This paper presents a recursive method for calculating the impulse response of an indoor free-space optical channel with Lambertian reflectors. The method accounts for multiple reflections of any order and enables accurate analysis of the effects of multipath dispersion on high-speed indoor optical communication systems. A simple algorithm for computer implementation is described, and simulation results for both line-of-sight and diffuse transmitter configurations are presented. The results show that multiple-order reflections are a significant source of intersymbol interference. Experimental measurements of optical multipath are also reported to verify the accuracy of the simulations. The paper describes a model for the source, reflectors, and receiver, and presents a recursive algorithm for calculating the impulse response. The algorithm is implemented numerically by discretizing the reflecting surfaces into small elements. The results show that higher-order reflections significantly affect the impulse response, leading to increased bandwidth requirements and power penalties. The paper also presents simulation and experimental results for different room configurations, showing good agreement between the two. The paper concludes that higher-order reflections are a dominant source of intersymbol interference in indoor optical communication systems. The results show that the power penalty increases with bit rate, and that diffuse systems can provide higher power than line-of-sight systems. The paper also discusses the importance of considering higher-order reflections in system design and the limitations of current models. The results are applicable to specific room configurations and highlight the need for further research in channel characterization.This paper presents a recursive method for calculating the impulse response of an indoor free-space optical channel with Lambertian reflectors. The method accounts for multiple reflections of any order and enables accurate analysis of the effects of multipath dispersion on high-speed indoor optical communication systems. A simple algorithm for computer implementation is described, and simulation results for both line-of-sight and diffuse transmitter configurations are presented. The results show that multiple-order reflections are a significant source of intersymbol interference. Experimental measurements of optical multipath are also reported to verify the accuracy of the simulations. The paper describes a model for the source, reflectors, and receiver, and presents a recursive algorithm for calculating the impulse response. The algorithm is implemented numerically by discretizing the reflecting surfaces into small elements. The results show that higher-order reflections significantly affect the impulse response, leading to increased bandwidth requirements and power penalties. The paper also presents simulation and experimental results for different room configurations, showing good agreement between the two. The paper concludes that higher-order reflections are a dominant source of intersymbol interference in indoor optical communication systems. The results show that the power penalty increases with bit rate, and that diffuse systems can provide higher power than line-of-sight systems. The paper also discusses the importance of considering higher-order reflections in system design and the limitations of current models. The results are applicable to specific room configurations and highlight the need for further research in channel characterization.