Image Archiving - HDR and RAW Formats

HDR and RAW formats are effectively digital files that are used only for storage.  Neither format provides a useful image for purposes of display.  So which is better for archiving my high dynamic range files.  It pays to know how these storage or capture formats differ in order to make an informed decision.

In an earlier article I wrote that a single RAW file captures around 10EV in dynamic range while a 32-bit HDR file captures a virtually unlimited tonal range through the application of floating point luminance values.  This translates into around 76 orders of magnitude in dynamic range, far exceeding the luminance found on even the brightest sunny day.  HDR files capture a true high dynamic range.

Because the HDR file is lossless, one might think that it provides a better storage solution than a RAW file.  Perhaps. The problem lies, however, in the simple fact that the creation of an HDR file from a RAW file depends on the conversion software one uses.  Merging three to five RAW files into a single HDR merged file will give different results depending on the software one uses and even between different versions of the same software as later versions generally provide more sophisticated results.  Merging algorithms are proprietary each having different working thresholds for pixel conversion; each algorithm, therefore, produces a slightly different result.

In the final analysis, archiving RAW files as source images is superior to archiving HDR merged files, however, RAW files are subject to degradation over time so I recommend that one use a redundant backup system.  I use an off-site backup as well as backing up my RAW and HDR files on DVDs.  While RAW source file storage and backup is ideal, I also recommend that one store converted HDR merged files, especially if one works with a single vendor's software.  I work with Photomatix Pro exclusively so I find that storing my HDR merged files as well as the RAW source files provides me with an ideal solution.  I don't believe one can take too many precautions when storing important image files.

All that being said, remember that the converted HDR file is not useful as an image file for purposes of display.  Displaying a 32-bit HDR file on a low dynamic range computer monitor one finds that the displayed image has apparent areas where the highlights are completely blown and the shadows are completely clipped; details of the HDR image are not apparent in the display of the HDR merged file on a standard computer monitor.  Tone mapping of the HDR merged file is necessary, not only for viewing the details on your monitor but is also a requirement if one wishes to print the merged image.  Tone mapping, in its simplest terms, is a way to convert the 32-bit HDR merged file into a 16-bit TIFF file or an 8-bit JPEG file that is now useful as an image for viewing and printing.