Apparatus (20) is provided for use with a fluid that contains biological cells. The apparatus (20) includes a tray (22) shaped to define (a) a flat upper surface (24) configured to support the fluid, the upper surface (24) not being completely surrounded by a wall and having a surface area that is less than 900 mm2, and (b) an underside (26). The apparatus (20) further includes a plurality of protrusions (30) protruding from the underside (26) of the tray (22). The tray (22) has a thickness between 0.4 and 0.8 mm, and a property selected from the group consisting of: transparency, and translucency. Other embodiments are also described.
