Dear Sir.

 In the world there is no uniform methods for measuring laser damage thresholds, which often leads to misunderstandings. For example, in the US there is one approach to measuring the damage thresholds of optical elements (http://www.regnum.ru/news/polit/1871217.html; https://play.google.com/books/reader?id=PJn-UrqMcU0C&printsec=frontcover&output=reader&hl=ru&pg=GBS.PA5), in Russia - other (OST11.070.802.80 "Materials and optical elements of solid-state lasers. The method of determining   the laserdamage thresholds").

Measurement of the damage threshold is greatly complicated by the presence of two effects:

The statistical nature of the destruction, caused mainly by the presence of absorbing laser radiation defects.

The dependence of the damage threshold of the diameter of the laser spot.

The cumulative nature of the destruction, meaning that the damage threshold with a single pulse is higher than when irradiated by a series of pulses.

However, all these methods have much in common, and they give similar results.

 In short, one of the possible procedures for measuring the damage threshold is reduced to the following sequence of actions. The point in question is irradiated by a laser pulse (or a series of pulses). If there was no failure, then the power level is raised in a pulse and so on until reaching the spark indicating the existence of destruction. Then move to the next point on the sample, and the procedure is repeated. The quantity of the studied points is set by requirements of statistical reliability. The size of the laser spot on the sample is chosen so large that during this one would get destruction. After that, the dependence of the laser damage threshold of the number of the test on the sample points. Next, again, there is a certain arbitrariness in determining the threshold. In most cases, the damage threshold for taking power density of laser damage at which 50% of the points has been a breakdown (in some cases it may be 99%, these figures must be indicated).

So that the measurement process - it is a complex process that requires a statistical approach in the treatment of the data indicating the conditions under which the measurements were made.

As I said earlier, there are other ways. For example, at a fixed power density required examines the number of dots and the number of fixing destruction. Then increases (decreases) the laser power and repeat the procedure for the same number of other points, and so on (method 1-on- 1 or N - on - 1).

And more. In the measurements, the damage threshold is extremely important to control the spatial distribution of the laser radiation (usually - a TEM00 mode, or Gaussian beam), the duration of the laser pulse, the spot diameter on the sample and incident on the sample capacity (based on reflection).

In the measurements, the damage threshold is extremely important to control the spatial distribution of the laser radiation (usually - a TEM00 mode, or Gaussian beam), the duration of the laser pulse, the spot diameter on the sample and incident on the sample capacity (based on reflection).  

As for your second question, it must be said that in general, when the threshold power density devastated area always has an irregular shape) is associated with the presence in it of microdefects absorbing laser radiation). Only in the case of a homogeneous absorption devastated area will follow the shape of the laser spot (eg, circle, if the beam is Gaussian).

Best Regards, Leonid.

...or you can simply characterize your material pre and post irradiation, i.e. SEM/EDX, XPS, XAFs... You should be able to easily see any physical damage due to, for instance, ablation (ripple patterning of the surface-optically visible). If damage is not quite evident (i.e. chemical changes) XPS, XAFS and XRD, for instance, should be helpful and straightforward. Do not overthink it.

In my previous answer, I missed one important detail. I mean the registration threshold of damage. Generally speaking, it is necessary to introduce the concept of a threshold of damage.  And different researchers damage threshold understand differently. Some consider laser damage is irreversible changes in the material, which led to the violation of his integrity. Thus the material may lose its functionality partially or fully. Others believe that the damage threshold - is generally any changes that have occurred with the substance. But this material can perform its functionality. Therefore, in the works usually indicate what is meant by the term "damage threshold" and the method of its registration. So it is not so simple.

Of course, the presence and knowledge of the standards for measuring the damage thresholds greatly simplifies the process of measurement and makes it possible to compare the results obtained by different researchers in different conditions.

Best regards, Leonid.

take the laser into consideration, while the sample is corrected factor!

and the laser heat effect is the main consideration.

so you can combined with TG-DSC with laser heat effect..