On Fri, 26 Aug 2016 04:05:56 -0700 (PDT), fred
wrote:

I want to measure the width of a groove which is only about 3.5mm wide and 1-3 mm deep.
Ordinary Mitutoyo calipers with pointed tips are not much good for this application due to the shallow nature of the groove.

Use a measuring magnifier such as
https://www.emsdiasum.com/microscopy...measuring.aspx