Abstract: An optical transparency assembly suitable for use in applications where substantial pressure, temperature and humidity differentials between the air spaces on its opposite sides must be withstood, with one of its external surfaces maintaining a precise contour. One embodiment of the transparency assembly meets these requirements by including two separate transparent panels joined together at their peripheries with a narrow air space defined between them, with a unit being provided for conditioning the air in this narrow space such that it has a pressure, temperature and humidity substantially the same as that in the space adjacent the surface whose contour is to be maintained. Pressure, temperature and humidity gradients across one panel of the transparency assembly are minimized and that panel's contour is thereby precisely maintained.