So let's focus on NADH. But before we actually measure NADH, we have to know some type of specific property of NADH that NAD plus doesn't actually have. Well, one major difference between NAD plus and NADH is the fact that NADH can actually absorb a specific type of light with a specific type of wavelength. So NADH can absorb light that has a wavelength of 340 nm. So basically, if we have some type of solution that contains these two reactants and we place our sample of enzymes into that solution, and if this enzyme is present in that mixture, then what happens is if we begin measuring how much light is absorbed where the light has this specific wavelength value. If the absorbedance increases over time, then that means is we produce more of this product because this product is able to absorb more of that light.