All about light intensity, luminous flux & illuminance

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Important photometric quantities and units

In light measurement, a distinction is made between various photometric quantities by which the unit of light intensity can be evaluated. The following table provides an overview of the most important photometric quantities and units:

Photometrische GrößeSI unit and calculationDefinition
Luminous fluxLumen (lm)The measure for the total amount of light emitted by a light source.
Light intensityCandela (cd) = lm/srThe ratio of lumen to radiation angle. Gives information about how much light is radiated in a certain direction.
IlluminanceLux (lx) = lm/m²The measure for the light that arrives at a receiver surface.
Luminancecd/m²The measure for the impression of brightness of a surface, perceived by the human eye.
Luminous efficacylm/WThe ratio of emitted luminous flux to required electrical power.
Light quantitylm*sThe total luminous flux emitted by a light source over a certain period of time.

What is light and how is it created?

Light consists of photons, also called light particles. They travel in waves and transmit energy impulses. Light is created when energy is converted. When visible light is emitted, this is also called luminescence. How bright and colourful the human eye can perceive a light depends on the wavelength of the radiation and the intensity with which the radiation hits the retina.

Short wavelengths are termed ultraviolet and longer waves are termed infrared.

What is the light flux (lumen)?

The light flux is measured in lumens (abbreviation lm). The unit lumen is an internationally standardized unit of measurement for the light flux of a light source. It indicates how much light is emitted by a radiation source in all directions, so it measures the total light output. The lumens of a luminaire therefore provide information about its brightness. Identical luminaire types can be compared by their wattage.

Different lamps, however, produce different amounts of light and therefore cannot be compared by wattage. To compare the high intensity of different lamps, luminous flux must be used.

The value Lumen does not take the brightness sensation into account. The perception of brightness is additionally influenced by the beam angle and the design of a luminaire. Also, the color temperature of the illuminant and the condition of the environment play a role in the perception of brightness.

Luminous flux

Two examples of typical luminous flux values:

- Incandescent lamp with 15 W electrical power: luminous flux Φ = 90 lm
- Compact fluorescent lamp/energy-saving lamp: with 15 W electrical power: luminous flux Φ = 900 lm

What is the luminous efficacy (η)?

The luminous efficacy is a measure that indicates how efficient a light source is. It is the ratio of lumens to power or watts, so it is measured in lumen per watt (lm/W) in the International System of Units (SI). The higher the value, the more efficient a light source is.

Exemplary lumen values:

Fluorescent tube (48 Watt)3000 lm
Energy-saving lamp (23 Watt)1400 lm
Light bulb (100 Watt)1340 lm
Candle12 lm

Examples of the luminous efficacy of different lamps:

Incadescent lamps6 - 19 lm/W
Halogen lamps13 - 23 lm/W
Fluorescent lamps52 - 85 lm/W
High-pressure mercury vapour lamps40 - 58 lm/W
High-pressure sodium vapor lamps70 - 140 lm/W

What is light intensity (candela definition)?

The luminous intensity is a photometric value that describes the radiation of light emitted in a certain direction. Since the radiation behavior of an optical signaling device is not only determined by the light source but also by the design of the domes, the luminous intensity is best suited to characterize the signaling effect of optical signaling devices.

Light intensity is one of the photometric quantities. It relates the light flux to the beam angle of a light source. The luminous intensity therefore indicates how concentrated the light is or what density the emitted light has.

What is the metric unit used for measuring luminous intensity? Candela (cd) is the metric unit of luminous intensity. As an example, the luminous intensity of a candle is approximately one candela.

How to measure light intensity?

Luminous intensity is an important value for comparing different lamps. Lamps with the same luminous flux can have completely different luminous intensities due to their beam angle. The beam angle indicates the angle at which the light is emitted by the lamp.

The luminous intensity or its candela value indicates how intensively the light is emitted. The more focused the light is emitted, the more intense it is. The diagram shows two lamps with the same luminous flux (lumen) but different beam angles. The luminous intensity of the lamp with the smaller beam angle is higher than that of the lamp with the wider beam angle.

Luminous intensity

How to measure light intensity?

To determine the luminous intensity of a lamp, you need the luminous flux and the beam angle or solid angle. The value of the luminous- or light intensity is given in candela. The units of luminous flux are lumens, for the solid angle it is steradians.

Luminous intensity [cd] = luminous flux [lm] / solid angle [sr].

If the solid angle is not known, but only the beam angle, this can be determined with a conversion formula:

Formula Luminous intensity

If the calculation according to the above formulae is not possible for the lamp, a light measurement can be carried out with an integrating sphere and a spectrometer. This creates a luminous intensity distribution curve. From this, conclusions can be drawn about the luminous intensity.

