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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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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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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, 2017 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. $\endgroup$
    – ProfRob
    Jul 2, 2017 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, 2017 at 12:21
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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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I thought this article looked credible and gave a thorough explanation:

Part of what makes this confusing for a lot of people is that all spectrophotometers incorporate a spectrometer. It’s also true that other analytical instruments use spectrometers. The spectrometer is the part of the spectrophotometer that is most responsible for measuring things. The spectrophotometer is a complete system that includes a light source along with a means to collect the light that has interacted with the things being tested, as well as a spectrometer for measurements.

Source: https://www.excedr.com/blog/spectrometer-vs-spectrophotometer/

I think the article explains both a spectrometer and a spectrophotometer in detail which may give more insight than simply the comparison.

The article addresses why it can be a little confusing (as well as a detailed answer), but in short, a spectrophotometer has a spectrometer in it, but the spectrophotometer is an entire system.

I believe this image/vector belows shows an entire spectrophotometer, where as the gauge at the end I believe is only a spectrometer.

Spectrophotometer

Furthermore, I thought providing a concrete example in industry may be useful. In laser safety, lenses are tested with a spectrophotometer. The spectrophotometer will test the amount of light being absorbed against the amount of light being let through at each wavelength in question. The amount being absorbed is a measurement known as Optical Density. See the diagram below for an brief overview on how a spectrophotometer works. Please note, the spectrophotometer depicted below is showing a spectrophotometer for visible light; different spectrophotometers test for different wavelength spans (UV and IR).

Spectrophotometer used in Laser Safety

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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, 2019 at 11:54

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