Lighting Terminologies Introduction

Transcription

1 Lighting Terminologies Introduction A basic understanding of lighting fundamentals is essential for specifiers and decision makers who make decisions about lighting design, installation and upgrades.

2 Radiometry the science of measuring light in any portion of the electromagnetic spectrum. In practice, the term is usually limited to the measurement of infrared, visible, and ultraviolet light using optical instruments. Irradiance is the intensity of light and is measured in watts per square meter.

3 Photometry the science of measuring visible light in units that are weighted according to the sensitivity of the human eye. It is a quantitative science based on a statistical model of the human visual response to light - that is, our perception of light - under carefully controlled conditions. The photometric equivalent of Radiance is called Illuminance and is measured in Lumens per square meter (Lux).

4 Lighting Units and Concepts

5 Radiations From a Hot Body The usual method of producing artificial light consists in raising a solid body or vapor to incandescence by applying heat to it As the body is gradually heated above room temperature, it begins to radiate energy in the surrounding medium in the form of electromagnetic waves of various wavelengths.

6 Thus, when the temperature is low, radiated energy is in the form of heat waves only but when a certain temperature is reached, light waves are also radiated out in addition to heat waves and the body becomes luminous. Heat waves are identical to light waves except that they are of longer wavelength and hence produce no impression on the retina Obviously, from the point of view of light emission, heat energy represents so much wasted energy.

7 Radiant Efficiency The radiant efficiency of a light source is the ratio of the energy radiated out in the form of light over the total energy radiated out by the hot body. It is found that the maximum radiant efficiency would occur above 6200 C and even then the value of this maximum efficiency will be 20%.

8 Solid Angle A solid angle (symbol: ω) is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large the object appears to an observer looking from that point. In the International System of Units (SI), a solid angle is a dimensionless unit of measurement called a steradian (symbol: sr).

9 The name is derived from the Greek stereos for "solid" and the Latin radius for "ray, beam A steradian is defined as the angle subtended at the center of a sphere by a part of its surface having an area equal to the square of its radius; A= r 2 ω = A r 2

10 Any area on a sphere, totaling the square of its radius and observed from its center, subtends precisely one steradian.

11 Definitions Terms employed in connection with illumination

12 Candela Unit of luminous intensity, or power emitted by a light source in a particular dimension. A common candle emits light of approximately one candela (cd). A source of one candela emits one lumen per steradian.

13 Luminous Flux (Φ). All the radiated power emitted by a light source and perceived by the eye Light energy radiated out per second from the body in the form of luminous light waves Commonly called light output.

14 Unit of measurement is lumen (lm), which is defined as the flux contained per unit solid angle of a source of one candela. 1 lumen = watt (approx.)

15 Mean Spherical Candle-Power Generally, the luminous intensity or candle power of a source is different in different directions. The average candle-power of a source is the average value of its candle power in all directions. M.S.C.P.= total flux in lumens 4π

16 Luminous Intensity (I) or Candle Power Luminous flux radiated out per unit solid angle in a direction Solid angular flux density of a source in a specified direction.

17 If d is the luminous flux radiated out by a source within a solid angle of dω steradian in any particular direction then I = d dω The unit of measurement is the candela (cd).

18 Illuminance (E) When the luminous flux falls on a surface, it is said to be illuminated. The illumination of a surface is measured by the normal luminous flux per unit area received by it. A measure of the amount of light falling on a surface.

19 The distance of the light source from the area being illuminated influences it. Unit of measurement is lux (lx) An illuminance of 1 lux occurs when a luminous flux of 1 lumen is evenly distributed over an area of 1 square meter.

20 Luminance (L) Brightness of an illuminated or luminous surface as perceived by the human eye. Unit of measurement is candelas per square meter (cd/m 2 ) Depends on the surface size seen and light intensity, reflected by the surface towards the eye

21 Luminous Efficacy (η). Luminous efficacy indicates the efficiency with which the electrical power consumed is converted into light. Refers to the number of lumens leaving the source compared to the amount of power The unit of measurement is lumens per watt (lm/w). Sources with higher efficacy require less energy to light a space or area.

22 Luminaire Efficiency Also known as the light output ratio An important criterion in gauging the energy efficiency of a luminaire. This is the ratio between the luminous flux emitted by the luminaire and the luminous flux of the lamp (or lamps) installed in the luminaire.

23 Quantity Quantity is a Measure of Symbol Unit in SI Definition of Unit Luminous intensity (Candlepower) Ability of source to produce light in a given direction I Candela (cd) Luminous flux Total amount of light Ø Lumen (lm) Illuminance (illumination) Luminous exitance Luminance (brightness) Amount of light received on a unit area of surface Density of light reflected or transmitted from a surface Intensity of light per unit area reflected or transmitted from a surface Approximately equal to the luminous intensity produced by a standard candle Luminous flux emitted in a solid angle of 1 steradian by a 1 candela uniform point source E Lux (lx) One lumen equally distributed over one unit area of surface M Lm/m 2 A surface reflecting or emitting 1 lumen per unit area L Cd/m 2 A surface reflecting or emitting light at the rate of 1 candela per unit of project area