  We present the design and validation of a variable temperature cryogenic
blackbody source, hereinafter called a cold load, that will be used to
characterize detectors to be deployed by CMB-S4, the next-generation
ground-based cosmic microwave background (CMB) experiment. Although cold loads
have been used for detector characterization by previous CMB experiments, this
cold load has three novel design features: (1) the ability to operate from the
1 K stage of a dilution refrigerator (DR), (2) a 3He gas-gap heat switch to
reduce cooling time, and (3) the ability to couple small external optical
signals to measure detector optical time constants under low optical loading.
The efficacy of this design was validated using a 150 GHz detector array
previously deployed by the Spider experiment. Thermal tests showed that the
cold load can be heated to temperatures required for characterizing CMB-S4's
detectors without significantly impacting the temperatures of other cryogenic
stages when mounted to the DR's 1 K stage. Additionally, optical tests
demonstrated that external signals can be coupled to a detector array through
the cold load without imparting a significant optical load on the detectors,
which will enable measurements of the CMB-S4 detectors' optical time constants.
