Colour filters and channels

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Photographic Techniques Index

With film cameras, filters are often used when working in black and white to contrast different colours. For example, a red filter can be used to lighten reds or to darken a blue sky and make white clouds stand out. A green filter will lighten foliage. In both film and digital photography, a polarising filter can be used to darken a blue sky or to eliminate unwanted reflections.

Normal colour images can be manipulated on a computer too. With appropriate software, the different colour elements of an image can be isolated (e.g.  the red, green or blue channels) and the colour removed to produce an image similar to what can be achieved with a camera and filter.

To illustrate the effect on contrast, the photo on the right (part of site 2) is reproduced below in black and white using only the blue and red components of the image.

Depending on the subject matter, such techniques can be a significant aid in image analysis.

Taking photos outside the visible spectrum (infra-red / ultra-violet) can be even more useful especially in aerial photography (see below).

Vertical shot of archaeological site 2  KAP October 2007

Blue channel

(Good for highlighting stones on grass)

Red channel

For archaeological applications, David Stott, of the Dart Project, is using a spectroradiometer to cover a wide range of wavelengths.

Image Inversion

Delineation can be improved by inverting an image to its negative.

KAP June 2011