PHOTOMETRIC SIZES

Basics of photometry: Important photometric units of measurement

Photometric quantity in a nutshell:

Photometric quantities are fundamental concepts in lighting technology that measure the power and distribution of light when analyzing light sources.

The luminous flux, measured in lumens, indicates the total output of a light source, while the luminous intensity, measured in candelas, describes the part of the luminous flux that is emitted into a specific solid angle. The lighting intensity, measured in lux, provides information on how much luminous flux falls on a certain area. These parameters are crucial for the planning and evaluation of lighting systems in various areas of application.

Luminous flux (lumens, lm) describes the total light output emitted by a light source in all directions.

Light intensity I (candela, cd) describes the part of a luminous flux which is emitted in a solid angle.

Illuminance E (LUX, lx) describes how much a light flux incides on a given area.

What is luminous flux?

**SI unit* : Lumen (lm)**

The luminous flux is the total light output emitted from a light source in all directions. The lumen already considers the sensitivity of the eye. That is to say, two light sources with the same luminous flux are perceived as equally clear regardless of their light color.

Lumen ist das neue Watt!

„Lumen ist das neue Watt “ bedeutet, dass man Leuchtmittel nicht mehr aufgrund der Watt -Angabe sondern anhand der Lumen-Angabe bewertet. Wie in der Tabelle ersichtlich erreichen LED Leuchtmittel mit viel weniger Watt deutlich höheren Wert beim Lichtstrom (ausgestrahlte Lichtleistung).

* Definition SI: SI ist das internationale Einheitensystem für physikalische Größen.

LEUCHTMITTEL IM LEISTUNGSVERGLEICH


15W	≙120lm	-	-
25W	≙220lm	18W	≙217lm
40W	≙415lm	28W	≙410lm
60W	≙710lm	42W	≙702lm
75W	≙930lm	52W	≙950lm


3W	≙135lm	3W	≙136lm
5W	≙229lm	6W	≙249lm
12W	≙432lm	8W	≙470lm
14W	≙741lm	12W	≙806lm
18W	≙970lm	15W	≙1055lm

What is luminous intensity?

**SI unit* : Candela (cd)**

Lamps radiate in several directions - but with different strength. Light intensity indicates the luminous flux that is emitted at a solid angle (thus in a certain direction). Luminous intensity is a characteristic of the light source and has no relation to human perception. An ordinary household candle has a luminous intensity of 1 cd and therefore corresponds to one lumen per solid angle.

* Définition SI : SI est le Système International d'unités pour les grandeurs physiques.

VERGLEICHSTABELLE CANDELA

CANDELA CD	HALOGEN GU10
15W	≙120lm
25W	≙220lm
40W	≙415lm
60W	≙710lm
75W	≙930lm

CANDELA CD	HALOGEN GU10
15W	≙120lm
25W	≙220lm
40W	≙415lm
60W	≙710lm
75W	≙930lm

Photometric quantities are measured quantities that quantify the brightness and lighting intensity of light sources based on human perception. Units such as lumen and lux are used to describe light intensity and lighting level.

What does lighting intensity indicate?

**SI unit* : Lux (lx)**

Illuminance indicates how much luminous flux incides on a defined area and is therefore calculated as the quotient of the luminous flux (lm) by the illuminated area (m²).
The illuminance developed by a light source over a given area decreases as a square of the increasing distance.

OTHER EXAMPLES OF TYPICAL ILLUMINANCE FOR ORIENTATION:
5 mW laser pointer, green (532 nm)	427.000 lx
Lightning in a modern OP	160.000 lx
Clear sunny day	100.000 lx
Cloudy summer day	20.000 lx
Cloudy winter day	3.500 lx
Sports stadium lightning	1.400 lx
Office lightning	500 lx
Street lightning	10 lx
Candle 1 meter away	1 lx
Full moon	0,25 lx
CALCULATION: Luminous fl ux (lm) / Area (m2) = Illuminance (lx)

ILLUMINANCE LEVELS ACCORDING TO WORKPLACE REGULATIONS
REQUIREMENT ON VISUAL TASK	RATED LUMINOUS STRENGTHEN	EXAMPLE
very low	50 lx	Storage rooms, storerooms
low	100 lx	Break rooms, traffic zones
low	150 lx	Traffic zones with vehicles, loading areas
moderate	200 - 300 lx	Work on the workbench, machine tools, rough work, reception desk at hotel
medium	500 lx	Office
high	750 - 1000 lx	Technical drawing, precision engineering, printing
very high	1500 lx	Watchmaker‘s workshop, electronics workshop
exceptionally high	2000 lx	Engraving, invisible mending

Luminance: What is the difference to lighting intensity?

The illuminance expressed in Lux is a recipient variable, i.e. it describes the light output inciding on a given surface. The luminance, however, describes the perceived light that is emitted by a surface (whether this is used as light source or as a reflector). Luminance is the quotient of the luminous intensity (cd) and the surface perpendicular to the radiation direction (A).

CALCULATION: Luminous intensity / (cd) Area (m 2) = Luminance (LV)

CALCULATION: Luminous fl ux (lm) / Power (W) = Luminous efficiency (s)

THE LIGHT YIELD IN COMPARISON:
Bulb 60 W	10 lm/W
Bulb 100 W	15 lm/W
Energy-saving lamp	50 lm/W
Fluorescent tube 40 W	55 lm/W
LED	200 lm/W

Understanding luminous efficacy: Effectiveness of illuminants

The luminous efficiency describes the efficiency of a lamp and is derived from the emitted luminous flux (lm) and the added electric power. Its unit is therefore lm/W.

Photometric quantities such as luminous flux (measured in lumens), luminous intensity (measured in candelas) and lighting intensity (measured in lux) play a decisive role in the evaluation of lighting systems in terms of their light quality and quantity.

Color temperature: Insights into light colors

**SI unit* : Kelvin (K)**

Colour temperature helps determine quantitatively the colour impression of a light source. Colour temperature is the temperature of a black body** belonging to a particular colour of light emanating from the lamp. Upon heating a black body, the light colour changes from dark red to orange and yellow white to blue white.

CHARACTERISTIC LIGHT COLOURS ACCORDING TO DIN 5035

LUMINOUS SOURCE	COLOUR TEMPERATURE IN KELVIN
Warm white	< 3.500 K
Neutral white	< 5.300 K
Daylight white (also cool white)	> 5.300 K

* Definition of SI: SI is the International System of Units for physical quantities.
** A black body is a body that absorbs all radiation incident upon it.

Colour rendering index CRI (Colour Rendering Index)

The Colour Rendering Index (CRI) is specified in Ra and expresses the colour rendering quality of luminous sources. Unlike neutral and cold white light, warm white light has a high amount of red. This leads to different colour sensations. The Ra value indicates how much of the natural colour spectrum of sunlight is reflected by a lamp. A light bulb reaches a value of 100 Ra, while a white LED lamp attains Ra values between 70 and 98. The higher the colour rendering index, the better the colours of an illuminated object are perceived.

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