Luminous intensity

The luminous intensity Iv is the power emitted by a light source per unit solid angle, weighted by the spectral sensitivity of the eye (more precisely: a standardized 2 ° standard observer ), ie the luminous flux per unit solid angle. It is the photometric analog of the radiation intensity.

By measuring the light intensity in candelas over the entire solid angle in steradian, one obtains the luminous flux in lumens. For point light sources or measurement locations outside the photometric limiting distance, the light intensity is linked via the photometric distance law with the illuminance in lux.

Unit

The light intensity is a basic parameter in the SI unit system. In the past, served different light sources as a reference until in recent times, a cavity radiator ( black body ) of platinum: the luminous intensity of a 1/600000 square meters of board space in a cavity radiator at a temperature of 2042.5 K ( solidification temperature of platinum ) and an ambient pressure of 101,325 Pa corresponds to perpendicular to the surface of the luminous intensity of 1 cd.

Since 1979, the candlepower unit is returned to the derived SI unit watts. The currently accepted definition

Here is not to be understood as a realization procedure [ eg Example, via the use of ( yellow-green ) laser radiation ]. The specified frequency in air is just happened to be near the maximum of the light sensitivity of 2 ° standard observer. Rather, the specified frequency is the intersection of two of the CIE defined and published by the BIPM Light sensitivity curves for photopic vision (tags marriages) and scotopic vision ( night vision) again, so the unit candela is valid for both excitation conditions of the human eye.

Other derived quantities with the same reference to a radiation source, the luminous flux (unit: lumen ), the luminance (unit: cd · m -2) and the light intensity (unit: lux). For energy comparisons, the radiation intensity (light intensity ) instead of the light intensity must be used as a benchmark.

Perceived intensity

The light intensity is a property of the light source and is not dependent on the distance of an observer. It quantifies the amount of luminous flux (unit: lumen ) emitted in a certain direction (per solid angle ). The spectral perception of the human eye is used as a reference. For example, the luminous intensity of a source of infrared radiation of arbitrary radiation intensity equal to zero because it is invisible to the human eye.

The perceived by the eye brightness of a light source is only partially consistent with the physical light intensity. The contrast with the environment influences the physiological perception. A light source having a small surface area is regarded as a bright (or blinding ) as a light source with light of the same physical thickness, but a greater surface area. This impression can be observed for example in car headlights different size or up or sinking of the moon or sun.

Conversion Examples

Obsolete units

Previously available light intensity units were:

• The old light unit, defined by a 83 g heavy wax candle with a flame height of 42 mm
• The unity of the German Association of Gas and Water professionals, DVGW, defined by a paraffin candle of 20 mm diameter at 50 mm flame height
• The Berlin light unit, defined by a spermaceti candle with flame height 44.5 mm and a consumption of 7.77 g per hour
• The Violle unit, named after the French physicist Jules Violle, 1889 defined as the luminous intensity of one square centimeter of platinum at a hardening temperature of 2042 Kelvin,
• The " bougie DECIMALE " was taken over a unit of measurement of luminous intensity and was founded in 1909 as the " International Candle " ( IK ) of Great Britain and the United States in France before 1901
• 1896 Hefner candle (HK ) was used in Germany

All units were replaced in 1942 by the "new candle " (NK ), which was renamed in Candela in 1948 and since then is considered as SI base unit for luminous intensity.

Isotropic light sources

Isotropic light sources are characterized by the independence of direction ( isotropy ) of the light intensity.

For simplicity, one can imagine the luminous flux as coming from the source beam and the light intensity as the density of these beams. An isotropic light source (eg, a point source ) that is emitted in all directions the same number of rays (energy). This means that the density of the beam ( the luminous intensity ) in the space surrounding the light source, all the same (constant).

The luminous flux of an isotropic light source of the light intensity corresponds to the integral of the light intensity over a complete sphere.

The intensity of the flame of a household candle. When:

• = 12 lm ( luminous flux of a conventional household candle. )
• = 4 · π sr ( The entire space surrounding the source. The full solid angle. )

Then:

• .

Anisotropic light sources

A light source is referred to as anisotropic when its intensity depends on the viewing direction.

Thus, a light source has the maximum light intensity and the light intensity in the viewing direction ().

Example: household candle.

Example: Lambertian Emitter.

The relationship between the viewing angle and the light intensity shown is also referred to as a radiation characteristic.

The emission of the isotropic property candle flame is spherical and the Lambertian emitter follows the mathematical function. The exact radiation characteristics of real light sources (eg flashlight ) are partly provided by the manufacturer.

The light intensity can be increased when the radiation is not evenly illuminates the surrounding space. Limiting example reflectors the light cone of a household candle (about 12,566 lm) on a steradian (1 m2 at 1 m distance ), this light source has a brightness of 12,566 cd.

If we calculate the integral for a light source with the opening angle, we obtain the flux:

Conversion of light intensity in luminous flux

A directional light intensity in candelas can be converted into the luminous flux in lumens, as the luminous flux is defined as Candela × steradian. To this end, the opening angle in steradian must be converted, this is the ratio of the area of ​​a spherical cap to the square of the radius Selbiger. Optionally the radius can be reduced or the unit circle are accepted.

Depending on the calculation of the angle above must be converted from degree measure to radians.

Link Overview of the calculation bases "Light / shine "

To better find their way within the basic light units