07-Lighting Concepts. EE570 Energy Utilization & Conservation Professor Henry Louie

Transcription

1 07-Lighting Concepts EE570 Energy Utilization & Conservation Professor Henry Louie 1

2 Overview Light Luminosity Function Lumens Candela Illuminance Luminance Design

3 Motivation Lighting comprises approximately 25% of building electrical energy use How much light is needed? How do we quantify light? How do we design effective and efficient lighting systems?

4 Light Light is electromagnetic radiation of varying wavelengths that are visible to the human eye Generally nm wavelengths

5 Light Longer wavelengths have lower frequencies, and less energy Electromagnetic power and energy are measured in Watts and joules, respectively

6 Electromagnetic Radiation Wavelength and frequency are related by: f c l where f= frequency (Hz) c= speed of light (299,792,458 m/s) l = wavelength (m) Higher frequency radiation has shorter wavelengths

7 Electromagnetic Radiation Energy of a single photon is related to wavelength and frequency by: E hf where E: energy (joules) h: Planck s constant x10-34 (joule-second)

8 Example What is the energy of a photon with wavelength 600nm? Recall: c= speed of light (299,792,458 m/s) Planck s constant x10-34 (joule-s)

9 Example What is the energy of a photon with wavelength 600nm? f c l hz 19 E hf joules

10 Electromagnetic Radiation Blackbody radiation Power is distributed across a range of wavelengths, but we are only interested in the those in the visible range "Black body" by Darth Kule - Own work. Licensed under Public Domain via Commons -

11 Luminosity Function Need to distinguish power and energy in the visible spectrum from power and energy in the entire spectrum Human eyes have different sensitivities to different wavelengths in the visible spectrum Most sensitive: ~550nm (yellow-green) Power of light is found by the weighted sum of the power associated with each frequency Weighting accounts for human vision sensitivity and is determined by the luminosity function V(l)

12 weighting Luminosity Function V(l) Yellow (photopic) curve is commonly used Wavelengths outside the visible spectrum wave a weighting of 0 wavelength Tabulated values are provided the CIE: "LuminosityCurve2" by Raoul NK - Own work. Licensed under CC BY-SA 3.0 via Commons -

13 Power density Perceived power density Luminosity Function arbitrary EM power density spectrum V(l) Sum (integrate) for total power x = wavelength wavelength Multiply the power density at each wavelength by the weighting function evaluated at that wavelength.

14 Example What is the perceived energy of a photon with wavelength 600nm? Assume the luminosity function at 600nm is

15 Example What is the perceived energy of a photon with wavelength 600nm? Assume the luminosity function at 600nm is E p = 3.31x10-19 x = 2.088x10-19 Compare this result to the previous example.

16 Luminous Flux (f v or F) Visible (perceived) power in a spectrum of electromagnetic radiation is called luminous flux Units: lumens (lm) Measures luminous energy/time (power of visible electromagnetic radiation)

17 Typical Lumen Output Source:

18 Typical Lumen Output Source Lumens 37 mw "Superbright" white LED mw green laser (532 nm wavelength) W high-output white LED Kerosene lantern W incandescent lamp W high-output white LED W fluorescent lamp W incandescent lamp W fluorescent lamp W xenon bulb W fluorescent lamp 8000

19 Luminous Flux Luminous flux from a spectral power distribution is found from: inf F 683 J( l)v( l)d l 0 Why 683? It is arbitrary. It was selected to link the old unit of candela with the new unit of candela where F: luminous flux (lm) J(l): spectral power distribution of the light source (power per unit wavelength) (w/m) V(l): luminosity function (unitless)

20 Luminous Efficacy (h L ) Measurement of how well a lamp converts input power to perceived power (luminous flux) F hl P in h L : luminous efficacy (lm/w) P in : input power Maximum value is 683 lm/w Efficiency is not used, as we do not care about power output to the EM spectrum not visible to the human eye

21 Example A incandescent light bulb consumes 100W. It outputs 1600 lumens. Compute the luminous efficacy.

22 Example A incandescent light bulb consumes 100W. It outputs 1600 lumens. Compute the luminous efficacy hl

23 Source Luminous Efficacy candle W tungsten incandescent (230 V) W tungsten incandescent (120 V) W tungsten glass halogen (5.2 V) 19.2[19] white LED (raw, without power supply) W LED screw base lamp (120 V) W LED PAR20 (120 V) W compact fluorescent (with ballast) T8 tube with electronic ballast T5 tube metal halide lamp low pressure sodium lamp Plasma display panel 2-10[48]

