How is the intensity of light determined?

Question

And what is the difference with light radiation densitiy ?

what are lumens and candlepowers how is it related to the number of photons that falls on a surface?So howmany photon come out of a candle?

Answer 1

Intensity is a measure of the time-averaged energy flux. To find the intensity of light, take the energy density (that is, the energy per unit volume) and multiply it by the velocity at which the energy is moving. The resulting vector has the units of power divided by area (i.e. watt/m²). So here the "light radiation density" you referred to is the energy density or energy per unit volume.

Lumens is luminous flux or luminous power, which is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to represent the sensitivity of the human eye.

Candela is the unit of luminous intensity, which is a measure of the wavelength-weighted power emitted by a light source in a particular direction, and also adjusted to represent the sensitivity of the human eye.

So lumens is related to candelas in that they are both SI photometry units, but lumens measures flux or power, and candela measures intensity.

So if you wish to determine the number of photons that falls on a surface (from a point source), or how many photon come out of a candle, then you will need to measure radian flux or radian intensity.

N = (IAt)/E

where N = Number of photons, I = Radiant Intensity, A = Surface Area, t = time duration, and E is the energy of a single photon, which by the way depends on the phton's frequency. So if the light source emits photons of all different frequencies, then we will need to use an average.

Answer 2

The SI unit for the intensity of light is the candela (abbreviated cd).

1 cd is defined as "the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian".
A steradian is similar to the radian as a type of unit-less unit (its own units, m^2/m^2, cancel to 1). A steradian is a measure of a solid (3D) angle.

So the intensity of light is measured by, essentially, the amount of light energy in a given area.

Answer 3

The unit of luminous intensity is candela.

1 candela is the unit of energy per second per steradian = 1.464 x10^(-3) joules per( second x steradian) by a wave of frequency 540 x10 ^ 12 hertz.

The energy of one of photon, having this frequency, is 3.578 x10 ^ (-19) joule

Therefore the number of photons emitted is {1.464 x10^(-3) / 3.578 x10 ^ (-19)} = 4. X10 ^15 photons per (second x steradian)

Intensity is determined by the formula I = N h n.

N the total number of photon, h is the Planck’s constant, and n is the frequency of the light.

Answer 4

The intensity of light is measured in lumens

See http://en.wikipedia.org/wiki/Lumen_%28unit%29

Answer 5

Intensity is measured by CAndle Power.

Answer 6

Military Grade Tactical Flashlight : http://FlashLight.uzaev.com/?vhOm

Answer 7

by looking at it

Answer 8

by its magnitude.