How Do Brightness and Luminosity Differ in Energy Terms?

[sancharsharma]

I cannot get a feel of what is luminosity physically...

Can anyone explain me the difference between energy transfer in terms of brightness (which is measured in Watts) and luminosity (which requires a new definition of units, candela)??

 

[pallidin]

Try not to be confused when dealing with Watts.
It's easy to think that a 100 watt light bulb is brighter than a 60 watt bulb, but it's only true for bulbs of the same type. A 25 watt bulb of a different type can match or even exceed the brightness of the original 100 watt bulb.

A "watt" is a relationship of voltage and current. It should not be considered a measure of brightness.

 

[Andy Resnick]

Radiometry has (possibly) the most horrid system of units in all of science, except for Photometry. "Brightness" is a photometric quantity (how your vision responds) related to the radiometric quantity 'radiance'. Radiance has units of W/sr*m^2 and is the most basic way to describe a source: how much power is emitted (W/m^2) and in what direction that radiation propagates. Radiance is conserved for all optical systems that do not have absorption present.

The fundamental relationship for radiative energy transport takes into account not only the amount of radiated power, but also the relative orientations of the two surfaces (which is why there's steradians floating around)

A candela (Cd) is a radiometric unit of "radiant intensity" has units of W/sr: in what direction the radiated power propagates. "Radiant flux" is a radiometric quantity with units of W: how much energy flows through a surface per unit time.

Edit: photometric units, like brightness and luminance, take into account the fact that your eye does not respond to all wavelengths the same- you are much more sensitive to green than far-red (or far-blue). There's a 'standard curve', the luminosity function, that allows you to convert from radiometric to photometric units, but it's far more difficult to convert a photometric unit to a radiometric unit.

Hope this helps...

 

[sancharsharma]

thankyou... but I am still not able to grasp that why is candela a fundamental unit if it can be derived from power and angles. explain please...

 

[PJC01]

Luminosity is a measure of the total amount of energy that a star or other object emits per unit time. It is a physical property that is related to the brightness of an object, but they are not the same thing. Brightness is a measure of the amount of light that reaches our eyes from an object, and it is affected by both the luminosity of the object and its distance from us. Luminosity, on the other hand, is an intrinsic property of the object that does not depend on our observation or perception of it.

To better understand the difference between energy transfer in terms of brightness and luminosity, it is helpful to think of an analogy with a light bulb. The brightness of a light bulb is like the intensity of light that we see when we look at it, while the luminosity is like the total amount of energy the bulb consumes and emits in the form of light. A 100-watt light bulb and a 60-watt light bulb may have the same brightness when viewed from the same distance, but the 100-watt bulb has a higher luminosity because it is consuming and emitting more energy.

In terms of units, brightness is typically measured in watts (or sometimes in lumens, which is a measure of the amount of light that reaches our eyes), while luminosity is measured in watts or solar luminosities (the amount of energy emitted by the Sun). Candela, mentioned in the question, is actually a unit of luminous intensity, which is a measure of how bright a light appears to the human eye. It is not a unit used to measure luminosity.

In summary, luminosity is a physical property that describes the total amount of energy emitted by an object, while brightness is a measure of the amount of light that reaches our eyes. They are related, but not the same thing, and have different units of measurement.