24 Luminous Coefficient Another way of expressing luminous efficacy Scaled so that the maximum value (h L = 683) is 1.0 Example: a white 7W LED has an output of 450 lumens. Its luminous coefficient is (450/7)/683 = (or 9.4%)

25 Luminous Flux Luminous flux (lumens) describes the visible power from a light source Lumen value alone is not enough to characterize a light source Consider the difference between a torch and a florescent light tube A torch concentrates (focuses) the light on a smaller area it appears brighter within that area

26 Candela (I V ) Luminous intensity is known as the candela Accounts for how the lumens are emitted from the light source Luminous flux (power) per unit solid angle as emitted by a light source A common candle emits approximately one candela of luminous intensity

27 Candela 1 Candela: luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x hertz (550 nm) and that has a radiant intensity in that direction of 1/683 watt per steradian (1 lumen per steradian)

28 Steradians and Solid Angles Geometric concept Steradian (square radian) is a unit of solid angle Watts is to power as steradians is to solid angle Solid angle (W): ratio of the area of the sphere subtended and the square of the distance from the vertex Unitless, but steradians (sr) commonly used for clarity Solid angle is not expressed in degrees or radians Area Subtended W 2 radius

29 Steradians and Solid Angles Consider a sphere with radius r Let a ray originate from the vertex and sweep out a cone Let the spherical surface area swept out by the cone have area A The solid angle is found from: A W (solid angle) (in sr) 2 r r A If the area A happens to equal r 2, then the solid angle is 1 steradian

30 Steradians and Solid Angles r 0.5r 2 r r 2 2r 2 r r 2 r 2 1 r 1 r Steradians 1 steradian 2 Steradians Both 1 Steradian Note: figures are to show relative differences they are not exact

31 Steradians and Solid Angles The surface area of the sphere corresponding to 1 steradian depends on the radius of the sphere Example: For a sphere with radius of 2m, the surface area corresponding to 1 steradian is 4m 2 For a sphere with radius of 3m, the surface area corresponding to 1 steradian is 9m 2 Remember: solid angles are measured in steradians, which is unitless. The area on the surface of the sphere is the area subtended by the solid angle

32 Steradians and Solid Angles Recall the surface area of a sphere is 4pr 2 The maximum solid angle is therefore W= 4pr 2 /r 2 = 4p

33 Example Consider a globe whose radius is 2m. Compute the area subtended by 1.5 steradian.

34 Example Consider a globe whose radius is 2m. Compute the area subtended by 1.5 steradian. Area Subtended W 2 radius 2 W radius Area Subtended m 2 2

35 Example A light sensor is held 5m from a light source. The light sensor reads 0.35 lumens and has a surface area of 0.001m 2. What is the luminous intensity?

36 Example A light sensor is held 5m from a light source. The light sensor reads 0.35 lumens and has a surface area of 0.001m 2. What is the luminous intensity? Area subtended by 1 steradian of a 5m sphere is 25m 2. Area of the sensor is therefore steradians. The luminous intensity is 0.35/ = 8750 cd

37 Example What is the lumen value of the source in the previous example if the lumens are emitted in every direction? Assume the source is spherical.

38 Example What is the lumen value of the source in the previous example if the lumens are emitted in every direction? Assume the source is spherical. Source is 8750 candela (8750 lumens per steradian). A sphere has 4p steradians, so p lumens

39 Candela Why use Candela and not lumens to describe a light source? Candela can be used to describe the light source in a particular direction Two sources may have the same lumens but be focused differently

40 Candela Point source of 1000 lumens Emitting light in all directions (three dimensional) Source of 1000 lumens Emitting light in a focused pattern. Which is perceived to be brighter?

41 Candela Relating linear angles to solid angles is done through: W 2 2pr 1 cos Example: what solid angle corresponds to a linear angle of 20 degrees in a unit sphere? Answer: W = 2p(1-cos(20 o ))= Angle is defined from the center line

42 Candela A 1000 lumen light source emitting light in every direction has an luminous intensity of 1000/(4p) = 79.6 cd If the light from the same source was focused in a beam of 2 steradians, it would have a luminous intensity of 500 cd More appropriate for a torch A 1000 lumen light is focused in a =10 o beam. The luminous intensity is: 1000/ = cd

43 Luminous Intensity and Candela Common to see lamps rated in both lumens and candela Candela is often more appropriate when illuminating a large space Lumen is often more important when the light will be focused (torch, spot light, etc.)

