The Total Irradiance Monitor (TIM), an active-cavity solar radiometer, is to be launched in 2002 on the SORCE mission of the Earth Observing System (EOS). The relative uncertainty (1 σ) will be better than 10⁻⁴, i.e. 100 parts per million (ppm) (1 σ) with a noise level of < 1 ppm each 500 s. Sunlight passes through a shutter, a 50 mm² aperture, and is then absorbed into a silver cavity blackened inside with nickel phosphorus. High thermal-conductivity diamond insulators at the electrical terminals help to localize thermal nodes. Four cavities are aligned side-by-side at the rear of a 2 kg heat sink. The flight "standard digital watt" is derived from a high-precision voltage and a pulse-width-modulator; this eliminates the need for a square root in the servo loop. We determine the irradiance from the in-phase sinusoidal component at the shutter frequency. This phase-sensitive detection allows more accurate characterization than traditional time-domain methods. We calibrate the aperture transmission integral over area against a chrome-on-quartz ruling, transferring by charge-coupled device (CCD) images. In flight, we recalibrate the servo-loop gain, pointing variations, and dark signal. We determine the equivalence ratio from models and by laser measurements of the cavity parameters.