Effect: This spell is similar to invisibility,
but the recipient is able to attack,
either by missile discharge, melee combat,
or spell casting and remain unseen.

Note, however, thot there are sometimes telltale traces, a shimmering,
so that an observont opponent con attack the invisible spell recipient.
Such attacks ore at -4 on the "to hit" dice, and all saving throws are made at +4.

DMG: Improved invisibility has the effect of moving considerations for detection of such invisibility downwards by two places
compared to other forms of invisibility. See INVISIBILITY for commentary.

Improved Invisibility: PCs should be
aware that this is less easily detected by
monsters with an innate ability to detect
invisibility as a function of HD and
INT than an ordinary invisibility
spell.  They might not know by how much;
details are given on p47 of the DMG.
(Imagine #29)
 

Wu Jen: This spell functions as the invisibility spell, but in addition the recipient may attack, use spells, and fire missiles and remain unseen.
A flickering form can be detected, and attacks on the invisible opponent are at -4, and saving throws are at + 4.