Direct-to-video or 2013's "Sharknado". As DVDs gradually replaced VHS videocassettes, the term "direct-to-DVD" replaced "direct-to-video" in some instances. However, the word "video" does not necessarily refer to videocassettes; many publications continue to use the term "direct-to-video" for DVDs or Blu-ray discs. Both disc-based release types may also be referred to as "direct-to-disc". A new term sometimes used is "DVD premiere" (DVDP). Such films can cost as little as $20 million, about a third of the average cost of a Hollywood release. According to "Variety", "" sold a million copies in one week, despite retaining only two actors from the original trilogy.
Optical medium Optical medium An optical medium is material through which electromagnetic waves propagate. It is a form of transmission medium. The permittivity and permeability of the medium define how electromagnetic waves propagate in it. The medium has an "intrinsic impedance", given by where formula_2 and formula_3 are the electric field and magnetic field, respectively. In a region with no electrical conductivity, the expression simplifies to: For example, in free space the intrinsic impedance is called the characteristic impedance of vacuum, denoted "Z", and Waves propagate through a medium with velocity formula_6, where formula_7 is the frequency and formula_8 is the wavelength