44 Luminance and Illuminance We have discussed measures of light coming from sources Next we consider the light received by a surface, and the light seen by a person Two commonly confused quantities: luminance and illuminance

45 Luminance (L V ) Luminous intensity from a source passing through a given area and falls within a given solid angle (often the pupil of an eye) Luminous intensity per steradian projected onto a flat surface Indicator of how bright an object will appear by a person s eye at a given angle Units: cd/m 2 (also known as a nit )

46 Luminance Candela/m 2 = Lumens/steradian/m 2 Think if this as an eyeball and the green area (area subtended) as the pupil r If the pupil receives 1 candela (1 lm/sr) from a TV whose area is 0.25 m 2, the luminance is 4 cd/m 2. m 2 lumens steradian The person could change their viewing angle and either increase or decrease the luminance. Moving closer or farther would also influence luminance.

47 Illuminance (E V ) Amount of light incident to a surface (e.g. a desktop, roadway, floor, artwork) Surface properties does not affect the illuminance Independent of the observer Unit: Lux (lumen/m 2 ) surface

48 Luminance and Illuminance A photograph may have a lot of light incident to it (illuminance), but may have low luminance if, for example, it is viewed from an oblique angle or there is little reflection. Illuminance is used in building lighting design.

49 Typical Outdoor Lux Values Condition Lux (lm/m 2 ) Sunlight Full Daylight Overcast Day 1075 Very Dark Day 107 Twilight 10.8 Deep Twilight 1.08 Full Moon Quarter Moon Starlight Overcast Night

50 Typical Indoor Lux Values Condition Lux (lm/m 2 ) Public areas with dark surroundings Simple orientation for short visits Working areas where visual tasks are only occasionally performed Warehouses, Homes, Theaters, Archives 150 Easy Office Work, Classes 250 Normal Office Work, PC Work, Study Library, Groceries, Show Rooms, Laboratories Supermarkets, Mechanical Workshops, Office Landscapes Normal Drawing Work, Detailed Mechanical Workshops, Operation Theaters Detailed Drawing Work, Very Detailed Mechanical Works Performance of visual tasks of low contrast and very small size for prolonged periods of time Performance of very prolonged and exacting visual tasks Performance of very special visual tasks of extremely low contrast and small size ,

51 Lighting Design Lighting design itself is a field of study Basic problem is to design lighting of a room, facility or space to achieve a prescribed level of illuminance Design is usually computer-aided and can be very specific We will cover basic concepts next

52 Luminaires and Lamps Luminaires: the complete lighting unit consisting of Lamp(s) Ballast (if applicable) Light distribution components Electrical components Lamp: light-emitting component of a luminaire

53 Coefficient of Utilization (CU) Ratio of luminous flux (lumens) incident upon a plane (e.g. desk) to the luminous flux emitted by the luminaire Value between [0, 1.0] Accounts for how the luminous flux is emitted not all will be incident to the plane Factors influencing CU Room geometry Wall/floor/surface reflectance Luminaire design For more information on CU tables see:

54 Lighting Loss Factor Accounts for losses associated with the luminaire (and dirt on surfaces of the room) Causes of loss Dirt (on lamp and surfaces) Ballast variations Temperature Aging Etc. Lighting Loss Factors (LLF) generally less than 1.0

55 Method of Lumens Procedure to compute the average illuminance of a working plane for a given design n N F CU LLF EV A where: n: number of lamps per luminaire N: number of luminaires A: area of working plane (m2) F: luminous flux (lm)

56 Example Illuminance of 500 lux is required at desk height for an office. The room is 50m 2. The CU of the room and luminaire is The LLF is The luminaire chosen has 6 lamps, each rated for 2500 lumens. How many luminaires are needed?

57 Example Illuminance of 500 lux is required at desk height for an office. The room is 50m 2. The CU of the room and luminaire is The LLF is The luminaire chosen has 6 lamps, each rated for 2500 lumens. How many luminaires are needed? Lumens required at desk height: 500 x 50 = lumens Accounting for CU and LLF: 25000/0.50/0.85 = lumens Each luminaire produces: 4 x 2500 = lumens We need ceil(58824/10000) = 6 luminaires

58 Example Illuminance of 300 lux is required at desk height for a schoolroom. The room is 30m 2. The CU of the room and luminaire is The LLF is The luminaire chosen has 4 lamps, each rated for 2350 lumens. How many luminaires are needed?

59 Example Illuminance of 300 lux is required at desk height for a schoolroom. The room is 30m 2. The CU of the room and luminaire is The LLF is The luminaire chosen has 4 lamps, each rated for 2350 lumens. How many luminaires are needed? Lumens required at desk height: 300 x 30 = 9000 lumens Accounting for CU and LLF: 9000/0.44/0.85 = lumens Each luminaire produces: 4 x 2350 = 9400 lumens We need ceil(24064/9400) = 3 luminaires