Transimpedance-gain, loop-gain, and voltage-gain requirements of photodide short-circuit-current meters are summarized for ac and dc optical radiation measurements. Design considerations to implement the gain equations in silicon radiometers are discussed. Silicon radiometers with improved dynamic characteristics were built, and tested. the frequency dependent photocurrent gains were measured and th noise performance was tested. the noise floor was optimized in ac measurement mode using photodides of different shunt resistance and operational amplifiers with low 1/f voltage and current noise. In dark (without any signal), the noise floor of the optimized radiometers was dominated by the resistor noise of the source resistance. The resistor noise was decreased and equalized to the amplified 1/f input noise at 9 Hz chop frequency and 30 s integration time constant, resulting in an equivalent photocurrent noise of 8 x 10-17 Arms. The lowest noise floor of 5 x 10-17 Arms was obtained at 100 s ontegration time constant. The silicon radiometers, optimized for ac measurements modes were compared. In ac mode, ten times shorter (40 s) overall measurement time was needed than in dc mode (400 s) to obtain the same 10-16 Arms noise floor.