A standard candle, for example, emits a luminous intensity of 1 cd, i.e., it emits about 12 lumens (lm) in all directions.

What is illuminance (lux)?

Lux provides information about the illuminance. It is a measure of the brightness with which an area is illuminated. Lux indicates how much luminous flux (lumen) of a light source arrives per unit area of a receiver surface. The value lux is purely a receiver quantity.

Illuminance is calculated with the following formula: Lux [lx] = luminous flux [lm] / area [m2].

The illuminance is 1 lux if a luminous flux of 1 lumen falls uniformly on an area of 1 m².

Another formula for calculating illuminance at greater distances is as follows: Lux [lx] = luminous intensity [cd] / radius or distance squared


The further away the area is from the light source, the lower the illuminance. The lux value determined can be used to determine whether certain areas are sufficiently well lit. For example, there are labor-law requirements for how brightly a work area should be illuminated for employees.

What does a luxmeter measure?

A luxmeter is a light inensity meter and measures the illuminance (lux). The value indicates how bright it is at the measuring point. The luxmeter consists of a photo sensor and a display. The photo sensor usually consists of photo diodes that detect the light. The measured lux value then appears on the display.

Exemplary lux values

Sunlight40.000 lx
Office workplace300-500 lx
Living space50-200 lx
Full moon night0,3 lx
Starry night0,1 lx

How are lumen, candela and lux related?

The terms lumen, candela and lux occur very frequently in light measurement. They are all photometric quantities. The following diagram shows the relationship between the three terms.

How are lumen, candela and lux related

The unit lumen stands for the total light output of a luminaire emitted in all directions. However, since the light emitted by luminaires is not emitted evenly in all directions, the luminous intensity is given in candela. So candela is the unit of light intensity. This value indicates how much light is emitted in a particular direction. In contrast to these two emitted quantities, there is also the receiver quantity lux. The lux unit does not measure how much light is emitted, but how much of the emitted light reaches a certain surface.

To assess the brightness of a luminaire or illuminant, it is essential to consider all three values. The units lumen and candela are quoted by most manufacturers. Manufacturers cannot quote the lux value because this value depends on the ambient conditions of the area of application.

What is the solid angle?

The solid angle is the three-dimensional size of the light cone. If a luminaire emits light, the angle of the emitted light is three-dimensional. The unit of the solid angle is the steradian (sr). The light intensity indicates the amount of light that a light source emits per solid angle.

The solid angle is calculated by dividing the area (A) by the radius (r²).

solid angle

What is the density of luminance (cd/m²)?

The density of luminance provides information about the impression of brightness of a luminaire. It is expressed in luminous intensity per area (cd/m²). The density of luminance describes how bright a surface appears to us. This factor is also influenced by other circumstances, such as the condition of the illuminated surface.

What is colour temperature?

The colour temperature of a lamp determines whether the light gives a warm or cool impression. The colour temperature is given in Kelvin and can be assessed on a scale. The lower the colour temperature, the warmer and darker the light appears. The higher the colour temperature, the cooler and brighter the light appears.

The colour temperature influences the room atmosphere. For living rooms, a lower colour temperature is preferred, for laboratories or factories, light with a higher colour temperature. The scale below shows the colour temperature and its three ranges: Warm white, neutral white and daylight white.

Colour temperature can be measured with a colourimeter. Besides colour temperature, the colour rendering index is also significant for the spatial atmosphere.

Colour temperature

What is the colour rendering index?

The colour rendering index, abbreviated CRI (Colour Rendering Index) or RA (General Reference Index) tells us what quality the emitted light has.

When an object is illuminated, it emits colours. The colour emitted is not only determined by the colour of the object itself, but also by the light source. The illuminant emits different wavelengths that are absorbed or reflected by the illuminated object. Those wavelengths that match the illuminated object are reflected, the rest is absorbed. The colour rendering index therefore depends on which wavelengths are emitted by the light source.

Natural sunlight has an RA value of 100, which is also the highest RA value. The closer the RA value is to 100, the higher the quality of the lighting.

What is the reflection factor?

The reflection factor indicates the percentage of the luminous flux that hits a surface that is reflected. Depending on the nature of the illuminated surface, the light is reflected, absorbed or transmitted.

If light is reflected, it is reflected back. Mirrors have a reflectance of 1. Light surfaces have a value close to 1, dark surfaces have a value below 0.1. In a room with dark walls, more light is needed than in a room with light walls to create sufficient illuminance on the working plane.

Examples of the reflectance from different surfaces:

  • White ceiling or wall reflects up to 85 % of the light,
  • light wood panelling up to 50 %,
  • red bricks up to 25 % and a
  • black floor 0 %.

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