Measurement of Absolute Reactor Flux and Spectrum at Daya Bay

Understanding of the absolute reactor flux has a strong impact on short baseline reactor experiments. Measurement of the absolute flux at Daya Bay Near detector gives a precise test on reactor anomaly, that has been seen from many experiments. While Daya Bay's $\theta_{13}$ measurement utilizes relative Far to Near ratio as well as spectrum ratio, the absolute spectrum shape provides test on our knowledge of reactor flux prediction. Various studies were performed to catch unaccounted detector or background effect on the spectrum measurement.