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I have been researching about the difference of a spectrometer and a spectrophotometer. They both sound the same. What is the difference?

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In astronomy, a spectrometer takes the light from an object, passes it through a slit, which then defines the input to a dispersive element. As a result a (generally unknown) amount of light is lost outside the slit, so it becomes difficult to estimate the absolute flux as a function of wavelength.

In a spectrophotometer you do not use an entrance slit, so all the light is collected and it becomes easier to estimate the absolute flux spectrum. The penalty is that the resolution is worse and is dependent on the size of the input image (or "seeing " in the case of stars).

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  • $\begingroup$ A small point which I am unable to comprehend: without the collimating slit, how can one get parallel rays from a broad source of light, e.g. a tungsten bulb. Or are you saying that spectrophotometers need different light sources, having a thin cylindrical shape only, to ensure that all light ( at least in the forward direction) passes through unobstructed? $\endgroup$ – 299792458 Jul 2 '17 at 4:59
  • $\begingroup$ @TheDarkSide In astronomy, the light source is a distant star. In any case, it isn't the slit that collimates the light, it is the collimator, which normally sits in front of the dispersive element. Unless you "stopped down" a non-stellar light source to look like a star, there wouldn't be much point in looking at it with a spectrophotometer. The term is used completely differently in laboratory science and the path is open for someone to write an answer that addresses that. $\endgroup$ – Rob Jeffries Jul 2 '17 at 8:11
  • $\begingroup$ Yes, that's a valid point. Thanks for the clarifying. Also as an aside, do you mind having a look at this once? I'm a bit reluctant to post this as a question on A.SE since it is a very silly reference request only. $\endgroup$ – 299792458 Jul 2 '17 at 12:21
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A spectrometer measures emission spectrum, as the light is focused on its entrance slit, dispersed and registered at different wavelengths.

A spectrophotometer measures absorption spectrum of a sample placed inside. The light from a built-in broadband light source is dispersed, sent through a sample and registered at different wavelengths.

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I really like this description straight from the optical spectrometer wikipedia page:

Spectrometer is a term that is applied to instruments that operate over a very wide range of wavelengths, from gamma rays and X-rays into the far infrared. If the instrument is designed to measure the spectrum in absolute units rather than relative units, then it is typically called a spectrophotometer. The majority of spectrophotometers are used in spectral regions near the visible spectrum.

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A spectrometer tells you which wavelengths of light is absorbed and which wavelengths of light is reflected. A spectrophotometer measures the relative intensity of the light absorbed or reflected at a particular wavelength of light.

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  • $\begingroup$ This looks inconsistent with the other answers. Have you got specific examples of literature that use these terms in this way? $\endgroup$ – Emilio Pisanty Dec 4 '19 at 11:54

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