Need an equivalent of isosurface for all values under a certain threshold

Hi, I have a large 3D matrix and I want to make a plot equivalent to an iso-surface but for values under a specific threshold and not just all values that are 'equal'. Is there such a function and if not what might you suggest I do instead?

Regards,

-E

[Patrick Kalita]

It's not clear to me how the proposed plot would look different than a standard isosurface. The regions corresponding to values less than the threshold would be in the interior of the surface whose boundary is defined by the equal values and therefore not visible.