2.6.1. Power

Power is the parameter that influences the most the photocoagulation effect. Selecting a certain power is relative, as it depends on individual factors: retinal pigmentation, edema, pre-retinal fibrosis, the presence of vitreous hemorrhage. Therefore, the key element in choosing a certain laser power is clinical (retinal reaction), not technical (mW). Usually, treatment is initiated with low powers (150–200 mW) in order to test retinal reaction, never on pigmented retina. If there is no visible mark, power is increased with 20–50 mW, until the obtainment of the desired effect: whitish lesion. When high powers are used, the lesion appears gray with well-defined margins and sometimes in its center a vapor stream is emitted. This effect can be identified when passing from a thicker to a thinner retina, for example from the posterior pole toward the periphery, without reducing the power accordingly. During retinal photocoagulation, power must be varied in order to obtain the appropriate tissue reaction [5].

### 2.6.2. Exposure time

Exposure time can vary between 50 ms and 1 s. At low exposure times, the effect in the tissue is cylindrical: the area of the lesion is the same at the superficial (RPE) and deep (choroid) levels. However, at low exposure times, high powers are required in order to obtain a clinical effect. This is associated with the risk of explosive effects. Usually, exposure times between 200 and 500 ms are preferred [5].
