The primary chromophore for the CO 2 lasers and Er:YAG lasers is water. Whenever using a laser, consideration should be given to other competitive chromophores that may absorb the wavelength. These competing chromophores may give additional benefit or create additional risks associated with using a particular laser wavelength.For example, at a wavelength of 532 nm, there is absorption of laser energy by both oxyhemoglobin and melanin. Thus, this type of laser can affect both vascular and pigmented lesions. Therefore, in patients with higher Fitzpatrick skin types there is an increased risk of developing hypopigmentation if treated with this laser wavelength.The possibility of competing chromophores should be considered whenever a laser is being selected. This should also be considered whenever using an IPL device, which can produce a spectrum of wavelengths. The spectrum of emitted wavelengths will depend on the light source as well as any associated filters.Examples in which competing chromophores can be advantageous are the current hair reduction lasers, which are designed to exert their effect on melanin, the target chromophore. These lasers may also be absorbed by hemoglobin, which is a competing chromophore. This absorption by hemoglobin, the competing chromophore, can also result in injury to the blood vessels supplying the hair follicle, which is desirable.Ninety percent of the epidermis is water. Consequently, water is the primary chromophore for the current resurfacing lasers, including the Er:YAG and CO 2 . During resurfacing treatments, the intercellular water absorbs laser energy and instantly boils or vaporizes. The amount of energy that a laser delivers to the tissue and the time over which it is delivered determines the amount of tissue vaporized. For skin resurfacing, the objective is to vaporize the primary chromophore, water, while transferring a limited amount of heat to the surrounding collagen and other structures. This is because type I collagen is extremely heat-sensitive and becomes denatured at 60°C to 70°C. These lasers are designed to avoid excessive thermal injury to the collagen as this can produce scarring.The fluence of a laser represents the amount of energy (joules) that is applied to the surface area of tissue (centimeter squared). Therefore, fluence is expressed as J/cm 2 . For CO 2 lasers, 0.04 J/mm 2 is the critical energy needed to overcome the tissue ablation threshold. For skin resurfacing, this is can be accomplished with 250 mJ per laser pulse using a 3-mm spot size. After each pulse, the tissue is allowed to cool before the delivery of the next pulse. The tissue thermal relaxation time is the amount of time necessary between pulses for complete cooling to occur. The pulse duration is the time the tissue is actually lasered.