The argon wavelength, provided through a 1 mm spot size and a pulse duration of 50 -100 msec, would be predicted to overheat the epidermis of the leg. This would occur due to melanin interaction and a long period of heating a focal portion of a small blood vessel so that non-specific thermal damage would occur to the surrounding collagen as well. This explains why electrocautery is still employed on facial vessels. While argon laser is a blue-green laser emitting just above the ultraviolet range which melanin favors, longer wavelength lasers have been developed in the hopes of achieving greater selectivity for hemoglobin. The longer the wavelength, the deeper the penetration.
Deeper vessels require a longer wavelength to allow penetration.
However, large diameter vessels also require a longer pulse duration to effectively thermocoagulate the entire vessel diameter from front to back wall, this principle being called thermokinetic selectivity. Optimal pulse durations have been calculated for various diameter blood vessels. It is best to examine each laser proceeding from lowest to highest wavelength. Its relatively short wavelength, combined with a spot size of 1mm, prevents its penetration much beyond 0.5 mm. When the patient is pigmented or tanned, epidermal melanin will selectively absorb the laser energy preventing penetration below the epidermis.