What quantity does a unit called lux have

Everything about the unit of luminous intensity (photometry)

One of the many units that can be determined from electromagnetic radiation called light among other things the light intensity. she will given in candela and provides information about the strength of the luminous flux in a certain beam angle. More about the unit of luminous intensity (how to define them, how to calculate them and with which units they should not be confused) you will find out in this article.

Contents overview

Definition: light intensity simply explained

The light intensity describes the amount of light emitted by a light source in a certain solid angle.

Mathematically, the Luminous flux divided by the angle (also called steradian, abbreviated to sr). If a light source radiates in all directions, the number of lumens can be divided by 4π (corresponds to the solid angle of an entire spherical surface) and the light intensity is obtained.

A certain luminous flux is emitted and bundled through a more or less large angle. After all, the light intensity denotes the Amount of light, which within the light cone is located and thus not only takes into account the total amount of light emitted, as is the case with luminous flux.


The light intensity also does not take into account how much light hits a certain surface. She pulls that Direction and angle into consideration, however, as a whole.

That means that you don't measure at a certain distance on a certain surface, but that all light within the cone accepts.

So while the light scattered depending on the distance to the source becomes and thus loses intensity, the light intensity always remains the same.

Nevertheless, the physiological aspect, i.e. the Light sensitivity of the human eye, not left out.

For example, if you wanted to measure the light intensity of an infrared lamp, the result would always be 0, since infrared radiation is not visible to the human eye.

Unit of luminous intensity: Candela (cd)

The unit with which the luminous intensity is specified is called candela (abbreviated: cd). So that you can roughly imagine how bright 1 cd is, we have listed some comparative values ​​in the following table.

Light sourceCandela
Firefly10 microcandela (mcd)
candle1 cd
Incandescent lamp (60 W)58 cd
Mercury vapor lamp (125 W)430 cd
Motor vehicle high beam225 kcd (kilocandela)

Since “Candela” is Latin for “candle”, it is not surprising that a candle as a guideline or as 1 candela Is accepted. In practice, there are of course even more precise methods to find out the guideline value.

Luminous intensity (candela) in relation to plant lighting

If you shop around for plant lighting, you will be hard pressed to find a model that lists a candela. This size is namely not very informative for the effectiveness of the luminaire in relation to the growth of the plant. This is because it does not take into account the frequency of light or the density on a certain surface.

Measure light intensity

The light intensity cannot be measured directly, but very much simply calculate if you have previously determined the luminous flux. This is measured with the help of a so-called integrating sphere in which the light source is placed. The value determined in this way can then divided by the beam angle will and gives the light intensity (Candela).

Not to be confused with ...

Attention: risk of confusion! You can measure and determine a lot when it comes to light. Therefore, there are not just a few light measurement values ​​that can be determined from the light. So that you have one Overview of the different measurement units that are still there in addition to the light intensity, let's go over some of them here.

descriptionUnit namesymbol
Luminous fluxLumenslm (= cd * sr)
Illuminanceluxlx (= lm / m2)
LuminanceCandela per m2cd / m2
Amount of lightLumen secondlm * s
Light outputLumens per wattlm / W

... the luminous flux (unit: lumen)

In contrast to the light intensity, the denotes Luminous flux is the amount of lightthat the light source emits. It pours in all possible directions and its value does not change with a focus of the light.

That means that even if the light emitted by a lamp passes through a very narrow screen concentrated in a cone of light becomes, this does not change the number of lumens of the light sourcealthough the light may then appear brighter.

If the Candela value is the quotient of the lumen and the angle is, the luminous flux, i.e. the number of lumens, is the product of candela and angle (steradian, sr). The lumen number can therefore be calculated from cd * sr.

... the illuminance (unit: lux)

The illuminance, or luminous flux density, refers to the Luminous flux that hits a certain surface. Its unit is lux, also abbreviated to "lx", and can be calculated as lumen per square meter. The angle of radiation is included here indirectly - only this time counts in contrast to the light intensity, also the distance to the light source.

Old illuminance unitsthat were displaced by Lux Nox (nx) and Phot (ph). In North America, the unit foot-candle (fc) is also used, which represents the Light intensity to one square foot indicates.

... the luminance (unit: candela per square meter)

What the foot-candle is in the Anglo-American system of measurement and indicates the illuminance, is the luminance in our latitudes. Here are the Candela values ​​added to a certain area. With that, the Intensity of light must be taken into account.

This measurand has the most direct expressiveness about the perception of light, since all parameters are included that could influence the perceptible brightness of the emitted light.

In addition, it is taken into account that a light source does not emit exactly the same amount of light radiation at all points. The Luminous intensity is, so to speak, the sum of all luminance levels, since it denotes all light emitted in a certain direction.

... the color temperature (unit: Kelvin)


The color temperature stands out in the context of this list, because it is more concerned with the Frequency and wavelength, so the spectrum, the radiation than with its intensity.

The Kelvin number of a light source indicates what hue the lightHas.

For example, if you compare the sunlight at sunrise with that at noon, you will notice that there is something warmer (i.e. in orange tones) and once a little colder (i.e. more in blue tones) shines.

The same applies to differences between the various artificial light sources. Their hue and thus their Kelvin value can be measure using a spectrometer.

Summary: unit of luminous intensity - bundled luminous flux

It is summarized in a nutshell the light intensity is the amount of light emitted in a certain cone of light. The amount of light emitted (= luminous flux, specified in lumens) is divided by the direction (= steradian, sr). We learn “how much light” there is in this cone of light. The candela number is there, however not necessarily information about the brightness of the light.

As a reminder: in plant lighting this unit is almost never used, since it does not provide any information about the usefulness of light for the plant. The photon flux density (PPFD) is definitely better suited for this.