Which illumination pattern is fired perpendicular to the observer target line?

The choice of the illumination pattern that is fired perpendicular to the observer target line is indeed lateral spread. This method is designed to effectively illuminate an area by deploying rounds in a way that expands laterally, rather than along the line of sight toward the target. By firing laterally, the illumination created can enhance visibility across a wider area and provide greater situational awareness for the observer.

In contrast, other patterns such as range spread or multi-gun illumination might focus on depth or coverage along the target line, without achieving the broad lateral effects found in lateral spread. This specific characteristic of the lateral spread is crucial for operations where illumination needs to cover more area across the battlefield, particularly for engagements or reconnaissance where visibility is paramount.