The holmium laser, which operates in a wavelength of 2.1 microns, is highly absorbed by water and has a shallow penetration depth of approximately 0.4 mm. As a pulsed mode laser, the Holmium reduces deeper thermal effects and charring which are more prevalent in Continues Wave lasers (e.g. Thulium, KTP).
In statistical theory, covariance is a measure of how much two random variables change together. In other words, covariance is a measure of the strength of the correlation between two random variables. Also, it can be considered as a generalization of the concept of variance of two random variables.
This scaling makes correlation invariant to changes in scale of the original variables, (which Akavall points out and gives an example of, +1). The scaling constant is the product of the standard deviations of the two variables. (2) If two variables are independent, their covariance is $0$. But, having a covariance of $0$ does not imply the variables are independent. This figure (from Wikipedia)