The transmission for the modeled surface is most likely too high. Earlier versions of DIRSIG do not check for conservation of energy (reflectance + emissivity + transmission = 1). Therefore, it is possible possible for a surface to reflect 100% of the incident photons and also transmit 100% of the incident photons. When this happens, the reflectance or emitted radiance of the surface is augmented by a large amount of transmitted radiance.
To resolve this problem, check that your effective transmission is what you intended based on the provided extinction coefficients and the material thickness. Secondly, check that the transmission and reflectance or emissivity do not add up to greater than 1 (depending on which wavelength region you are modeling). More recent versions of DIRSIG will automatically correct the transmission and issue a warning to the user if energy is not conserved.
In DIRSIG version 4, a "more correct" treatment of reflectance, emissivity and transmission will be introduced that will avoid these issues.