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Theintensity emitted by the surface area of one cm² or one m² of thelight source is known as Luminance (L). Mathematically it can bewritten as;



I= Luminous intensity

S=Area of the source surface

Luminancecan be expressed as cd/m² or cd/cm² or lm/cm² or Lambert. Thevalue of luminance indicates glare and discomfort to see a lightingsource. For example the luminance value for sun is very high or veryless for a small lamp. The quantity of the energy of the lightemitted per second in all directions is known as Luminous flux whichcan be expressed as lumen (lm). The ability to emit light into agiven direction is known as Luminous Intensity. In other words; thisintensity can be defined as the luminous flux which is radiated bythe light source in a given direction within the unit of the spatialangel and can be expressed as candela.



Ø= Luminous flux

ß=Small spatial angel

I= Luminous intensity 

Let’sdefine Luminosity. The Definition ofLuminosity states that it is the criteria that howmuch energy is coming from the object in one second. It is expressedin watts (W). Generally astronomers use another term for measurementthat is known as absolutemagnitude which is based on aratio scale.

Thevalue of luminosity for the Sun is 3.9 x 1026 Wwhile for starts, it is around5.2 x Lsun,therefore the star has 5.2 times more energy output per second of theSun.

The intensity ofthe starlight is called as apparentbrightnessor the flux oflight. It is measured in watts per square meter (W/m2).

TheLuminosity Equation for star can bewritten as;

L= (4Pid2)b


L= luminosity ofthe star

d =distance to the star

b=brightness ofthe star

TheLuminosity Distance can calculate byusing given equation;

b="L/" 4Pid2

Ord2=L/ 4Pib

Or d="√L/4Pib

Fundamentallythe ‘L’ value for a body or object like for star depends upon twofactors; the effective temperature, Teff andthe size or radius (R) of the star. The temperatureof a black body depends uponthe radiations emitted by it. As the temperature increases; the rateof radiates power output per unit surface area also increase. You canobserve this in the filament of light bulb; as it gets hotter it getsbrighter and emits more energy. The relation between the powerradiated by a black body per unit surface area and effectivetemperature, Teffis as given below;

l ∝ T4 or l = σT4 

Whereσis a constant known as Stefan-Boltzmannconstant and its value is 5.67× 10-8 Wm-2 K-4.

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