What type of illumination is primarily used for viewing the edge light of soft contact lenses?

The correct choice is conical beam illumination, which is specifically designed for assessing the edge of soft contact lenses. This type of illumination provides a focused light that helps in highlighting the contour and surface of the lens, making it easier to assess parameters such as thickness, edge shape, and overall fit on the eye.

Using a conical beam allows practitioners to observe how the lens interacts with the tear film at the edges, offering insights into potential issues like lens movement, edge clearance, or even deposition of material. This focused, circular beam enhances visibility of the lens' edges compared to other forms of illumination, making it the preferred method for detailed evaluation in a clinical setting.

Other illumination techniques, while useful in their respective contexts, do not provide the same level of precision for edge analysis of soft contact lenses. For instance, direct illumination lights the entire surface of the lens, which can mask edge details. Cylindrical light is more suited for objects requiring illumination from specific angles, and parallel beam illumination offers a broader viewing area but lacks the targeted focus necessary for detailed edge examination. Thus, conical beam illumination stands out as the most suitable option for this particular application.