The instruments used to detect the aperture of a high-precision smooth ring gauge in practical work can be divided into two categories: one is a dedicated high-precision aperture measuring instrument; the other is a general-purpose measuring instrument. They have their own advantages and each has its own limitations. Common special aperture measuring instruments include: image point meter, small aperture measuring instrument, Abbey hole length meter, etc. These instruments are not conducive to widespread application in production due to high price and harsh conditions.