Photographic Techniques

(available to us)

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see also Archaeology Index and 3D Kite Aerial Photography

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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 a Hoya U-360 25mm UV-pass filter - Data. We also have a 55mm screw-fit B+W 403 filter (www.SchneiderKreuznach.com/index_e.htm).  Note that both filters transmit some light beyond ~700nm, at the red end of the spectrum.

We also have a 25mm Schott BG39 UV-pass / IR-block filter.

Other UV filters (1, 2). See also ultra-violet KAP below.

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

ISO 800, f2.8, 1/4 sec. Blue channel (to reduce residual infra-red) with colour removed. Nov 2007

Fuji F30 with Hoya U-360, 25mm UV filter, attached with adhesive tack

ISO 400, f2.8, 1/4 sec.

ISO 1600, f2.8, 1/4 sec.

Kite aerial photography in the near infra-red and ultra-violet

Infra-red KAP

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

We have chosen these photos to illustrate how lighting conditions affect the images (Colour removed).

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.

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

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

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

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

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

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

Ultra-violet KAP

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 (using an R72 filter) and also combined IR+UV KAP through a B+W 403 filter, at high ISO in good light. The sensitivity of the S5600 to filtered UV light alone is probably insufficient for KAP.

The usefulness of 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 not too large and thereby not masking any UV effects.  The Hoya U-360 filter lets through much less IR but appears not to be readily 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. Cheaper combinations of a Schott BG38 with either a U-330 or Tiffen 18A have also been suggested or U-330 with a BG-40.

High shutter speed UV KAP would probably require a dedicated digital camera with UV optics and a higher UV sensitivity than currently available. We may return to consider this option if a clear application presents itself. The limited continuous shooting mode of the S5600 means that we are unlikely to use it for KAP.

The modified Optio E-35 and Fuji S5600 cameras will 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.

Modified Fuji S5600 and B+W 403 IR/UV pass filter.

ISO 200, f3.2, 1/60 sec (NB IR foliage and UV sky)

A manuscript (dated 1422) photographed under light from a UV LED flashlight.

Modified Fuji S5600 and B+W 403 IR/UV pass filter.

An uninspiring first attempt at IR (dominant)+UV KAP

Update June 2009

First attempt at UV pole aerial photography (PAP)

This PAP photo was taken with an unmodified Fuji F30 camera using the continuous shooting mode. However, at this shutter speed (1/4 sec), most images were blurred. This is not a problem with a digital camera where you are capturing hundreds of images in a short period of time.

Light seepage is visible at the top corners because of the simple way the filters were mounted. This will be corrected.

The pole was mounted on a ground stake. Motion blur is minimised if the shutter fires when the camera is at, or near, the maximum amplitude of swing.  Conditions for this are optimal if the shutter speed is fast (and the rate of taking photos is high) compared with the period of oscillation of the camera.  This is also true for lateral movement of a camera suspended from a kite line.  A rigid pole would be perfect, as would most poles in wind-free conditions, but a near rigid pole would have a higher frequency of oscillation than a less rigid one.

Our pole photos suggest that if a higher shutter speed is used with the Fuji F30 camera, similar UV photos should be possible using a kite, albeit with a low success rate.

We will attempt to obtain some UV kite aerial photos in the near future using the Hoya U-360 UV-pass + Schott BG39 IR-block filter combination.

We have now upgraded to a Fuji IS1 infra-red camera for non-aerial work.

Fuji F30 with Hoya U-360 UV-pass + Schott BG39 IR-block, 25mm filters, ISO 800, f2.8, 1/4 sec

•           Verhoeven, G 2008. Exploring the Edges of the Unseen: An Attempt to Digital Aerial UV Photography, in: Lasaponara, R. & Masini, N. (Eds.), Remote Sensing for Archaeology and Cultural Heritage Management. Proceedings of the 1st International EARSeL Workshop, CNR, Rome, September 30 – October 4, 2008, Rome, pp. 79-83.

•           Verhoeven, G. 2008. Imaging the Invisible. Using modified Digital Still Cameras for Straightforward and Low-Cost Archaeological Near-InfraRed Photography. Journal of Archaeological Science 35(12), pp. 3087-3100

•           Verhoeven, G. & Loenders, J. 2006, Looking through Black-Tinted Glasses – A Remotely Controlled Infrared Eye in the Sky, in: Campana, S. & Forte, M. (eds.) 2006, From Space to Place. 2nd International Conference on Remote Sensing in Archaeology. Proceedings of the 2nd International Workshop, CNR, Rome, Italy, December 4-7, 2006, Oxford, pp. 73-79. (=BAR International Series 1568)