The use of modern computer programs is essential for the rapid creation of high-quality lighting, flexible transformation of systems at the design stage to obtain the best engineering solution in every case-reliable, safe, comfortable for performing visual work and acceptable for real installation and subsequent maintenance. The key criterion for choosing light engineering programs is the reliability of the design values, which is expressed by the coincidence of illuminance at control points as a result of measurements and computer calculations. In this case, it is important with what degree of reliability the program calculates the distribution of illuminance in the scene. However, setting of the calculated parameters depending on the complexity of the scene and the tasks performed by the user can significantly affect the accuracy of the results of computer calculations. The objective to increase the reliability of the results of computer simulation of the lighting installations was achieved by solving the following tasks: identifying the reason for the unreliability of the results of existing calculation methods in comparison with the measured and set values of the errors of determining the parameters of lighting systems on the basis of analysis of existing calculation methods; introduction of the generalized coefficient of maximum error, which takes into account the coincidence of the simulation results with the experimental values obtained when measuring the illuminance at the control points of the room with a digital luxmeter; the proposed expansion of the error of determining the quantitative parameters of the light environment ensures an increase in the reliability of the results obtained using light engineering computer programs, which, in turn, determines their effective application and reduction of the period of reconstruction of obsolete lighting systems.