November 28, 2021, 06:39:12 AM
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Topic: What is the difference betwwen photometer, spectrophotometer & spectrometer  (Read 46536 times)

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Offline Ibin-Huan

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 Hi

Can anyone help me

What is the  basic difference betwwen photometer, spectrophotometer & spectrometer?

Thank you for any reply

Offline marquis

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The photometer just measures the intensity of light.  Think of a solar cell hooked up to a voltmeter.  The higher the intensity of the light, the higher the reading on the voltmeter ( a simplification, but you get the idea).

A spectrophotometer has a more sophisticated system of separating the light into the individual wavelengths and measuring the intensity at each individual wavelength.  Typically, a spectrophotometer has a monochromator (for separating the individual wavelengths of light) and is a scanning instrument.  As an example, it will scan from something like 400 nm to 200 nm at 100 nm per minute.

A spectrometer is a scaled down spectrophotometer.  It can use diffraction gratings to separate the individual wavelengths.  Commonly, this type of instrument is used in fourier transform  (FT) instruments. 

These instruments can be for many ranges of light, including IR, UV, VIS, etc.

Theoretically, a spectrophotometer is the best.  Don't always count on this.  Many of the spectrometers offer faster speed and similar resolution to spectrophotometers.  They are often cheaper, using computer power instead of optics to separate the wavelengths of light.
 

Offline marquis

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There is another instrument in this series that you didn't mention.

In the spectrophotometer, the monochromator is the part of the instrument that breaks the light into very small segments.  The monochromator can easily resolve to 1 nm in the vis range. Depending how fast you need the scan done, fractional nm can be resolved.

An easier version of the same instrument is a colorimeter.  Instead of using a monochromator, it uses filters and measures the light that penetrates the filters.  Where a spectrophotometer can resolve to 1 nm, a colorimeter would be lucky to get to 10-15 nm (again in the visible range).

I wanted to give a few examples of spectrometers.  One is a diode array instrument.  It sends the light through the sample and then to a prism (easier to imagine, usually it is a diffraction grating). This spreads the different wavelengths out, kind of like what you see in a rainbow.  The diode array is a line of very small photodiodes.  Each diode represents a specific wavelength.  The computer reads the light intensity of each wavelength by seeing the signal at the photodiode.

Another type is a fourier transform instrument.  It runs the sample through an interferometer (usually a Michaelson interferometer for IR instruments).  The analogue signal that resulsts can be converted into a spectrum.

Depending on the instrument design, spectrometers can rival or outperform spectrophotometers. They can also be much worse, again depending on design.

Offline Ibin-Huan

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Thank you marquis and you seam experienced in this field, but please could you explain me the basic different between monochromator and filter as wavelength selector.

Thank you again

Offline marquis

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For a filter, think of colored film, say red colored.  When you look through the film, the light appears red.  This is a filter.  The filter limits the range of light that passes through it.  But only very broadly (many nanometers, in the case of light).

Now for the monochromator.  Think of a prism and the way it splits light into a range of colors.  On the other side of the prism is a piece of black paper with very narrow slit cut in it.  By rotating the prism, you can change the wavelength of light that gets through.  And by changing the width of the slit, you can change the spectral width of the light let through. This light represents a very narrow range of light, often only a part of a nanometer.  It also dramatically reduces the amount of light that passess through.  The detector on the other side must be very sensitive.  Often, a photomultiplier tube is used.  And the scan time can be long.

This is a crude description.  The typical monochromators are using diffraction gratings, not prisms.  But a prism is easier to imagine.

It's difficult to describe, but hopefully this gets the basic idea across.

Offline ykprakash

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Monochromation means selecting a single wavelength out of multiple wavelengths of the incident light. The Filter is a coarse and low cost option for monochromation. It does not not pass exactly a single wavelength. It passess thru a narrow band of wavelengths. Say a 400nm Filter may pass thru wavelength of 400nm + or - 20 nm. This is called bandwidth of the filter.

For better monochromation of white light, we use gratings. You may change the source of light itself and use an LED or Laser!

So a Filter is a kind of monochromator!

YK Prakash

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