Photographic Techniques

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see also Archaeology Index and 3D Kite Aerial Photography

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Updated 11 December 2009

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).

Vertical shot of archaeological site 2  October 2007

Cameras dedicated to taking photos in the near infra-red (not to be confused with thermal imaging (2) 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.

* What is visible depends on both the individual and the intensity of the light

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.)

Thermography

Although thermal imaging cameras are coming down in price, they are still relatively expensive for use by amateurs. However, an individual thermopile can be used in conjunction with a scanner.

An alternative technique is to use a non-contact thermal infra-red (~8-14 um) thermometer in conjunction with a fine grid. The grid can be placed (printed or projected) directly onto the object/surface of interest and the directional laser on the thermometer (a very approximate guide) directed through or at the grid.

Although using a grid is tedious, it is similar to carrying out a ground resistance or magnetometry survey and the results can be visualised in an identical way using a program like snuffler.

The usefulness of this grid approach, and particularly the size of the grid, is limited by the rate of change of the the target's temperature. Consequently, the full benefits of a high quality thermometer like the Everest Model 6110.4ZL may not be realised in practice.

We are interested in the potential application of this cost-effective approach to small, superficial, archaeological sites but it is dependent on very specific environmental conditions. We will investigate if conditions are ever appropriate for the use of cheap, low accuracy, fixed emissivity (0.95) thermometers like the CEM DT- 880B (~£35), which has a continuous monitoring mode.

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 (365nm 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

UV image with residual IR, ISO 400, f2.8, 1/4 sec. June 2009

UV image, ISO 1600, f2.8, 1/4 sec.  June 2009

(Simply looking at the blue channel from a normal image may also be useful)  Cathlaw, July 2009

Fuji F30 with Hoya U-360 UV-pass + Schott BG39 IR-block, 25mm filters.

Less obvious features can also be revealed

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.

Looking vertically down on St. Ninian's Chapel, Isle of Bute  December 2008

The two photos above were taken in overcast conditions. These light pink images were produced from the original red/deep pink ones (eg here) by using the 'auto levels' function in Photoshop.

Near infra-red can also reveal features in grass which later show up as

parch marks in drier warmer weather  April 2009

The two photos above were taken in direct sunlight

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 IR and/or UV through a B+W 403 filter, in artificial light. The sensitivity of the S5600 to filtered UV light alone is insufficient for KAP at reasonable shutter speeds.

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, and a specific application. 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 (or XNiteBP1).

High shutter speed UV KAP would probably require a dedicated digital camera with UV optics and a higher UV sensitivity than currently available. The limited continuous shooting mode of the S5600 means that we are unlikely to use it for KAP.  However, an unmodified Fuji F30fd has proved useful in low shutter speed PAP and KAP - see June 2009 update below.

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)

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

 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 when 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 has been 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.

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

July 2009

Fuji F30 with Hoya U-360 UV-pass + Schott BG39 IR-block, 25mm filters, ISO 1600, f2.8, 1/20 sec  July 2009

First attempt at UV kite aerial photography (KAP)

The image to the left is one of the best of 600 images we obtained in our first ~20 minute flight with the UV filter assembly.  The wind was gusting, and the sun was often obscured by clouds, giving a 'success' rate (at this level of quality) of about 1%. The shutter speed was set independently of the photometry, which indicated 1/4sec and f2.8 at ISO1600. 

Taking kite aerial photos of manmade objects (buildings, roads etc) or rocks is much easier than flora because of the higher levels of reflected UV light.  Differential imaging of foliage is more problematic, but not impossible.

We are looking at the use of other filter combinations to maximise the shutter speed. Then we will consider other available compact cameras. For investigative archaeological KAP, the problems of a high ISO (with associated image noise) are less important than achieving a higher shutter speed. Fortunately, cameras are continuously improving, with lower noise/higher sensitivity, image stabilization and wider apertures.

The difficulties associated with using compact digital cameras can be resolved by using a non-digital SLR with UV optics and UV sensitive film, but our objective is a simple, workable, low-cost, accessible system.

In our set-up to the right, the 25mm filters are joined around their edge with a strip of aluminium adhesive tape. Adhesive tack was used to attach the filter assembly to the front of the lens mount, taking care not to impede lens movement.

For suitable, unmodified cameras, this low shutter speed approach can be applied to infra-red photography (using an R72 filter) where images will be much clearer than in the UV.  Also, sensitivity to IR is usually higher than to UV.

Fuji F30fd ultra-violet set-up, with Hoya U-360 UV-pass + Schott BG39 IR-block, 25mm filters.

Links

Other IR / UV KAPers

Do you know any links to IR or UV kite aerial photos?  e-mail Rosie  

General IR aerial photography

General UV aerial photography

General IR/UV photography

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