Spherometers are small precision instruments for measuring the radius of curvature of spherical surfaces. This item is printable using any 3d printer and the printed part must be supplemented using a domed bolt or a regular bolt in combination with an acorn nut. You will also need a cheap dial gauge to take measurements. I have supplemented the STL file with an excel spreadsheet which calculates the characteristics of the lens based on the readings of the spherometer. To get this working accurately you must calibrate the distance between the domed legs B. To do this, use a pencil to mark the legs and place the spherometer on a piece of paper. The legs will leave lead marks which can be measured using a set of calipers. The distance B is the average of all 3 readings.