The slit lamp illumination that provides an overall view of the cornea but limits detail is:

Diffused illumination is a technique used during slit lamp examinations to provide a broad, overall view of the cornea and the anterior segment of the eye. This method involves using a wide beam of light that scatters as it hits the cornea, which allows the practitioner to see general characteristics and any surface irregularities. However, because of this scattering effect, it limits the ability to see fine details or specific features of the cornea, such as the layering or fine pathology that may be present.

In contrast, techniques like direct illumination provide a more focused beam of light, enabling a detailed view of specific areas, whereas retro-illumination and specular reflection are utilized to assess the corneal layers or the endothelium. These methods enhance detail but do not offer the broad overview that diffused illumination provides. Therefore, the answer emphasizing the ability to observe the overall corneal structure while sacrificing fine detail is accurately identified as diffused illumination.