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Basic concepts and definitions in lighting technology
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Light and emission

Light is basically electromagnetic emission, which causes a visual sensation in human eyes. In this case emission has a 360 to 830 nm diapason, which is a tiny part of all the known electromagnetic emission.

Light path F

Unit of measurement: lumen [lm].

Light path F is the amount of energy flowing through an area unit in a time unit. Light path characterizes the power of the light source. An ordinary light bulb (100 W) at a 220 V voltage creates a light path equal to approximately 1300 lm. A compact daylight lamp (26 W) at a 220 V voltage creates a light path equal to approximately 1600 lm. The sun’s light path is equal to 3,8 × 1028 lm.

Luminous intensity I

Unit of measurement: candela [Cd].

Light source emits light path F in different directions and with varying intensity. The intensity of light emitted in one definite direction is called luminous intensity I.

Luminous density Е

Unit of measurement: lux [lx].

Luminous density Е reflects the correlation between the light path and the illuminated area. Density is equal to 1 lx if light path is evenly distributed on a 1 square meter area. 

Brightness L

Unit of measurement: candela per m2 [Cd/m2].

Brightness L of a light source or an illuminated area is the main factor for the level of visual sensation of a human eye.

Light output h

Unit of measurement: lumen per Watt [lm/W].

Light output h shows how efficiently consumed electric power is transformed into light.

Color temperature

Unit of measurement: Kelvin [K].

A light source’s color temperature is defined in comparison to a so-called “black body” and is displayed with the help of a “black body line”. If the “black body” temperature goes up, the blue component increases and the red one decreases. For example, a standard light bulb with a warm white light has color temperature of 2700 К, whereas a luminescent daylight lamp – 6000 К

Light color grade

Light color grade is best describes with the help of color temperature.

There are three main color grades of light:

Warm white < 3300 К

Neutral white 3300 — 5000 К

White day light > 5000 К.

Lamps with equal color grades can have quite different color reproduction characteristics, depending on the spectral composition of emitted light.

Color reproduction

Color reproduction characteristics describe how natural surrounding objects look in the light of this lamp. It is defined by a common Ra color reproduction index. In order to define Ra measures, 8 colors are picked out from the surrounding environment. They are being lit by the tested lamp, and then by a standard lamp, which has the same color temperature (from “black body” temperature to day light temperature). The smaller the difference between the tested colors, the better is the color reproduction of a lamp. Maximum Ra measure is 100 (as an average of the 8 test colors). Depending on the lamp’s position and function artificial lighting has to ensure the best color perception (similar to natural light). This is defined by the light source color reproduction characteristics (Ra). Color reproduction coefficient reflects the level of correlation between an object under natural light and the same object under tested light. Light source with Ra = 100 emits light, which optimally reflects all surrounding colors. The lowe the Ra measure, the worse color reproduction it shows.

Basic concepts and definitions in lighting technology
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