A key requirement on color printers is maintaining accurate color reproduction in output images. Reproducing the same colors more precisely on the same printer or between different printers requires a color calibration process involving precise measurement of the output sheet's chromaticity and subsequent chromaticity adjustments. This sort of color calibration has conventionally been a complicated offline process, involving placing and measuring the output sheet in a standalone colorimeter and then editing and adjusting print data based on the colorimetric data. Usability improvement and space saving in this process have been needed. And with the growth seen in the digital color press market in recent years, expectation for the inclusion of more accurate colorimetric sensors and more sophisticated automatic color matching are increasing. In response, we developed a spectrophotometric sensor with excellent colorimetric precision that is small enough to fit inside a color printer. In this paper, we report on the optical technology that was a key to reducing the size of spectrophotometric sensor and examples of applications using the digital press device, imagePRESS C10000VP, which makes use of the developed sensor.