spectrophotometer: a photometer for comparing two light radiations wavelength by wavelength
photometer: measuring instrument for measuring the luminous intensity of a source by comparing it (visually or photoelectrically) with a standard source
secretase: a set of enzymes believed to snip pieces off a longer protein producing fragments of amyloid protein that bunch up and create amyloid protein plaques in brain tissue (the pathological hallmark of Alzheimer's)