TlGaSe2 is ferroelectric – semiconductor crystals having an incommensurate (INC) phase. It has centrosymmetric monoclinic space group at room temperature. On cooling from ambient temperature, TlGaSe2 undergoes successive phase transitions: from paraelectric phase to INC – phase at Ti ~ 120 K and from the INC - phase to polar ferroelectric C - phase at Tc ~ 110 K.  Ferroelectrics with INC - phase are well known to exhibit pronounced memory effect. If a crystal is kept inside INC - phase at a certain stable temperature for a sufficiently long times then, on passing this temperature again, anomalies of various physical properties of crystals are observed [1]. This is caused by the formation of the long - live metastable states, connected with the defects density wave (DDW) which is formed in the crystal, as a result of the spatial redistribution of impurities and structural defects in the periodic field of the INC – phase. Since the relaxation time of the mobile defects, determined by their diffusion mobility, considerably exceeds the time, required for the optical measurements, it is possible to assume the conservation of DDW out of the INC - phase region.  The optical properties of the ferroelectric - semiconductor TlGaSe2 near the fundamental electronic band gap have been investigated after the thermal annealing of crystals inside the (INC) phase for a few hours. It was shown that optical absorption properties of TlGaSe2 are drastically changed after annealing of the crystal within the INC - phase. It was found that DDW may form the new defect energy state in the electronic band gap of material which can be easily identified from optical absorption investigation. The material remembers this defect energy state, induced by memory effect, after temperature treatment of TlGaSe2 within INC – phase. The optical response of TlGaSe2 material on DDW - modulated structure corresponding to the memory effect have been established and explained.  The self-consistent ab initio calculations were performed on the structural, electronic and optical properties of TlGaSe2 to model and study the DDW. The Full Potential Linearized Augmented Plane Wave (FP-LAPW) method was applied to solve the Kohn-Sham equations. Results were obtained by using the PBE-GGA and mBJLDA exchange correlation potentials by WIEN2k program [2]. The general profiles of the optical spectra and the optical properties, including the real and imaginary part of dielectric function, absorption coefficient of TlGaSe2 were calculated. The optical anisotropy was studied through the calculated optical constants, namely dielectric function along three different crystallographic axes.