Well, you want W/m². but you seem to have only a number of photons.

Obviously you need two things:

- the surface of the spot, to obtain an intensity in xxx/m²

- to convert photon into W, you need to know the amount of photon which arrive per unit of time. Then each one of your photon has an energy, in eV, or in J. With the time normalization, J becomes W.

Is that clear ?

Light intensity can be  express in J/c. cm2  or W/cm2.. Therefore, when You measure the emission  of CW - laser You know a beam power. To take intensity connected with this power You should determine form-factor of a beam or, other words, the spatial distribution of the power. Impossible due to diffraction  homogeneous distribution could be give You chance to find intensity after division power on beam cross-section. The beam cross-section You will determine with a modern camera (but be careful not to fire last). Very often laser operates on lowest axially symmetrical main mode with the same Gauss type spatial distribution in near and far diffraction zone. In this case, You get possibility calculate intensity just on average, for example, on half maximum. If You need quite good homogeneous intensity You should use small beam aperture to limit the central part of the Gauss beam and to determine its  intensity to measure separately  its  power and diameter. In typical many mode laser operation there is rather nonhomogeneous intensity distribution. It means that precision determination intensity is getting difficult for the similar cases.

In spite of circumstances, I wish any success.

Other than photon counts, you also need to know the following parameters: 1)wavelength of the photons; 2) photon counts per second; 3) S= area of illumination. The photon energy can use this equation to calculate: E=hc/lambda. Here, h is Planck constant, c is the speed of light, and lambda is the wavelength of the photon.  hc=12400 ev*A, A is a length unit=0.1nm.  Once have E, with number of photon per second N and area of illumination S, the quantity you want to calculate will be: a*E*N/S, a will be the conversion parameter between W and ev/second.  Hope this explains.

I Thank you everyone for your great answers.. Special thanks to Muhammad Aslam Baig...  Have a good day...... Cheers..

Dear Shivaraj Maidur 

Have a nice time and good luck

Really Dr. Muhammad Aslam Baig gave good elaboration of how to obtain the the laser intensity. This is what we do. In our practical works, the used laser has a spot of  diameters 4.6 mm. The powers of the laser source were 10 Watt., so the peak power energy density was about 60 W/cm2.

May this help you.

Thanks

Dear Sam Keramati!

The "fluence" is given by N/(effective spot area),

where N is the number of particles.

Dear Sam Keramati!

Perhaps I am very old, since the use, McGraw-Hill DICTIONARY OF SCIENTIFIC AND TECHNICAL TERMS, Second Edition, Daniel N, Lapedes, 1974, p-622.

The diameter at the focal point will be:

d = (4/Pi)(laser wavelength)(lens focal length/D)

But there is another answer for spot radius..

w0 : Gaussian beam spot radius at focus (half width at 1/e2 of maximum of the irradiance)

Which one is suitable or correct.

Kindly tell us.. If possible give us an example calculation also..

I found that diameter of the focal spot on the surface is 

d=2(focal length)(wavelength)/D..

Then which will be more experimentally correct?

I derived an intensity of light as photon by I=P^2/(16 alpha h c^2), alpha is the fine structure constant, h Planck constant, c speed of light.

There is a simple conversion formula from the number of photons per second Iph to light intensity P in w/m^2 as follows:

P= Iph. Eph/ A

where Eph is the photon energy in Joule and A is the area of the laser beam.

Best wishes

Do it using pointing vector

Please read the book

Electromagnetics

by

Kraus

from

Ohio state universiy

A correction to Abdelhalim abdelnaby Zekry .

Eph is the photon energy in Joules.

Iph. Eph is the beam power in watts.

Thank you Malcolm, the unit is corrected.

I don't think that scan speed or spot beam diameter has to do anything with number of pulses. It is inside electronics which produces the laser pulse. Usually pulse frequency gives the number of pulses coming out from the laser source per unit time. Now you can relate it with scanning speed to find out time of contact and then count number of pulses. Spot beam diameter can help you to determine pulse overlapping percentage.

Dear Prof. Baig,

I have some laser unit conversion calculation below for your check and requires your endorsement if it is correct. The detector I've is power meter from Thorlabs (S370) which calculate the power or irradiance. I ran my pulsed laser (pulse width 5 ns) at 532 nm at 10 Hz and got the power of 650 mW with the laser beam size is 6.5 mm in diameter. I wanted to calculate the fluence, energy per pulse (Not the peak power density) and her I go:

Power = 650 mW = 0.65 W;

Area of the laser beam = Pi x (6.5/2*10)^2 = 0.33 cm^2

Irradiance (Power Density) = 0.65/0.33 = 1.97 W/cm^2

Fluence (Energy Density)=1.97( J/s/cm^2)/(10 s^-1)=0.197 J/cm^2=197 mJ/cm^2

Energy Per Pulse = 0.65/10= 65 mJ/pulse.

Is the calculation correct?

Mamun

W/M2 IS CALLED energy density and can be calculated by no. of photons per second.

P=E*photons/second

I = P hfv

Where

I = light intensity in W/m2

P= photon density in photons/ m3

h= plank's constant

v= velocity of light

hello everyone,

Can anyone help me with this question?

How to calculate the power of photons falling on a detector which is kept at 2cm distance from a 1mm circular light source?

You might probably need the luminance figure of the light source and the surface area of the detector. The inverse law would be helpful in calculating the incident power.