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.
covariance(X,Y)/(Standard Deviation(X)*Standard Deviation(Y)). In other words, the correlation is proportional to the the covariance of the two variables. The divisor in the equation acts has a scaling effect on the covariance so that the resulting correlation will lie between -1 and +1.