Light Transmission Method

The light transmission method measures the attenuation of light transmitted from a light source as it passes through a sample to the light receiving element at the other end.
Absorbance is calculated from the transmittance and converted to a concentration that the absorbance is directly proportional to.
To ensure that only the absorbance is measured and that changes in the light source intensity are not included, it is common to measure and correct the light intensity along a path that does not pass through the sample. Zero calibration is also necessary to correct for light absorbed due to changes in the optical path when the sample is in a state that does not absorb light (zero state).
Depending on the sample, the light transmission method can be used to measure various components by changing light wavelength and pretreatment.
UV meters used to measure organic pollutants take advantage of the fact that ultraviolet light at a wavelength of 253.7 nm is absorbed by organic matter. However, in addition to organic matter, suspended matter can also block the light and affect absorbance. Therefore, when measuring samples containing suspended matter (such as turbid water), the absorbance of another wavelength of visible light may be measured and used for correction.
Additionally, when measuring total nitrogen, substances that interfere with the measurement are first removed by adding reagents or oxidizing and breaking down the sample with ultraviolet light. Then the total nitrogen concentration is determined by measuring absorbance at a wavelength of 220 nm.
For total phosphorous or silica, reagents are added to the sample to induce a color change that corresponds to the concentration of the substance being measured. The concentration is then determined by measuring the intensity of this color as absorbance.
In this way, the light transmission method can be combined with other technologies to measure various components.