I wish to use ENVI 4.5 for processing of SeaWiFS L1 A data downloaded from Ocean color web;

The data S2004304080548(1).L1A_MLAC was downloaded and opened in ENVI. The image opens with its eight bands. Since the data is level 1A, the bands give only DN values. To convert the respective DNs into radiance, gain and offset is needed.

Could please I be guided from where I can get the gain and offset values.

In ENVI the header file of SeaWiFS L1A data shows four Gain values for each band like saturated gain, unsaturated gain, mean gain (please see the attachment) . Which gain value should be used for processing the DN values to radiance.

regards
Paras Sidiqui

Converting SeaWiFS L1A to L1B calibrated radiances is not as simple as applying a gain and offset to the counts.  If you insist on using ENVI, I suggest you use SeaDAS first, and convert to L1B.  There is also a forum for ENVI users on this site, where you might find info or assistance. 

-- bryan