A perfect white is an object that has a reflectance equal to unity throughout the visible spectrum. And also, it should be isotrophic , i.e. it should reflect light in all directions uniformly. Then k is called percent luminance factor . If k is chosen such that Y value of a perfect white object is , it is called luminance factor values . A colorimeter is an instrument that can provide direct measurement of the CIE XYZ tristimulus values of a color stimuli.
Y corresponds to luminance which is the measurement of the perception of brightness .
If X and Z are equal, stimuli with higher Y will look brighter.