a device is provided which passes light of preselected wavelengths through a fluid or vapor to be tested . the light is detected after it has passed through the fluid or vapor . upon detection , a voltage is created which is proportional in amplitude to the detected light strength for each wavelength of light . these voltages are compared to produce a ratio which represents the condition of the fluid or vapor which is tested . when the ratio is outside of preselected boundaries , control signals are generated which may trigger alarms or otherwise respond to the condition .