A method for rescaling a light signal to remove an absorption offset from said light signal in an optical measurement, wherein said absorption offset is due to a gas mixture in a housing through which said light signal has been transmitted, said method comprising: a first mode comprising: measuring a first light signal from each of at least one light source (1, 2, 3) located in said housing and said first light signal has been transmitted through, or back-scattered from, a reference object located outside of said housing using an external detector; measuring a second light signal from each of said at least one light source located in said housing using a reference detector; calculating a scaling factor related to each of said at least one light source using said detected first light signal and said detected second light signal; and a second mode, such as a consecutive second mode, comprising: measuring a third light signal from each of said at least one light source located in said housing and said third light signal has been transmitted through, or back-scattered from, a measurement object inserted instead of the reference object located outside of said housing using said external detector; measuring a fourth light signal from each of said at least one light source located in said housing using a reference detector; applying said scaling factor related to each of said detected fourth light signal which is then subtracted from each of said detected third light signal transmitted through, or back-scattered from, said measuring object. a first mode comprising: measuring a first light signal from each of at least one light source (1, 2, 3) located in said housing and said first light signal has been transmitted through, or back-scattered from, a reference object located outside of said housing using an external detector; measuring a second light signal from each of said at least one light source located in said housing using a reference detector; calculating a scaling factor related to each of said at least one light source using said detected first light signal and said detected second light signal; and measuring a first light signal from each of at least one light source (1, 2, 3) located in said housing and said first light signal has been transmitted through, or back-scattered from, a reference object located outside of said housing using an external detector; measuring a second light signal from each of said at least one light source located in said housing using a reference detector; calculating a scaling factor related to each of said at least one light source using said detected first light signal and said detected second light signal; and a second mode, such as a consecutive second mode, comprising: measuring a third light signal from each of said at least one light source located in said housing and said third light signal has been transmitted through, or back-scattered from, a measurement object inserted instead of the reference object located outside of said housing using said external detector; measuring a fourth light signal from each of said at least one light source located in said housing using a reference detector; applying said scaling factor related to each of said detected fourth light signal which is then subtracted from each of said detected third light signal transmitted through, or back-scattered from, said measuring object. measuring a third light signal from each of said at least one light source located in said housing and said third light signal has been transmitted through, or back-scattered from, a measurement object inserted instead of the reference object located outside of said housing using said external detector; measuring a fourth light signal from each of said at least one light source located in said housing using a reference detector; applying said scaling factor related to each of said detected fourth light signal which is then subtracted from each of said detected third light signal transmitted through, or back-scattered from, said measuring object.