Light increase of the Commander of the Faithful (pbuh), which is qualitatively of the kind and accompanied by the Holy Prophet’s light increase, is among the usages of the term “light” in the Islamic written heritage. Data for this have been presented in the two large scale fields of reports and its scholars’ approaches. Reports concerning this pointـalso wellـknown as the hadith of lightـmay be seen in many authentic written Imami texts (though their authors followed various hadith and theological movements). Documents for this hadith may be traced back, through eighteen ways, to the Holy Prophet (PBUH) and some Infallible Imams, and according to the criteria to evaluate authenticity of a hadith, this one is classified under the most authentic ones, “hadiths with a successive chain of narrators”. In the Imami scholars’ approach to the aboveـ mentioned heritage, in addition to agreement with reports, some functions of it have been recorded among which we may mention historical, theological, and interpretive ones.