OpenEXR is a high dynamic-range image format developed by ILM and opensourced for use by other projects. Whilst TIFF has support for HDR float images, OpenEXR is able to store data more efficiently because it employs a 'half' float datatype. OpenEXR images also allow for arbitrary named channels.
To render image to OpenEXR format you must turn off quantize and select apropriate display:
Quantize "rgba" 0 0 0 0 Display "imagename.exr" "openexr" "rgb" "string compressinon" "algorithm"
Where algorithm can be RLE, ZIPS, ZIP, PIZ or PXR24.
To view exr image you can use exrdisplay from ILM's OpenEXR package.