Interband optical transitions happen at the scale of the lattice constant which is small compared to the exciton radius. Therefore, for large excitons in direct-gap crystals the oscillator strength f e x of exciton absorption is proportional to | Φ Φ e x ( 0 ) | 2 which is the value of the square of the wave function of the internal motion inside the exciton Φ Φ e x ( r e − − r h ) at coinciding values of the electron r e and hole r h coordinates. For large excitons | Φ Φ e x ( 0 ) | 2 ≈ ≈ 1 / a e x 3 where a e x is the exciton radius, hence, f e x ≈ ≈ v / a e x 3 ≪ ≪ 1 , here v is the unit cell volume. The oscillator strength f i for producing a bound exciton can be expressed through its wave function Ψ Ψ i ( r e , r h ) and f e x as