Highlight headroom
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Highlight Headroom is a subject that has always been important to photographers, and is becoming more so with the proliferation of digital photography today. This entry comes from the viewpoint of a semi-professional photographer, with regards to how Highlight Headroom impacts photography and why it's important, rather than from the more scientific angle discussed under Sensitometry.
[edit] What it is
Highlight Headroom is a measure of how much more capacity any given photographic media (such as film or digital) has to record detail within the white part of the visible spectrum.
Imagine you're trying to photograph a white wedding dress against a white background. If you have limited Highlight Headroom, you'll find it hard to appreciate the intricate details within the fabric of the dress. The higher the available Highlight Headroom, the more subtle shades of white will be captured.
In practice, photographers are often faced with photographing outdoors on a cloudy day, but with the sun shining through a gap onto the subjects forehead. This can often lead to a bright reflection which records as bright white. If this bright spot (called a highlight) lies at the edge of the persons outline (from the cameras perspective) then it may end up blending into the cloudy sky if there is insufficient Highlight Headroom. This is a particular problem with digital cameras which only have a limited gamut of discreet colour values.
