Colour filters and channels

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).

Cameras dedicated to taking photos in the near infra-red (not to be confused with thermal imaging where equipment is expensive but cheaper systems are, hopefuly, on the way) and ultra-violet are now readily available. Some cameras are easily modified but others can be more complex (1, 2).

However, if you are prepared to use a tripod, most normal digital cameras can work effectively in the near infra-red part of the spectrum. See flickr (1, 2).

 As illustrated on the right, if a camera can 'see' the infra-red output from a TV controller, then it may be suitable. A Hoya R72 (small unmounted or conventional mount ) infra-red filter will be needed to block out visible light.

See the 'infra-red' link above for more details. 'Hot spots', focussing and exposure can be problematic.

Armadale left, Standhill/Brownclair centre (R72 infra-red filter). Colour removed.

(Fuji S6500fd, R72 infra-red filter, ISO 200, f2.8, 1/2 sec. Colour removed.)

The photo to the right illustrates the sensitivity of three cameras to UV+violet light, the Fuji S6500fd and F10 (in the picture) and the Fuji F20 which took the picture. The UV lamp (366nm peak) in front of the cameras looks dull violet to the unaided eye, when ambient light levels are low, and near black at normal light levels.

For the photos below we used an unmounted (hand-held) Hoya U-360 25mm UV filter - Data. We also have a screw-fit B+W 403 filter (www.SchneiderKreuznach.com) - Data.  Note that both filters transmit light beyond ~700nm, at the red end of the spectrum.

Other UV filters (1, 2).

Fuji F10 with a  Hoya U-360, 25mm UV filter

ISO 800, f2.8, 1/4 sec. Blue channel with colour removed. Nov 2007

Infra-red KAP

These photos of Armadale were taken in July 2008 with a Pentax Optio E35 modified for use in the infra-red by Mila Pravda (irdigi.com).

We have chosen these photos to illustrate how lighting conditions affect the images.

To the left, the image has a uniform lighting of soft sunlight.

Lower left, harsh evening sunlight.

Lower right, strong sunlight on the horizon with the foreground in shadow (similar to the Barbauchlaw Glen photo above) taken from the same position as the photo on the lower left but looking approximately northeast rather than south.

Compare the clarity of these images with the ones here which illustrate the effect of haze.

Our first outing with the modified Optio E35 (Larger version)

Power Sled 24 kite, continuous shooting mode, sports setting, ISO 80, 1/500sec f5.4

Our second outing with the modified Optio E35  (Larger version)

Power Sled 24 kite, continuous shooting mode, sports setting, ISO 80, 1/400 sec f2.

Our second outing with the modified Optio E35 (Larger version)

Power Sled 24 kite, continuous shooting mode, sports setting, ISO 80, 1/400 sec f2.7

Ultra-violet KAP

Preliminary ground-based tests with a Fuji S5600, modified for both IR and/or UV (also by Mila), suggest that the camera would be sufficiently sensitive for IR KAP work and possibly borderline for combined IR+UV KAP through a B+W 403 filter, at high ISO.

The usefulness of such UV (+IR) images in relation to our archaeological interests is not known but would probably only be applicable where the IR component of the image is minimal. The Hoya U-360 lets through much less IR but appears not to be available in larger sizes.  The ideal 2 inch Baader U-filter at $265+ is a little expensive, but worth it, especially if you have dedicated UV optics.

The sensitivity of the S5600 to filtered UV light alone may be insufficient for KAP.

The cameras will also be useful aids in our work deciphering old manuscripts using either IR or UV.

We would like to thank Mila for enabling us to take up low-cost KAP in the near infra-red.