A light source comprising: an optically reflective hollow cavity comprising: one or more input ports for receiving light and an output port for transmitting light; and one or more lamps disposed at the one or more input parts located on an edge of the optically reflective hollow cavity; and an optical stack disposed at the output port and comprising: disposed at the a substantially forward scattering optical diffuser disposed at the output port and having an optical haze that is not less than about 20%; an optical film disposed on the substantially forward scattering optical diffuser for enhancing total internal reflection at an interface between the optical film and the substantially forward scattering optical diffuser, the optical film having an index of refraction that is not greater than about 1.3 and an optical haze that is not greater than about 5%; and a reflective polarizer layer disposed on the optical film, wherein substantial portions of each two neighboring major surfaces in the optical stack are in physical contact with each other; wherein the substantially forward scattering optical diffuser has a transport ratio that is not less than about 0.2. an optically reflective hollow cavity comprising: one or more input ports for receiving light and an output port for transmitting light; and one or more lamps disposed at the one or more input parts located on an edge of the optically reflective hollow cavity; and one or more input ports for receiving light and an output port for transmitting light; and one or more lamps disposed at the one or more input parts located on an edge of the optically reflective hollow cavity; and an optical stack disposed at the output port and comprising: disposed at the a substantially forward scattering optical diffuser disposed at the output port and having an optical haze that is not less than about 20%; an optical film disposed on the substantially forward scattering optical diffuser for enhancing total internal reflection at an interface between the optical film and the substantially forward scattering optical diffuser, the optical film having an index of refraction that is not greater than about 1.3 and an optical haze that is not greater than about 5%; and a reflective polarizer layer disposed on the optical film, wherein substantial portions of each two neighboring major surfaces in the optical stack are in physical contact with each other; disposed at the a substantially forward scattering optical diffuser disposed at the output port and having an optical haze that is not less than about 20%; an optical film disposed on the substantially forward scattering optical diffuser for enhancing total internal reflection at an interface between the optical film and the substantially forward scattering optical diffuser, the optical film having an index of refraction that is not greater than about 1.3 and an optical haze that is not greater than about 5%; and a reflective polarizer layer disposed on the optical film, wherein substantial portions of each two neighboring major surfaces in the optical stack are in physical contact with each other; wherein the substantially forward scattering optical diffuser has a transport ratio that is not less than about 0.2.