Measurement of Light

If we want to know more about light, we should start know the basic metrics to describe ‘tlight’. Let’s make it as simple as we can.
There are different units for measuring quantity of light and it sometimes can get complicated.

A candle the original light source is emitting an intensity to all directions. That intensity is one Candela (cd).  The amount of light what you get from in any and all directions. This unit of intensity is actually used to describe and create base for all the light sources.

Luminous flux
is defined as the time rate of flow of light. The amount of total luminous energy – light emitted from a source in unit time is measured in lumen.  It does not imply direction, distance, or intensity. Simply the total light output of a source.   The more lumens the fixture or the source gives off, more light it emits.

The force of the
luminous flux in a specified direction is luminous intensity. Luminous intensity measures the strength or (we rather to called it as) “punch” of a light source in a specific direction  Very often, a light source will have different intensities when measured at different directions.

Measured in candelas (lumen / stredian), the luminous intensity of a light source is commonly shown on a candlepower distribution curve.   The polar axis on this chart graphically indicates the luminous intensity in any given direction from the source position.

You can find most of the successful luminaire producers share their products candlepower distribution curves for you to understand the basic light distribution characteristics of the luminaire you are planning to use. An experienced eye can understand how light will behave with looking to these polar distribution intensities.  

The intensity of light falling to a surface in unit time is called
Lux, in which is also called as illuminance. Illuminance is the density of incident on a surface. Simply how much light is hitting a surface. It changes according to the distance and angle of the surface.  It can be measured inexpensively by a lightmeter called luxmeter.
We can easily measure amount of light falling to a surface.

Density of light varies according to the distance and the angle to the square of the distance.
That is called inverse square law.

S represents the light source, while r represents the measured points.

We do not see illuminance when a surface is illuminated we see its brightness defined as
Luminance. Luminance, is measured brightness which is defined as the intensity of visible brightness of a source or surface in the direction of the observer, divided by the area of the source or surface seen. The specific way the light comes off of a surface. It allows us to see things. The unit is candela/squaremeter .

The perceived brightness of an object depends on the light reflected from itself and its background, brightness adaptation. Its brightness relative to other objects appears the same in bright light or dark.



Korhan Şişman

PLANLUX is an architectural lighting design studio, based in Istanbul, with its IALD member designers, search and study the architectural details of projects in deep to use light and shade to create attractive and unique atmospheres.