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Physical and Luminous Units of

Energy vs

Paul Avery Dept. of Physics University of Florida

PHY 3400 Light, 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 • Basic unit: = joule/sec Luminous ⇒ subjective brightness • Measured by observer ⇒ • Same energy flow at different wavelengths gives different visual response and thus different brightness • Basic unit:

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= in

• LE is maximum for a light source emitting at λ = 555 nm. • To achieve desired lumens for lowest $ ⇒ maximize LE • Projectors, indoor/outdoor , displays, floodlights

Paul Avery (PHY 3400) 4 Physical and Luminous Units Luminous efficiencies for different sources Light Source LE (lumens/watt) 0.1 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 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 • 90% of radiation from 100 W lightbulb in

Paul Avery (PHY 3400) 5 Physical and Luminous Units Other criteria besides LE are important • Quality ⇒ sodium 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 (watts/) ()

Light per unit (watt/m2) ( = lum/m2)

Paul Avery (PHY 3400) 7 Physical and Luminous Units