A polarimetry method for determining the presence and/or character of a polarizing medium in a test region, comprising: receiving light via a first optical path through the test region, from a first polarized light source having a first light source polarization axis, at a first polarized light receiver having a first light receiver polarization axis being generally +45 degrees relative to the first light source polarization axis; receiving light via a second optical path through the test region, from the first polarized light source, at a second polarized light receiver having a second light receiver polarization axis being generally -45 degrees relative to the first light source polarization axis; receiving light via a third optical path through the test region, from a second polarized light source, at the first polarized light receiver, the second polarized light source having a second light source polarization axis being generally 90 degrees relative to the first light source polarization axis; and receiving light via a fourth optical path through the test region, from the second polarized light source, at the second polarized light receiver; the method comprising the sequential steps of: (a) turning off the first and second polarized light sources (12, 14) and waiting for a predetermined settling time; (b) measuring the light received at the first and second polarized light receivers (16, 18); (c) turning on the first polarized light source keeping the second polarized light source off, and waiting for a predetermined settling time; (d) measuring the light received at the first and second polarized light receivers (16, 18); (e) turning on the second polarized light source and turning off the first polarizing light source, and waiting for a predetermined settling time; (f) measuring the light received at the first and second polarized light receivers (16, 18); and (g) deriving optical transfer functions for each of the first to the fourth optical paths from the measurements of the received light in order to reduce or eliminate the effect of ambient light. receiving light via a first optical path through the test region, from a first polarized light source having a first light source polarization axis, at a first polarized light receiver having a first light receiver polarization axis being generally +45 degrees relative to the first light source polarization axis; receiving light via a second optical path through the test region, from the first polarized light source, at a second polarized light receiver having a second light receiver polarization axis being generally -45 degrees relative to the first light source polarization axis; receiving light via a third optical path through the test region, from a second polarized light source, at the first polarized light receiver, the second polarized light source having a second light source polarization axis being generally 90 degrees relative to the first light source polarization axis; and receiving light via a fourth optical path through the test region, from the second polarized light source, at the second polarized light receiver; the method comprising the sequential steps of: (a) turning off the first and second polarized light sources (12, 14) and waiting for a predetermined settling time; (b) measuring the light received at the first and second polarized light receivers (16, 18); (c) turning on the first polarized light source keeping the second polarized light source off, and waiting for a predetermined settling time; (d) measuring the light received at the first and second polarized light receivers (16, 18); (e) turning on the second polarized light source and turning off the first polarizing light source, and waiting for a predetermined settling time; (f) measuring the light received at the first and second polarized light receivers (16, 18); and (g) deriving optical transfer functions for each of the first to the fourth optical paths from the measurements of the received light in order to reduce or eliminate the effect of ambient light. (a) turning off the first and second polarized light sources (12, 14) and waiting for a predetermined settling time; (b) measuring the light received at the first and second polarized light receivers (16, 18); (c) turning on the first polarized light source keeping the second polarized light source off, and waiting for a predetermined settling time; (d) measuring the light received at the first and second polarized light receivers (16, 18); (e) turning on the second polarized light source and turning off the first polarizing light source, and waiting for a predetermined settling time; (f) measuring the light received at the first and second polarized light receivers (16, 18); and (g) deriving optical transfer functions for each of the first to the fourth optical paths from the measurements of the received light in order to reduce or eliminate the effect of ambient light.