Zoom lenses Following a request in the late 1940's from the
BBC, who wanted a single lens to replace the classic 'turret' of different focal length lenses, he produced the now familiar
zoom lens. Although there had been earlier attempts to produce a lens which could achieve continuously varying magnification without re-focusing, none of them could provide a good quality image throughout their zooming and aperture ranges. The design of a zoom lens is more complicated and difficult than that of a fixed focal length lens. The performance of the Hopkins' zoom lens revolutionized television images, especially outdoors broadcasts and opened the way to the ubiquitous use of zooming in modern visual media. It was even more remarkable for being produced pre-computer, the
ray-tracing calculations instead being performed on desk top electro-mechanical machines such as the
Marchant Calculator. Even so, the early zoom lenses fell short of the image quality and brightness of fixed lenses. The application of computer design-programs based on his Wave Theory of Aberrations, in conjunction with new types of glass coatings and manufacturing techniques, has transformed the performance of all types of lenses.
Coherent fiber optics, fibroscopes and rod-lens endoscopes These inventions of Hopkins draws upon the
ancient Roman technique of heating and drawing out glass into thin and flexible fibres. They also observed that light falling on one end was transmitted to the other due to successive reflections from the internal surface of the fibre. These multiple reflections mix the light beams together, thereby preventing an image from being transmitted by a single fibre – (more accurately, the different path-lengths experienced by individual light-rays alter their relative phases rendering the beam
incoherent and thus unable to reconstitute the image). The end result is that the light emerging from a single fibre will be an average of the intensity and colour of the light falling on the incident end.
Coherent fibre optics If a bundle of fibres could be arranged such that their ends were in matching positions at either end, then focusing an image on one end of the bundle would produce a '
pixelated' version at the far end which could be viewed via an
eyepiece or captured by a camera. A German medical student,
Heinrich Lamm produced a crude coherent bundle in the 1930s of perhaps 400 fibres. Many of the fibres were misaligned and it lacked proper imaging optics. It also suffered from leakage where adjacent fibres touched; which further degraded the image. To produce a useful image, the bundle would need to contain not a few hundred but tens of thousands of fibres all correctly aligned. In the early 1950s, Hopkins devised a way to accomplish this. He proposed winding a single continuous length of fibre in a figure-of-eight around a pair of drums. Then, when sufficient turns had been added, a short section could be sealed in resin, cut through and the whole straightened to produce the required
coherent bundle. Having polished the ends, he was then able to add the optics he had designed to provide an objective and eyepiece. Once enclosed in a protective flexible jacket the 'fibroscope' (now more commonly called a fiberscope) was born. Details of this invention were published in papers by Hopkins in
Nature in 1954 and
Optica Acta in 1955. However, the bare fibres still suffered from light leakage where they touched. At the same time a Dutchman, Abraham van Heel, was also trying to produce coherent bundles and had been researching the idea of cladding each fibre to reduce this 'cross-talk'. He published details of his work in the same issue of
Nature. Eventually, a system for cladding fibres with a layer of glass of lower refractive index was developed by Larry Curtis et al., which reduced the leakage to such an extent that the full potential of the fiberscope was realised.
Fibroscopes and borescopes Fibroscopes have proved extremely useful both medically and industrially (where the term
borescope is commonly used). Other innovations included the use of additional fibres to channel light to the objective end from a powerful external source (typically a
xenon arc lamp), thereby achieving the high level of full spectrum illumination needed for detailed viewing and good quality colour photography. At the same time this allowed the fibroscope to remain cool, which was especially important in medical applications. The prior use of a small filament lamp at the tip of the endoscope had left the choice of either viewing in a very dim red light or increasing the light output at the risk of burning the inside of the patient. In the medical application, alongside the improvement to the optics, came the ability to 'steer' the tip via controls in the endoscopist's hands and innovations in remotely operated surgical instruments contained within the body of the endoscope. This beginning of keyhole surgery which he presented in 1962 when he delivered the
Thomas Young Oration of the
Institute of Physics, was one of the first to establish the
modulation transfer function (MTF) – sometimes called the
contrast transfer function (CTF) – as the leading measure of image quality in image-forming optical systems. Briefly, the contrast of the image of a sinusoidal object is defined as the difference in intensities between the peaks and troughs, divided by the sum. The
spatial frequency is the reciprocal of the period of the pattern in this image, normally measured in cycles/mm. The contrast, normalised to make the contrast at zero spatial frequency equal to unity, expressed as a function of spatial frequency, is the definition of the modulation transfer function. MTF is still used by optical designers as the principal criterion of image quality, although its measurement in production is less widespread than it once was. It is calculated from the lens data using software such as
OSLO,
Zemax and
Code V.
'Laserdisc and CD' optics Originally an analogue video play-back system, the
Philips laserdisc format was adapted to digital in the late 1970s and was the forerunner of the
CD and
DVD. The digital data is encoded as a series of depressions in a reflective disc. They are arranged along a spiral path that a laser can read sequentially (similar to a stylus following the groove on a
vinyl record). The laser must be focused onto, and track this path and the reflected beam must be collected, diverted and measured. The prototype optics to achieve this was an expensive glass-lens arrangement. Hopkins was able to show by a thorough mathematical analysis of the system, that with a carefully calculated geometry, it was possible to use a single piece of transparent moulded-plastic instead. This continues to be a major factor in the low cost of laser disc-readers (such as CD players). ==The Hopkins Building, University of Reading==