An exposure chamber for reducing pathogens in a biological fluid such as whole blood or blood-derived products includes a serpentine-shaped UV-transparent flow path and closely positioned ultraviolet light emitting diodes configured for emanating UV irradiation towards the biological fluid at peak wavelength of 250 nm to 270 nm. The control system is provided to energize UV LEDs using various novel modes of modulating pulse width and current amplitude for LEDs so as to deliver higher UV intensity to the biological fluid but without overheating thereof or the LEDs.
