The optical zone width of a GP lens can be assessed with a:

The optical zone width of a gas-permeable (GP) lens can indeed be assessed using a magnifier with a millimeter grid. This tool allows practitioners to closely examine the lens and measure the distance across the optical zone accurately. By placing the lens under the magnifier, one can easily visualize the edges of the optical zone and read off the width directly using the grid, providing a straightforward and effective method for this measurement.

Other methods, such as using a radiuscope, are better suited for measuring the curvature and radius of the lens rather than the width of the optical zone. A topogometer is typically used for mapping the surface contours of the cornea or lens, making it less applicable for measuring the optical zone. The diameter v-gauge is designed for measuring the overall diameter of the lens rather than focusing on the specific optical zone width. Thus, the magnifier with a millimeter grid stands out as the most appropriate tool for this specific measurement.