDs 10–20 Fluorescent lamp 60–75 Metal halide lamps 65–90 http://www.phoenixlamps.com/mhl.html Blackbody, T = 6500 K 93 Monochromatic light, λ = 555 nm 683 Poor LE performance caused by light emitted outside visible spectrum • 90% of radiation from 100 W lightbulb in infrared Paul Avery (PHY 3400) 5 Physical and Luminous Units Other criteria besides LE are important • Quality ⇒ sodium lights for indoor? UUGGHH!! • Brightness of light • Cost of bulb • Lifetime • Cost and availability of fixtures Search for efficient light sources continues • Is LE > 100 high quality light possible? Paul Avery (PHY 3400) 6 Physical and Luminous Units Other Units Quantity Physical Units Luminous Units Total light output Radiant flux Luminous flux (watts) (lumens) Light per unit angle Radiant intensity Luminous intensity (watts/steradian) (candelas) Light per unit area Irradiance Illuminance (watt/m2) (lux = lum/m2) Paul Avery (PHY 3400) 7 Physical and Luminous Units.

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