The development of the Utrecht physics laboratory into a photometric institute In the period between 1920 and 1940 the physics laboratory of the university of Utrecht enjoyed international fame because of the measurements of intensity ratios of spectral lines, performed by L.S. Ornstein and his co-operators. In 1924 these investigations led to the so-called 'sum rules', which caused a lot of commotion in the ranks of the leading physicists. This article describes how this work originated and how the laboratory subsequently developed into a full-fledged research centre. After Ornstein accepted the chair of theoretical physics in Utrecht in 1915, he encountered the photometric techniques which were developed by the director of the laboratory W.H. Julius and his assistant W.J.H. Moll. Ornstein realized that the measurement of light intensities offered a wide field of applications and he decided to investigate the possibilities of the use of photographic plates. The measurement of intensity ratios in atomic and molecular spectra was only one of the applications of the photometric methods. The work of Ornstein and his pupils concerned a wide variety of subjects, most of which had nothing to do with atomic physics at all. But all investigations were based upon measurements of intensities of light and heat radiation. Ornstein had purposefully created a specialized institute for optical photometric work, in which pure science, technical physics and industrial research were brought together.