As spot size decreases, fluency must ______.

When the spot size of a laser decreases, fluency must increase to maintain the same energy density delivered to the target. Fluency is defined as the energy per unit area, usually represented in joules per square centimeter (J/cm²). The relationship between spot size and fluency is crucial because a smaller spot means the same amount of energy is concentrated over a smaller area, leading to a higher energy density.

For example, if a laser delivers a fixed amount of energy but the area over which it is delivered (the spot size) becomes smaller, the energy per unit area increases. This higher fluency can enhance the laser's efficacy, especially in applications like laser surgery or material processing, where precision and energy concentration are essential.

Larger fluency can be beneficial in achieving the desired effect on a material or tissue, such as in ablation or cutting. Therefore, to achieve the same operational effects with a smaller spot size, it's necessary to increase the fluency to ensure that the energy remains sufficiently high to complete the desired task effectively.