Science and Photography
On Tuesday 24 November John Ranson took the broad subject of Photography in Science as his theme. He noted that the pin-hole camera concept (an inverted image in a darkened room) was in use as far back as the 5th century BC in China. Canaletto is believed to have used this Camera Obscura method to produce images with perfect perspective. The largest pin-hole camera was produced in an aircraft hanger in California. Here the negative film was 107 feet long by 37 feet high and the pin-hole a quarter of an inch diameter. The pin-hole size is important. It determines the amount of light entering the system. A smaller diameter increases image sharpness until diffraction effects begin to dominate. A rule of thumb indication of the optimum diameter is given by 0.036 times the square root of the distance from the pinhole to the film plane. Thus for a 100mm gap the pinhole diameter should be about 0.36mm.
Zone plates, an alternative to the lens, make use of diffraction. John used several examples (water waves and sound waves in air) to illustrate waves constructively and destructively interfering with one another. This effect is the basis of the Zone Plate. Binary Zone Plates (alternate clear and opaque rings) have several points of apparent focus whereas the Sinusoidal plate (where the density of opaqueness varies with radius sinusoidally) has one clear focus point. Zone plates can be used at many frequencies outside the visible spectrum, where glass is not an option – X-Rays being a good example.
The Fresnel lens is an alternative; a flatter version of the usual glass lens, where the surface curvature is maintained, but the whole lens “flattened” by removing glass in the middle. However, using too many sectors leads again to diffraction problems. John gave several examples of the common use of the Fresnel lens; from the overhead projector to the “map reader” magnifier to the Lighthouse lens.
John discussed the importance of the anti-reflection coating of the camera lens and how, by gradually increasing the opaqueness at the edge of the lens diffraction effects can be reduced. Apodization was the name given to this process.
John reviewed the spectral response of the eye (effective over the range 400 to 700 nm wavelength) and compared it to the different response of the Cathode Ray Tube phosphors and the tri-colour filters used in colour film. He briefly discussed the Bayer matrix of RGB filters used with digital sensors. X-Ray photography has proved useful in recovering data hidden within documents and paintings, whilst infra-red photography has proved useful in detecting hidden features in the ground. Fibre optics have proved invaluable in medicine (the endoscope).
John discussed the advent of the liquid lens for camera phones, where the curvature of the oil/water interface can be altered to produce zooming effects, and considered how “autofocussing” was achieved by detecting an abrupt change of contrast at focus.
High Speed photography had been an important part of his job early on at Pilkington’s, where the strength of CRT glass, the shattering of car windscreens, and the growth of internal laser damage of optical glass were important problems to be solved.
John considered a range of early hoax photographs, and how the hoaxes had been uncovered before turning his attention to UFO’s and the Loch Ness Monster. He concluded with a famous shot of the largest cat in Canada, almost the size of its owner.
In his Vote of Thanks Graham Johnston said he had been interested, fascinated, and mesmerized, in turn, by the wealth of information presented. He felt the audience could all relate to parts of the talk. He himself had sat in the dentist’s chair, watching on a screen, as the dentist proceeded with his root filling (just one of John’s many examples). It had been good to sit back and take in the wider picture of scientific photography, and he called upon the audience to say thank you in the usual way.