IIT JEE , AIEEE Forum - Physics , Optics - optics: can you explain luminouus intensity,luminous flux,illuminance,luminance

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optics: can you explain luminouus intensity,luminous flux,illuminance,luminance

edison

These are photometric quantities, defined as follows

Photometric Quantities:

Radiant flux is usually measured in watts.
Luminous flux - the fundamental quantity measuring the rate of flow of radiant energy modified for its effectiveness in creating the sensation of seeing i.e. Luminous flux = radiant flux x relevant spectral sensitivity of visual system. In SI units, luminous flux is measured in
- lumens (lm). Luminous flux is useful for describing the total light output of light sources. However, to describe the distribution of light from a source, luminous intensity is used.
Luminous intensity - the luminous flux emitted per unit solid angle in a specified direction. The measure is the
- candela (cd) which is equivalent to the lumens per steradian (lumens steradian -1). Formally the candela is defined as "the luminous intensity in a given direction of a source emitting monochromatic radiation at 555 nm of which the radiant intensity is 1/683 watts steradian-1." Both these have AREA measures associated with them.
Illuminance - the luminous flux per unit projectedarea of a surface in a given direction (luminous flux/unit solid angle/unit area) measured in
- candela metre2 (cd m2) (1 footlambert = 3.43 candela m-2)

edison

Luminance/Reflectance/Apostilbs

With a non-luminous surface e.g. a wall, what the eye sees - the brightness or luminance of the surface - depends on the reflectance i.e. the ratio of reflected light to incident light. With an illuminance of 500 lux and a reflectance of 0.4 the luminance of the surface will be 200 apostilbs.

Illuminance (lux) x reflectance = luminance (apostilbs).
Apostilbs is not an SI unit. To convert this to SI (candelas m-2) divide by pi (or multiply by 0.318).
If the Munsell "Value" reference of a color is known the reflectance can be approximately calculated using Reflectance = V(V - 1) where V = value.
If Munsell "value" is 6, reflectance = 6 x 5 = 30% = .3.
Since in most rooms the different surfaces will have different colors, they will reflect differing amounts of light and this will affect the distribution of light in the room.

Reflectance

For a perfectly diffusely reflecting surface, the ratio of the reflected luminous flux to the incident luminous flux is the reflectance.
Luminance = illuminance x reflectance / pi
Reflectance = illuminance / luminance
When the surface is not perfectly diffusely reflecting, reflectance is replaced by a
luminance factor
Luminance factor is the ratio of the luminance of a surface viewed from a particular position and lit in a specified way to the luminance of a diffusely reflecting white surface viewed from the same direction and lit in the same way. Here Luminance = illuminance x luminance factor / pi
In lighting practice
illuminance
and
luminance
are most frequently used to characterise the effect of lighting.

edison

SI photometry units
Quantity	Symbol	SI unit	Abbr.	Notes
Luminous energy	Q_v	lumen second	lm·s	units are sometimes called talbots
Luminous flux	F	lumen (= cd·sr)	lm	also called luminous power
Luminous intensity	I_v	candela (= lm/sr)	cd	an SI base unit
Luminance	L_v	candela per square metre	cd/m²	units are sometimes called nits
Illuminance	E_v	lux (= lm/m²)	lx	Used for light incident on a surface
Luminous emittance	M_v	lux (= lm/m²)	lx	Used for light emitted from a surface
Luminous efficacy		lumen per watt	lm/W	ratio of luminous flux to radiant flux; maximum possible is 683.002 lm/W

edison

The response of a typical human eye to light

Ask & Discuss Questions with Community & Experts

Luminance/Reflectance/Apostilbs

Reflectance

luminance factor

illuminance

luminance