I'm a little confused by your question.  The whole FLAASH module to ENVI is designed to collect all of the information that it needs to then derive the values for those variables.  Are you having trouble figuring out how to properly set the FLAASH parameters?  Or are you somehow trying to apply a FLAASH correction in your own code, outside of the ENVI FLAASH module?  Peg

actually, i prefer to understand the science that flaash build on.i just have the equation in the guide of flaash and all papers is talking about the same equation. i need to know the relations between several parameters firistly, to understand what i'll do secondly, to make a batch or code.also, i stopped at apoint  "at-sensor radiance reflected by the target " and "at-sensor radiance scattered into the path by the atmosphere" are these information are given with the data?

I'm not sure that anyone on this forum is going to be able to explain the FLAASH equations any better than they are explained in the published articles that it sounds like you already have.  Maybe your main confusion, though, is that you don't understand that the input image to FLAASH is the radiance data that FLAASH requires.  Your image must be calibrated to radiance before you choose it as the input file for FLAASH.  That radiance image will include both "at-sensor radiance reflected by the target" and "at-sensor radiance scattered into the path by the atmosphere".  The point of FLAASH is to take that input radiance image, and convert it to surface reflectance, thereby removing the "at-sensor radiance scattered into the path by the atmosphere", among other things. In case you do not already have the FLAASH references, here they are: Adler-Golden, S. M., M. W. Matthew, L. S. Bernstein, R. Y. Levine, A. Berk, S. C. Richtsmeier, P. K. Acharya, G. P. Anderson, G. Felde, J. Gardner, M. Hoke, L. S. Jeong, B. Pukall, A. Ratkowski, and H.-H Burke, 1999. Atmospheric Correction for Short-wave Spectral Imagery Based on MODTRAN4. SPIE Proceedings on Imaging Spectrometry, Vol. 3753, pp. 61-69. Berk, A., L. S. Bernstein, G. P. Anderson, P. K. Acharya, D. C. Robertson, J.H. Chetwynd, and S. M. Adler-Golden, 1998. MODTRAN Cloud and Multiple Scattering Upgrades with Application to AVIRIS. Remote Sensing of the Environment, Vol. 65, pp. 367-375. Berk A., L. S. Bernstein, and D. C. Robertson, 1989. MODTRAN: a moderate resolution model for LOWTRAN7. GL-TR-89-0122. Air Force Geophysical Laboratory, Hanscom AFB, MA. p. 38. Matthew, M. W., S. M. Adler-Golden, A. Berk, S. C. Richtsmeier, R. Y. Levine, L. S. Bernstein, P. K. Acharya, G. P. Anderson, G. W. Felde, M. P. Hoke, A. Ratkowski, H.-H. Burke, R. D. Kaiser, and D. P. Miller, 2000. Status of Atmospheric Correction Using a MODTRAN4-based Algorithm. SPIE Proceedings, Algorithms for Multispectral, Hyperspectral, and Ultraspectral Imagery VI. Vol. 4049, pp. 199-207. Peg  

thank you for these informations and ref. but i would like to ask about the four unkowns A,B,S,La where are the equations or relations that indicate that these unknows are functions of e.g.(aerosol,watervapor,.....).also,i read the paper Anderson et al., 1995. G.P. Anderson, J. Wang and J.H. Chetwynd , MODTRAN3: an update and recent validations against airborne high resolution interferometer measurements. In: Summaries of the Fifth Annual JPL Airborne Earth Science Workshop, Pasadena, CA, USA. JPL-Publication 95-1 vol. 1 (1995)  and stopped at both "at-sensor radiance reflected by the target" and "at-sensor radiance scattered into the path by the atmosphere" from where can i get these information ??from the image itself???to determine the values of the four unkowns sherehan