Highlight headroom is the measure of how much additional capacity a given photographic media (such as film

Photographic film

Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...

 or digital) has in order to record the detail within the brightest parts of a scene.

As an example, consider a photograph of a white wedding dress

Wedding dress

A wedding dress or wedding gown is the clothing worn by a bride during a wedding ceremony. Color, style and ceremonial importance of the gown can depend on the religion and culture of the wedding participants.- Western culture :...

 against a white background. With limited highlight headroom, it will be hard to appreciate the intricate details within the fabric of the dress. The higher the available 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

Reflection (physics)

Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

 which records as bright white. If this bright spot (called a highlight) lies at the edge of the persons outline (from the cameras perspective

Perspective (visual)

Perspective, in context of vision and visual perception, is the way in which objects appear to the eye based on their spatial attributes; or their dimensions and the position of the eye relative to the objects...

) then it may end up blending into the cloudy sky if there is insufficient highlight headroom. This is a particular problem with digital camera

Digital camera

A digital camera is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor. It is the main device used in the field of digital photography...

s which only have a limited gamut

Gamut

In color reproduction, including computer graphics and photography, the gamut, or color gamut , is a certain complete subset of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a...

of discrete colour values.