The meter works by emitting two frequencies of light, one at a wavelength of 660 nm (red) and one at 940 nm (infrared). Leaf chlorophyll absorbs red light but not infrared, the difference in absorption is measured by the meter and termed “Optical Density Difference,” ODD. Therefore, the unit of measurement is ODD, a ratio that is provided by the meter. The value does not give an actual chlorophyll or nitrate count Rather than saying, “there is this number of chlorophyll,” the meter value can be interpreted as, “this is a ratio of reflection vs. absorption.”
Additionally, plants cease to create chlorophyll once a certain threshold has been reached. Consequently, the meter cannot indicate an abundance of nitrate, only a possible deficiency. In terms of estimating nitrate content, the measured value is most useful when compared to a well fertilized control group.