Compact head-up display

An optical system includes a reflective polarizer (20), and a display (10), a first mirror (30a) and a second mirror (30b) disposed on the same side of the reflective polarizer (20). The reflective polarizer (20) transmits an image emitted by the display (10) after the image is reflected by the first mirror (30a) and the second mirror (30b). An intermediate plane defined by the optical axis of the system and the intersection between the display (10), the reflective polarizer (20) and the second mirror (30b) has first to fourth pass regions having respective areas A1 to A4. The first-pass region comprises a portion of the emitted image radiation (15) traversing the first-pass region at least once, the second-pass region comprises a portion of the image radiation (15) traversing the second-pass region at least twice, and the third-pass region comprises a portion of the image radiation (15) traversing the third-pass region at least three times, and the quartic passage region includes a portion of the image beam