A spectrophotometer adds an electrical or electronic photometer to the system to measure light intensity much more accurately than the human eye can judge it and display it on a meter or on the more expensive models print it out on a graph. The investigation involves a grating spectrometer. The light passes through an adjustable slit and then through a diffraction grating in the eyepiece. The light is dispersed into a spectrum, which is projected onto the retina of the eye, giving the observer the impression that the spectrum is inside the instrument, although it is purely a virtual image. The instrument also projects an image of a wavelength scale into the eye, superimposing it on the spectrum so the observer can read what wavelength each color corresponds to.
Most light sources provide a fairly complete spectrum with virtually all of the wavelengths present.
If a solution is place between the light source and the spectrometer, only those wavelengths transmitted by the solution will be seen. Diagram Transmittance means exactly what it implies. It is a measure of the light that is transmitted. It is the decimal fraction of the incident light which is transmitted through a sample of solution. Per cent transmittance is simply the expression of that fraction as a per cent instead of a decimal fraction.