A novel setup for the direct determination of the quantum yield in photocatalytic and photochemical processes is reported. It combines the opto-electronic measurement of the absorbed amount of light with established quantitative chemical analysis of the products. High power visible LEDs are found to be convenient light sources for the illumination without the need for spectral filtering. The LED output can be imaged efficiently and in a controlled fashion into the sample. The residual transmitted light is continuously monitored by a dedicated and calibrated solar cell. The setup can be used under the conditions of a chemical synthesis laboratory. All information needed for the assembly and operation of the device is made available. The performance is validated by comparison to standard chemical actinometry and by the determination of quantum yields for reactions reported by others and ones investigated in our own laboratory.