A breath analyzer system and method for determining an estimate of blood alcohol concentration of a person. A light source projects light into a sampling region where breath from the person is expected. A first light detector and a second light detector are configured to detect light intensity of light having certain wavelengths at a first time and a second time. Signals from the light detectors at the first time and the second time are used to determine an estimate of the blood alcohol concentration of the person by determining a ratio of a first light detector signal change to a second light detector signal change.