You can receive information on the question interesting you from:

http://www.academia.edu/7882462/Optical_properties_of_CdSe_and_CdO_thin_films_electrochemically_prepared

It depends on  vector (k); for two different energy states of conduction and valence of  a semiconductor which the transition is occured, it is called "indirect band gap" and vice versa. This means as a change in momentum in addition to energy. DRS mechanism is based on such detection. I think Streetman's book entitiled "Solid state electronic devices and systems" is knows for all of us a reference. 

for a new material how can find that its band gap is direct or indirect?