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Superwhites refers to the (illegal) video values above white level. In most but not all video systems, superwhites will be clipped off. If they do make it to the viewer's TV, then CRT TVs will actually be able to display them. However, picture quality will be lower for superwhite values as they tend to cause distorted geometry and the electron beam may lose focus.
Superwhites can also cause buzzing in the audio if video and audio signals are carried on one cable. This is one reason why all broadcasters will clip them off.
In my opinion, video masters should never contain superwhites. If they do, some viewers will see a different picture than everybody else. They will see a different picture because some display technologies and some DVD players will clip superwhites while others don't. It's just not a good idea.
Unfortunately, almost all cameras will record video with superwhites in them. This creates problems. While I strongly disagree with the practice, almost all cameras do it and we have to live with it.
You can do one of two things:
Clip them off. Many post houses will run video through a video legalizer that does this.
Pull them into legal range. This makes the picture darker and rescues dynamic range. This is one of the reasons why I disagree with cameras recording superwhites... by default, they will throw away dynamic range because many people do option 1 (e.g. live production, many post houses, etc.).
In video editing systems, you can turn the exposure or gain down on the video signal to pull superwhites into legal range. In a 3-way color corrector, the third color wheel controls gain. In Final Cut Pro, the slider under the rightmost color wheel controls luma gain... adjust luma gain and saturation (chroma gain) by the same amount to adjust exposure. Here are instructions for dealing with superwhites in Sony Vegas.
Some FCP online editors don't have a video legalizer and will intentionally ingest the footage darker all the time. By doing this, there will not be any illegal superwhites and they've worked around having to buy a video legalizer.
Firstly, no monitor will display superwhites correctly. LCD monitors will likely clip superwhites... this is a reasonable way to design them (to show superwhites would waste contrast ratio on these illegal values). Broadcast CRT monitors will show them... but doing so will cause some loss of resolution, distorted geometry, and phosphors to wear out faster. (Granted, one could turn the contrast / white level on the CRT down.)
Secondly, you need to know what will happen to the signal in post. Will the editor do #1 or #2? I know of a TV series where the DP saw the image he wanted on set, only to receive DVD screeners of the show and see highlight detail get clipped off. And from the post house's perspective, the DP overexposed the image. So the parties involved are pointing fingers at each other. (And in my opinion, the party that screwed up is the camera manufacturer.) Communication is the key! The solution in this case was for the post house to do #2 outlined above. If you are a DP, make sure you get to see the final output as various things can happen in post.
For certain video editing systems and compositing applications, sometimes not all superwhite values are mapped into legal range. Some systems and codecs cannot handle any superwhites at all. One solution is to pull superwhites into the legal range and then output the legal video into the application.
Programs like Sony Vegas can decode video signals to 8-bit studio RGB levels. Code values 16-235 are used for legal values and 236-255 is alloted for superwhites. However, extreme superwhites can fall above 255 when mapped in this manner. Vegas will simply clip these superwhite values off. These extreme values will only occur if the highlights are colored- it does not happen with white highlights.
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