The 1591BTBU is an Ice Blue polycarbonate enclosure in a 112 x 62 x 31 mm case, a step up in length and width from the 1591A size. Designers of boards with LEDs, readouts or displays use this tint so that blue and white light passes through the case walls.

The Ice Blue version is almost totally transparent to blue, white and near-white LEDs, readouts and displays, so status lights and indicators inside remain visible through the case. The case is molded from flame retardant translucent polycarbonate that is easy to machine, and lap joint construction between box and lid provides protection against access of dust and splashing water. It is designed to meet IP54 and is listed as RoHS and REACH compliant.

The drawing gives overall dimensions of 112.00 x 62.00 x 30.80 mm (4.409 x 2.441 x 1.213 in). Inside it measures 105.78 x 55.78 x 26.90 mm, room for a slightly larger board than the 100 mm size while staying under 31 mm deep. Integral card guides accept 1.5 mm (0.062 in) PC cards, with a guide height of 21.90 mm and a slot width of 2.13 mm.

The purchased 1591BTBU assembly includes the box, lid, 4 cover screws and pre-installed brass inserts. The lid is secured with M3 x 10 mm Phillips machine screws threaded into integral brass bushings at a recommended torque of 42-57 ozf.in (30-40 cN.m); replacement screws are part number 1591MS100. The same size is also offered as 1591BTCL and 1591BTRD in the other translucent tints. Some chemicals can damage or weaken plastic, so check chemical compatibility before cleaning or modifying the case.