L=DN*gain +bias
求出各样点的辐射能量值。其中L为地物在大气顶部的辐射能量值，单位为；DN为样本的灰度均值，gain和bias分别为图像的增益与偏置，可从图像的头文件中读取。
头文件：
CORRECTION_METHOD_GAIN_BAND1 = "CPF"
CORRECTION_METHOD_GAIN_BAND2 = "CPF"
CORRECTION_METHOD_GAIN_BAND3 = "CPF"
CORRECTION_METHOD_GAIN_BAND4 = "CPF"
CORRECTION_METHOD_GAIN_BAND5 = "CPF"
CORRECTION_METHOD_GAIN_BAND61 = "CPF"
CORRECTION_METHOD_GAIN_BAND62 = "CPF"
CORRECTION_METHOD_GAIN_BAND7 = "CPF"
CORRECTION_METHOD_GAIN_BAND8 = "CPF"
CORRECTION_METHOD_BIAS = "IC"
BAND1_GAIN = "H"
BAND2_GAIN = "H"
BAND3_GAIN = "H"
BAND4_GAIN = "L"
BAND5_GAIN = "H"
BAND6_GAIN1 = "L"
BAND6_GAIN2 = "H"
BAND7_GAIN = "H"
BAND8_GAIN = "L"
BAND1_GAIN_CHANGE = "0"
BAND2_GAIN_CHANGE = "0"
BAND3_GAIN_CHANGE = "0"
BAND4_GAIN_CHANGE = "0"
BAND5_GAIN_CHANGE = "0"
BAND6_GAIN_CHANGE1 = "0"
BAND6_GAIN_CHANGE2 = "0"
BAND7_GAIN_CHANGE = "0"
BAND8_GAIN_CHANGE = "0"
BAND1_SL_GAIN_CHANGE = "0"
BAND2_SL_GAIN_CHANGE = "0"
BAND3_SL_GAIN_CHANGE = "0"
BAND4_SL_GAIN_CHANGE = "0"
BAND5_SL_GAIN_CHANGE = "0"
BAND6_SL_GAIN_CHANGE1 = "0"
BAND6_SL_GAIN_CHANGE2 = "0"
BAND7_SL_GAIN_CHANGE = "0"
BAND8_SL_GAIN_CHANGE = "0"
请问gain值是什么？
