T he fundam e nta l ab sorption s pectrum of a so lid yie lds information about criti c al points in the opti· cal d e nsity of states. This inform ati on ca n b e used to adjust parameters of the band stru ct ure. Once th e adju s te d band s tru c ture is kn own , the opt ica l prope rti es a nd th e de ns it y of states can be ge nerated by num e ri ca l integration . W e re vi e w in thi s paper the pa ra me trizati on tec hniques used for obtaining band structures s uitable for de ns ity of s tates calc ulation s. The calc ulate d optical cons ta nts a re compared with expe rim e nt a l res ult s. Th e e ne rgy d e riv ative of th ese op ti ca l co ns tan ts is di sc usse d in co nn ection with res ults of modulate d re fl ecta nce meas ure me nts. It is a lso s hown th a t inform at ion abou t de ns ity of e mpty s tates ca n be obt ain e d fro m opti ca l expe rim e nt s in vo lvin g excit ati on from d ee p core le ve ls to th e co nduc tion band.A de ta il e d co mpari son of th e ca lc ul ate d one·e lec tron opti ca l line s hapes with expe rim e nt revea ls dev ia tion s whi ch ca n be int er pre ted as e xciton effects. The acc umul atin g ex pe rim e nt al e vid e nce poin t· in g in this d irectio n is reviewed to ge the r with th e ex is tin g th eo ry of these e ffects .A numb e r of simpl e mode ls for th e co mpli cate d inte rband de ns it y of s ta tes of a n in s ul ator ha ve been proposed. We re vi e w in pa rti c u lar th e P e nn mod e l, whi c h ca n be use d to acco unt for res po nse function s a t zero frequen cy, a nd t he parabo li c mode l, whi c h ca n be use d to accoun t for the di s pers ion of res pon se fun c tion s in th e imm e di ate vi c inity of th e fundam e ntal a bso rpti on e dge. Key word s: C riti ca l po int s; de ns ity of sta tes; d ie lec tri c co ns tant; modul ated re Aect a nce; op ti ca l abso rpti on.1. Optical Properties and One-Electron Density of States osc illa tor s tre ngth ten so r F ef is rela te d to th e ma trix ele me nts of p throu gh F e/= 2 < flp le > < el p lf > w eI-I. Th e Bloc h fun c ti ons are normaliz ed over unitThe opti cal behavior of se mi co nductors and in s ula· tors in the n ear infrared , visible, and ultraviolet is determined by electronic interband transition s. An ad· diti onal intraba nd or free elec tron co ntributi o n to th e optical properties has to be considered for me tals. W e shall di sc uss here th e relationship between th e inte rband contribution and the de n sity of states. The inter· band co ntribution to th e imaginary part of th e diele c tri c con stant can be written as (in atomic units, Ii = 1, m = I ,e= I):(1) wher e w eF w e-WI is th e differen ce in e nergy be tw een the e mpty bands (e) and the fille d bands (E). The spin multiplicity mu s t be include d expli citly in eq (1). The volume. Degenera te statis ti cs has bee n ass ume d in e q (1) and spatial di s persion effects have bee n neglecte d.It is c ustomary to take th e slowly varyin g oscillator stre ngth out of the integral sign in e q (1...