A improved confocal optical detector is proposed for measurement of emission efficiency of the infrared up-conversion phosphor ink. By detecting up-conversion phosphor ink with original and improved confocal optical systems, advantages of improved system are analyzed and the influence of different sizes of focusing dots to the up-conversion efficiency are discussed.The reliability of detecting the IR inks with different contents has been given and discussed. This kind of IR ink can be used as the printing ink of the hidden bar-code.