He was born into a South Wales mining village; its culture and the influences of his family were never to leave him. His family's love of music and concern for the social causes of the Welsh valleys led to his abiding love of opera and to his passionate commitment to socialism.
His family had great ambitions for him and at West Monmouth Grammar School his exceptional mathematical abilities were recognised. In 1957 he became a student in the Physics Department at the Imperial College of Science and Technology, and there he spent the rest of his academic career. He came under the influence of Professor David Wright, the doyen of colour vision, which led him to devote his subsequent research to the study of vision. He became a leading and highly innovative exponent of the application of psychophysics (the investigation of the relations between physical stimuli and sensation) to the study of vision in human subjects.
Not content with extending understanding of normal vision he turned to studies of the nature of visual abnormalities exhibited by a variety of clinical patients, many of whom had suffered brain damage as a result of a stroke or tumour. His studies were carried out not only to enhance understanding of the normal mechanisms of visual processing, but to offer practical assistance to the patient. Ruddock was a brilliant experimentalist who when presented with such a patient would rapidly devise a series of experiments, which would be speedily executed following modifications of the hallmark of the visual psychophysicist, the optical bench.
This consists of a series of mirrors, lenses, filters and light sources, which when viewed by the uninitiated gives the impression of science performed with string and sealing wax, but in the hands of Ruddock provided measurements of the highest precision. For the subject the experiments could be fairly demanding, but here Ruddock's humanity was apparent. Several of his subjects returned time and time again to his laboratories over extended periods, in some cases in excess of 20 years. If the patient was unable to travel to London Ruddock would devise suitable portable equipment to take the laboratory into the patient's home. His patients became his friends as did almost everyone who entered his life.
In 1972 he was awarded a Royal Society Exchange Fellowship to work in Venezuela with Gunnar Svaetichin, a pioneer in single-cell electrophysiological recording. This enabled Ruddock on his return to establish, in Imperial's country establishment at Silwood Park, a separate research programme in which he recorded from within cells in the retina, the light-sensitive tissue at the back of the eye. Both his electrophysiological and psychophysical studies led him into controversial areas in the field of vision; he defended his position strongly whilst always being prepared to listen and debate with the opposition.
Throughout his life at Imperial he was committed to undergraduate and postgraduate education, and actively involved in the organisation of teaching. He was an excellent lecturer who delighted in being able to bring biology to physics students steeped in mathematics and the physical sciences. Every year, after hearing his lectures, students flocked to his door to carry out final-year projects or PhDs under his supervision. To them he was always accessible. In 1988 he was appointed Professor of Biophysics, and in 1991 Head of the Biophysics Group in the Physics Department.
Over the years Ruddock developed an extended international network of former students whom he greatly enjoyed visiting. This and frequent requests to lecture abroad fed his passion for travel, which was made all the more pleasurable by his gift for languages, which extended to Chinese - studied as a challenge.
Outside Imperial College he was on several editorial boards and national committees. He particularly enjoyed his work on the Vision Research Working Party of the Wellcome Trust. In 1985 with three other visual scientists and clinicians he formed the Neuro-ophthalmology Club and it was his enthusiasm and interest which has ensured its continued success.
Ruddock was an ardent follower of sport, especially Welsh rugby and cricket, although here he was a Lancashire supporter. An accomplished violinist, he was involved in the musical life of Imperial, but it was his burning love of opera that was evident to all around him. He regularly visited the Royal Opera House and had in recent weeks been contemplating its forthcoming temporary closure for redevelopment, and whether to sign up for opera seasons in houses as far afield as New York, Paris or Vienna.
In 1963 he married Joan Anthony, a teenage friend who had followed him to study at Imperial College. Coming from the same background their 30 years together were spent sharing similar ideals, particularly a passion for socialism which eventually led Joan to become a Labour Member of Parliament. They separated in 1990.
Keith Ruddock was a scientist and humanitarian who showed absolute loyalty to his friends. His untimely and sudden death came just hours after he remarked that he had had one of the most successful years ever in his scientific career with a stream of exciting new observations. There was an irony in his death's resulting from a motor vehicle accident; it was a form of transportation Ruddock disliked and he had never considered it necessary to learn to drive.
Keith Harrhy Ruddock, biophysicist: born Croesyceiliog, South Wales 12 March 1939; Professor of Biophysics, Imperial College of Science, Technology and Medicine 1988-96; married 1963 Joan Anthony; died London 20 December 1996.