Add to ORKGThe measurement of light and colour has always been a peripheral science. Light became a 'disciplined' quantity over the period of a century, but the specialist communities that measured it did not. The quantification of visible light (photometry), colour (colorimetry), and radiant intensity (radiometry) involved distinct communities of physicists, psychologists, technicians and engineers. This chapter of _Science in the Shadows_ examines how the measurement of non-visible light became the domain of post Second World War military engineers and classified development programmes