A system for disabling an oximetric device when radiation-carrying channels are exposed to excess ambient radiation is disclosed. It includes a transmitter channel adjacent the sample to be measured, means for isolating the channel from the sample so that it does not carry radiation reflected from or transmitted through the sample, a detector for receiving the transmitted signal, if any, and means for disabling the output in response to the transmitted signal. Preferably, the transmitter channel runs parallel to other transmitter channels in the device, and is isolated from undue ambient light. The system preferably includes means for determining when the second transmitted signal exceeds background noise and the output is disabled when the second transmitted signal exceeds background noise. In general, the sample is a fluid, usually blood, passing through a cuvette adjacent the fiber-optic channels, the cuvette defining an area of radiation absorption adjacent the transmitter channel so that the transmitter does not carry reflected or transmitted radiation from the sample. Also disclosed is a fiber-optic sensor for measuring components of a composition by detecting transmitted or reflected, usually reflected, radiation including such a system, and the cuvette therefore. Finally, a method of disabling the output of an oximetric device when the fiber-optic channels are exposed to undue ambient radiation is disclosed.