Abstract: A window unit (102), comprising: a first sheet (301); a second sheet portion (304); a third sheet portion (308); and an actuation means. The first sheet (301) defines first high transmittance regions (302A, 302B) and first low transmittance regions (303A, 303B). The second sheet portion (304) is positioned substantially parallel to the first sheet, and defines second high transmittance regions (306A, 306B) and second low transmittance regions (307A, 307B). The third sheet portion (308) is positioned substantially parallel to the second sheet portion, and defines third high transmittance regions (309A, 309B) and third low transmittance regions (310A, 310B). The actuation means (507, 523, 524) is configured to simultaneously move the second sheet portion and the third sheet portion from an open configuration, to a closed configuration. In the open configuration the second and third high transmittance regions are aligned with the first high transmittance regions.