Depictions of light-measurers are rare. Certainly their shrouded and blackened apparatus made photography awkward; but the reliance on human observers to make scientific measurements came to be an embarrassment to practitioners. The practitioners remain shadowy, too, because of the low status of their occupation, commercial reticence and—somewhat later—military secrecy.

The measurement of brightness came to be invested with several purposes. It gained sporadic attention through the 18th century. Adopted alternately by astronomers and for the utilitarian needs of the gas lighting industry from the second half of the 19th century, it was appropriated by the nascent electric lighting industry to ‘prove’ the superiority of their technology. By the turn of the century the illuminating engineering movement was becoming an organized, if eclectic, community promoting research into the measurement of light intensity.

The early 20th century development of the subject was moulded by organization and institutionalization. During its first two decades, new national and industrial laboratories in Britain, America and Germany were crucial in stabilizing practices and raising confidence in them. Through the inter-war period, committees and international commissions sought to standardize light and colour measurement and to promote research. Such government- and industry-supported delegations, rather than academic institutions, were primarily responsible for the construction of the subject.

Along with this social organization came a new cognitive framework: practitioners increasingly came to interpret the three topics of photometry (visible light measurement), colorimetry (the measurement of colour) and radiometry (the measurement of invisible radiations) as aspects of a broader study.

This recategorization brought shifts of authority: shifts of the dominant social group determining the direction of the subject’s evolution, and a shift of confidence away from the central element of detection, the eye. From the 1920s, the highly refined visual methods of observation were hurriedly replaced by physical means of light measurement, a process initially a matter of scientific fashion rather than demonstrated superiority. These non-human instruments embodied the new locus of light and colour, and the data they produced stabilized the definitions further.

The rise of automated, mechanized measurement of light and colour introduced new communities to the subject. New photoelectric techniques for measuring light intensity engendered new commercial instruments, a trend that accelerated in the 1930s when photometry was taken up with mixed success for a wide range of industrial problems. Seeds sown in those years—namely commercialization and industrial application, the transition from visual to physical methods and the search for fundamental limitations in light measurement—gave the subject the form it was to retain over the next halfcentury.

Nevertheless, changing usage mutated the subject. Light proved to be a valuable quantity for military purposes during and after the Second World War. A wholly new body of specialists—military contractors—transformed its measurement, creating new theory, new technology, new standards and new units of measurement.

Following this variety of players through their unfamiliar environments illuminates the often hidden territories of scientific change. And two themes run throughout this account of the measurement of light and colour from its first hesitant emergence to its gradual construction as a scientific subject. The first traces changing attitudes concerning quantification. The mathematization of light was a contentious process that hinged on finding an acceptable relationship between the mutable response of the human eye and the more readily stabilized, but less encompassing, techniques of physical measurement. The diffident acceptance of new techniques by different technical communities illuminates their value systems, interactions and socio-technical evolution.