Lots of different viewpoints here. Since you ask about intensity, and this is not well defined, start with the answer of Martens, W/m2, irradiance. This would be good for a laser beam that has a well defined cross-section and impinges on a target, for example a window or filter. This type of target can dissipate energy of a given wavelength at a certain rate and will not tolerate irradiance above a critical level. In addition, eye-safety specifications are typically in units of irradiance AND radiance (W/m2-sr). Therefore, if eye-safety is your interest, you must know both irradiance which poses a hazard for the cornea and lens of the eye and radiance which poses a hazard for the retina. If you are putting light into a fiber, then you will care about the total integrated power and not the irradiance or radiance and the relevant units will be W only as answered by Ayra. Since optical communications cares about the received power, we compare the input to output using dB and often reference the power to 1mW, hence the unit dBm. Beware of dB because in many fields, it is defined as 20*log10(P) but in optical communications 10*log10(P) -- all for good reasons, but just be careful. Finally, there's the issue of radiometric vs. photometric units raised by Zahra. LED intensity often uses photometric units that measure light perceived by human eyes since LEDs are used these days as efficient lighting sources. Lasers can also be rated by photometric units, e.g., illuminance and luminance, but are almost always rated in radiometric units of irradiance and radiance. All this was stated above -- just wanted to pull it all together for you. Hope this helps.

In photometry and radiometry intensity has a different meaning:

it is the luminous or radiant power per unit solid angle. This can cause confusion in optics, where intensity can mean any of radiant intensity, luminous intensity or irradiance, depending on the background of the person using the term.

Radiance is also sometimes called intensity, especially by astronomers and astrophysicists, and in heat transfer.

The laser used for optical communication source has the unit of dBm or milliwatt power. Power per unit area is the Intensity of light.

Dear Shehan,

in addition to the two answers above please have a look at tables of the link with respect to the photometric and radiometric quantities and units:

https://en.wikipedia.org/wiki/Intensity_(physics)

Your 'intensity' in units of W/(m2*sr) is commonly named as radiance (see also the term 'radiance' in table 2 of the above link. But please also have a look at the radiance wikipedia link:

https://en.wikipedia.org/wiki/Radiance

Hi everyone

Simply, it's the well known unit of any electromagnetic wave!

Thanks a lot Prof Gerhard Martens pointing our mistake. I have corrected it. Thanks again.

In Optics, the intensity of light is likened to the power which has the unit of dB. But the light propagates through a distance in the medium usually optical fibre, we have the unit as dBm

Lots of different viewpoints here. Since you ask about intensity, and this is not well defined, start with the answer of Martens, W/m2, irradiance. This would be good for a laser beam that has a well defined cross-section and impinges on a target, for example a window or filter. This type of target can dissipate energy of a given wavelength at a certain rate and will not tolerate irradiance above a critical level. In addition, eye-safety specifications are typically in units of irradiance AND radiance (W/m2-sr). Therefore, if eye-safety is your interest, you must know both irradiance which poses a hazard for the cornea and lens of the eye and radiance which poses a hazard for the retina. If you are putting light into a fiber, then you will care about the total integrated power and not the irradiance or radiance and the relevant units will be W only as answered by Ayra. Since optical communications cares about the received power, we compare the input to output using dB and often reference the power to 1mW, hence the unit dBm. Beware of dB because in many fields, it is defined as 20*log10(P) but in optical communications 10*log10(P) -- all for good reasons, but just be careful. Finally, there's the issue of radiometric vs. photometric units raised by Zahra. LED intensity often uses photometric units that measure light perceived by human eyes since LEDs are used these days as efficient lighting sources. Lasers can also be rated by photometric units, e.g., illuminance and luminance, but are almost always rated in radiometric units of irradiance and radiance. All this was stated above -- just wanted to pull it all together for you. Hope this helps.

