An applied back-pressure to a high pressure homogenization system influences the development of hydrodynamic cavitation and the collapse of cavitation bubbles. This paper focuses on the effect of back pressure on hydrodynamic cavitation and how the intensity of collapsing cavitation bubbles can be identified. Based on sono-chemiluminescence (SCL), an optical measuring method was developed to detect cavitation induced formation of center dot OH radicals, which indicates the intensity of collapsing cavities. The results are discussed with the introduced dimensionless SCL intensity number (I) over bar (SCL). The (I) over bar (SCL) number increases with increasing inlet pressure as well as with increasing back-pressure and confirms therewith theoretical based assumptions by other studies. A maximum (I) over bar (SCL) at a back-pressure of approximately 30% was found and reduced to an intensified collapse of cavities in or after the first high pressure disruption unit. The maximum in SCL intensity is in line with the minimum droplet size of corresponding emulsification experiments. (C) 2015 Elsevier Ltd. All rights reserved.