The photometry of coloured light sources, a thorny problem: The role of two little-known scientists, Jules Macé de Lépinay and William Nicati

Résumé 0

The field of optics has produced “brilliant and unexpected advances,” as François Arago told the French Academy of Sciences on 5 August 1833. Nonetheless, techniques in photometry, or the science of measuring the intensity of light, has changed little since the time when they were set down by Pierre Bouguer (1698–1758). They are based on visually equating the light source being measured to a source of known intensity, and equalisation is obtained by various techniques for graduating light. The second half of the century saw progress in instruments used for photometry that went hand in hand with the development of the lighting industry. However, comparing light sources of different colours is still an unsolved problem and prevents the establishment of a universal reference for light. The difficulty lies in the physiological characteristics inherent in the human system of vision, and especially the Purkynĕ effect. Professor Jules Macé de Lépinay (1851–1904) and an ophthalmological surgeon called William Nicati (1850–1931) devoted their efforts to the question, in Marseille. The circumstances of their collaboration which began in 1879 are worthy of consideration. The results of their research into the components of light in the solar spectrum are considered in comparison to publications from their own time and later studies which led to the establishment of the “visibility curve” for a “standard observer,” published in 1924.