AbstractAn apparatus is described which consists of an assimilation chamber mounted in the centre of a light‐integrating Ulbricht sphere, irradiated through two fibreoptic light pipes. This arrangement provides totally diffuse radiation in the sphere. The quantum flux density in die sphere is measured by an integrating quantaspectrometer connected to the sphere by eight fibreoptic light pipes. The quantaspectrometer gives the spectral and the total quantum flux density in the sphere. By measuring the quantum flux density in the sphere before and after the insertion of a plant the number of absorbed quanta per unit time and plant area can be determined. In addition, the spectral distribution of the absorbed quantum flux density is obtained.CO2‐assimilation per unit time and plant area is determined in the same apparatus. The totally diffuse radiation within the sphere minimizes mutual shading between branches and leaves. Hence, well‐defined light‐responsecurves of photosynthesis can be obtained by plotting the flux density of C02‐assimilation as a function of the absorbed quantum flux density, and the quantum yields of intact plants can be calculated.Examples of photosynthetic quantum yield determinations and quanta absorption spectra are given for some plan