As part of my work, I've been reading a lot about audio loudness standards recently.
There seems to be a bit of confusion between LKFS and LUFS, so I thought I'd explain the difference between these two measurements.
So, the short answer to the question, what's the difference between LKFS and LUFS is.....nothing. Not any more anyway. They used to be different but now they are effectively the same thing.
Here's the long answer:
About 4 or 5 years ago, the ITU (International Telecommunications Union) released a paper, 'ITU-R BS.1770 - Algorithms to measure audio programme loudness and true-peak audio level.' It described a way to measure the average loudness of a piece of audio and gave you a value for the audio as a whole. The units it used were LKFS, which stands for Loudness, K-Weighted, referenced to digital Full Scale, averaged out over the duration of the piece.
The EBU (European Broadcasting Union), took this and ran with it. However, over time they discovered a problem. It was too easy for long quiet sections of audio to bring down the average measurement. So they released a paper, 'EBU R128 - Loudness normalisation and permitted maximum level of audio signals', containing a few modifications.
They introduced some new units, namely LUFS (Loudness Units, relative to digital Full Scale), LU (Loudness Units), and LRA (Loudness Range, a measurement of the dynamic range, measured in LU). The LRA measurement was actually developed by TC Electronics, who are part of the EBU PLOUD group.
The big change was the introduction of a gate that would stop data being taken into account if the level dropped below a certain threshold. Because consumers judge the loudness of any particular piece of audio based on the loud parts, what the EBU called the foreground loudness, this gate would ignore any data that was 8 LU below the ungated measurment, so quiet sections would not skew the overall measurement.
So, at that point, there was a difference between LKFS and LUFS. LKFS was the ungated measurement, and LUFS was the measurement that included the effects of the gate. Generally, on your average TV programme (over a period of say 20-30 minutes), the difference between the LKFS and LUFS measurements is usually between 1-2 LU.
However, in March 2011, the ITU updated their paper with ITU-R BS.1770-2, incorporating the changes the EBU recommended in it's paper R128, namely the gate. Both the ITU and the EBU agreed that they would both change the threshold of the gate to -10 LU.
So, as of March 2011, LKFS = LUFS. The EBU were hoping that the ITU would drop LKFS and adopt the EBU's LUFS units, as Florian Camerer, the chair of the EBU's PLOUD group told me that the ITU spec doesn't follow standard naming conventions for units. However, they didn't, so we're stuck with both of them.
Personally I find it a lot easier to speak of LUFS, as it's easier to describe the differences in material as being 3 LU, as opposed to 3 L_lowercase K_. But hey! Anything is better than nothing!