In answer to your question about units in Equations 1 and 3, I believe that both A and AL are in units of irradiance since they characterize beam flux which, fundamentally, is photons / time / area or W/m^2. But the paper you refer to is not specific and you could interpret the entire paper using A and AL in units of Watts -- and not be wrong in the conclusions that you draw.

I agree with Prof. Ackerman, For quasi-monochormatic sources, irradiance is the term that describes Power/Area, and it is directly related to |E|^2. Intensity is rather a photometric unit (W/m2-sr) . I think you can look at the definitions by IUPAP

Depends on the field you're working. For applications like nonlinear processes the intensity (W/cm2) is more appropriate to use than the power.

As others mentioned fiber optics work with dBm.

The SI unit of intensity (of light) is W/m2 but W/m2.sr is a more relevant quantity because it is conserved i.e. it automatically accounts for changes in W/m2 caused by beam divergence or by focusing

@David A. Ackerman

I believe you are wrong about dB - it is always a measure of power (or power density or something proportional to power). The 20 is used instead of 10 when the ratio is a ratio of voltages, for instance, and the power ratio is the square of the voltage ratio 10log(P1/P2) = 20log(V1/V2). This assumes of course that the voltages are measured in the same impedance. This also applies in other fields where the power is proportional to the square of the amplitude. Sometimes dB is used for equivalent power, not real power - such as in acoustic measurements.

Noise is sometimes measured in dBmicroV/root Hz, but this has the factor of 20 in it so is again in dB relative to the noise power in W/Hz that exists when there is 1 V/root Hz in whatever the impedance is.

Here's what I referred to. Using the venerable HP power meter for light carrying fibers, a change of 10dB corresponds to a factor of 10 in power, e.g., 10mW -> 100mW. However, in referring to dynamic range of an image sensor, a factor of 10 improvement corresponds to 20x more range. I think these two seemingly contradicting scales do match to your comment about 20 for voltage and 10 for power because, as you point out, dB always refers to power. And if so, we agree. My original comment about caution regarding 10's and 20's was simply a warning about referring dB to power but I didn't say it so clearly. Thank you for clarifying!

To avoid such boring details, I think the answer of Selden is positive and convenient

The unit of W/m2.sr relates to brightness, and for any paraxial beams, the brightness is conserved, this is the special case for Lagrange - helmholtz invariants in Geometric Optics. So, if a laser is built, it's brightness is given and is constant in propagation, and its value is sufficiently large! That's just the difference between laser and other source. The unit of W/m2 relates to intensity, it can be changed by different focusing and has more real meaning in laser engineering and applications.

Right!

sure!

For nonlinear processes the unit is W/cm2

In optics, heat transfer, astronomy and astrophysics the terms brightness, radiance and intensity are used to describe the power falling on an area from a range of directions, so the units are watts (the power), per square metre (the projected area that it falls on or is emitted from), per steradian (the solid angle it arrives from or is radiated into).

This is not the same meaning as when intensity is used in most other electromagnetism, such as microwaves. That intensity is just the power (watts) per square metre, which is the units of the Poynting vector, for instance.

The simplest way to calculate the intensity of a laser falling on some surface is = Watt/ area. Suppose you have got a CW laser and the beam is Gaussian. It is focused to a spot size d, then radius= d/2; Area = Pi.(r)^2.

Knowing the Power = 5 mW; you can easily find out the Intensity.

However, if the Laser Pulsed; then the Power = Energy/Pulse width= J/(ns).

In both the cases, Intensity can be easily calculated.

It is also referred as Irradiance.

Aslam

In the optical wave incident is Watt/cm2

in the optical wave emitted from source is the solid angle inverse sr.

thank

Regards

Raad

In most of the high power laser produced plasma experiment, laser intensity is measured in the unit of W/cm2. This is the standard unit used in scientific literature also.

In most of the high power laser produced plasma experiment, laser intensity is measured in the unit of W/cm 2. This is the standard unit used in scientific literature also.