1. A method for determining a respiration rate of a subject, the method comprising: transforming, using a signal processor, a first physiological signal based on light transmission at a first wavelength to generate a first transformed signal; transforming, using the signal processor, a second physiological signal based on light transmission at a second wavelength to generate a second transformed signal; deriving, using the signal processor, a ratio surface from the first transformed signal and the second transformed signal, wherein the ratio surface comprises three-dimensional data with dimensions including time and scale; identifying, using the signal processor, a first region of interest on the ratio surface indicative of venous perturbation; calculating, using the signal processor, a representative value for the first region of interest on the ratio surface; making, using the signal processor, a determination as to whether the calculated representative value for the first region of interest indicates respiration or motion of the subject based on the calculated representative value, wherein determining whether the representative value for the first region of interest indicates respiration or motion of the subject comprises: calculating a representative value for a second region of interest of the ratio surface related to a cardiac pulse frequency; calculating a short-term difference between the representative value for the second region of interest and the representative value for the first region of interest; and comparing the short-term difference with a long-term difference between historical ratio surface values near the cardiac pulse frequency and an expected respiration frequency, wherein smaller deviations of the short-term difference from the long-term difference are indicative of baseline modulations in the first and second signals being due to respiration, and larger deviations of the short-term difference from the long-term difference are indicative of baseline modulations being due to motion; and calculating, using the signal processor, the respiration rate of the subject when the calculated representative value for the first region of interest indicates respiration.