There is an extensive and increasing demand for photometric measurements on low level light sources. To make such measurements low intensity standards are required. The tungsten filament lamps which form the normal standards of luminous intensity have a minimum useable intensity of the order of one candela. For low level photometry a standard lower than this by several orders of magnitude is required. The normal methods of photometric attenuation are either insufficiently powerful or have other serious drawbacks. For these reasons a device using a neutral white diffusing surface and a small calibrated aperture is used. It will give an intensity attenuation of up to the order of 106 times.