You're probably here asking "why 83"?  Or "why 85" dB as the magic monitor calibration number for film mixes. It's too long a story to tell you about how "85" became "83" for a while and eventually ended up at "85". And it's really an academic argument because in the end, consistency is the name of the game. If you'd like to hear that story, then pick up "Mastering Audio: The Art and the Science, 2nd Ed" and check out the appendix.

The actual method of calibration requires a calibrated source of pink noise at a specified digital level. The official monitor level calibration method is specified by the SMPTE in:

RP 200:2002  Relative and Absolute Sound Pressure Levels for Motion-Picture Multichannel Sound Systems – Applicable for Analog Photographic Film Audio, Digital Photographic Film Audio and D-Cinema  which can be purchased from the SMPTE store.

But if you'd like to learn why "85" was picked as the standard for monitor calibration, then read on.

In the late 1970's, Mr. Ioan Allen, (back then a major motion picture audio engineer/consultant/inventor, member of the Dolby team since 1969; today senior vice president of Dolby) came up with the idea of standardizing monitor calibration levels with pink noise at 85 dB SPL. At the time it seemed like a good number to him, but it has truly stood the test of time. Examination of the Fletcher-Munson equal loudness curves shows that the ear has the most linear response to frequencies at around 85 dB. Which means if you listen to forte passages around 85 dB and equalize your material to sound good at that level, it has the greatest chance of translating to all other sorts of systems. If you listen louder than this you run in danger of producing material which is perceived by others as bass-deficient, and vice-versa. In other words, the more linear your system, the more your product will translate to a wide-range of other systems. This doesn't mean you should not listen to your mixes at alternate levels, but by knowing the calibration point of your monitor system and returning to it regularly, you will have the most consistent results.

This calibration level was intended for use with large theatre reproduction. In fact, the closer the monitors are to the listener and the smaller the room, the louder this calibration sounds, because the sound becomes more dominated by direct sound, with greater transient impact and the apparent frequency response changes because there is less contribution from the room. So there must be a fudge factor and you may find that 77 or 78 dB produces a better translation when using nearfield monitors.

That's the short of it. The long of it can be found in Chapter 14 of "Mastering Audio" or in Tom Holman's book, "5.1 Surround Sound: Up and Running", both of which cover home theatre, music and/or broadcast applications.