# Definitions ;photometry

1. Feb 26, 2006

### sapta

1. What is luminous flux and what is luminous intensity? How are they related?

2. What is the definition of lumen (unit of luminous flux)? How is it related with candela?

There are a lot of definitions in my books but I am a bit confused, particularly over the definition of lumen. One definition says that the amount of light emitted by a source of luminous intensity 1 cd and passing through a unit solid angle is 1 lumen and hence a source of 1 cd luminous intensity emits luminous flux of 4pi lm. But luminous flux is the total amount of light emitted by a source per second and luminous intensity is luminous flux per solid angle, right? So, aren’t the definitions of the quantity luminous flux and its unit lumen self-contradictory?

2. Feb 27, 2006

### nicp

As you have said, flux is the total amount of light emitted by a source per second. Light is energy, so flux is power, and so the best way to measure flux is using watts.
Lumen is an other unit, which corresponds to a certain number of watts depending on wavelenght. For instance at 550 nm wavelenghts, 1 watt is equivalent to 638 lumen. There is a table of conversion from lumen to watts and viceversa.
So now it should be clear what is a lumen.
Definition of candela follows from that of lumen, as you might already have understood. A source is said to emit 1 cd if it emits 1 lumen per solid angle unit.
If you still have dubts, please tell me.

3. Feb 27, 2006

### sapta

hmm,so flux is power. never thought it that way.Okay,from now on I shall define lumen in terms of watt.

But I am still confused over the definitions I gave.Take a look at the following relations that I have deduced from the first two posts in this thread-

1)Luminous flux=4pi >< luminous intensity[as the latter is flux per solid angle]

2) 1 cd=4pi >< 1 lm [as the latter is measured per unit solid angle]

Isn't this confusing? It seems candela should have been the unit of flux and lumen should have been the unit of intensityfor instance if I define candela as luminous intensity of a source emitting 4pi lm flux, then 1 cd=1 lm
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4. Feb 28, 2006

### nicp

Relation 1) is correct. The total flux emitted by a source is 4pi times the intensity of that source.
So a source of intensity 1 cd emits total 4pi lumen, 1 lm for each solid angle unit.
In a certian way we could write 1 cd = 4pi lm, but it's not completely correct because cd and lm are not two units for measuring the same thing: they meassure two different things flux and intensity so there might not be an equivalence relation.

5. Feb 28, 2006

### sapta

Okay,1 lm is emitted each second for each solid angle,but isn't that intensity (which is light emitted per second through a unit solid angle)?.so, how can lumen be a unit of flux?candela is measured for 4pi sd solid angle,so candela should be the unit of flux

6. Mar 1, 2006

### nicp

Flux is an amount of light. We can say a light beam has a certain flux, through a window pass a certain flux, on a given surface arrive a certain flux, etc.
Intensity is a characteristic of a source: a source which emits a certain flux in unit solid angle is said to have a certain intensity.

7. Apr 2, 2006

### sapta

I understand(to some extent).The confusion was probably arising because of the definition of luminous intensity given in my books which say-

The amount of light passing through a unit solid angle per second is the intensity of the source emitting the light.

I think the part in italics should be changed to assigns a certain intensity to the source emitting the light(as you have said)