  We examine a method to detect the light speed variation from gamma ray burst
data observed by the Fermi Gamma-ray Space Telescope (FGST). We suggest new
criteria to determine the characteristic time for low energy photons by the
energy curve and the average energy curve, and obtain similar results compared
with those from the light curve. We offer a new criterion with both the light
curve and the average energy curve to determine the characteristic time for low
energy photons. We then apply the new criteria to the GBM NaI data, the GBM BGO
data, and the LAT LLE data, and obtain consistent results for three different
sets of low energy photons from different FERMI detectors.
