When calibrating a lens clock, what surface should be measured to obtain a zero reading?

To obtain a zero reading when calibrating a lens clock, a flat surface must be measured. A lens clock is commonly used to measure the radii of curvature of lens surfaces, and it operates based on the principle of pressure exerted by its curved measuring tip against the surface being measured.

When the measuring tip is placed on a flat surface, it creates no curvature for the clock to gauge. This allows the clock to read zero because there is no radius of curvature to measure. Conversely, if the clock were placed on convex or concave surfaces, it would register a positive or negative curvature, indicating that the surface being measured has a radius which is not zero. A spherical surface, like convex or concave, would similarly yield a reading that does not equal zero. Therefore, measuring a flat surface is essential for correctly calibrating the lens clock to achieve an accurate zero reference point.