Everything about Spectroscope totally explained
A
spectrometer is an
optical instrument used to measure properties of
light over a specific portion of the
electromagnetic spectrum, typically used in
spectroscopic analysis to identify materials. The variable measured is most often the light's
intensity but could also, for instance, be the
polarization state. The independent variable is usually the
wavelength of the light, normally expressed as some fraction of a meter, but sometimes expressed as some unit directly proportional to the
photon energy, such as
wavenumber or
electron volts, which has a reciprocal relationship to wavelength. A spectrometer is used in
spectroscopy for producing
spectral lines and measuring their
wavelengths and intensities. Spectrometer is a term that's applied to instruments that operate over a very wide range of wavelengths, from
gamma rays and
X-rays into the
far infrared. If the region of interest is restricted to near the
visible spectrum, the study is called
spectrophotometry.
In general, any particular instrument will operate over a small portion of this total range because of the different techniques used to measure different portions of the spectrum. Below optical frequencies (that is, at
microwave and
radio frequencies), the
spectrum analyzer is a closely related electronic device.
Spectroscopes
They are used often in
astronomy and some branches of
chemistry. Early spectroscopes were simply a
prism with graduations marking wavelengths of light. Modern spectroscopes, such as
monochromators, generally use a
diffraction grating, a movable slit, and some kind of
photodetector, all automated and controlled by a
computer. The spectroscope was invented by both
Gustav Robert Georg Kirchhoff and
Robert Wilhelm Bunsen.
When a material is heated to
incandescence it emits
light that's characteristic of the atomic makeup of the material.
Particular light frequencies give rise to sharply defined bands on the scale which can be thought of as fingerprints. For example, the element
sodium has a very characteristic double yellow band known as the Sodium D-lines at 588.9950 and 589.5924 nanometers, the colour of which will be familiar to anyone who has seen a low pressure
sodium vapor lamp.
In the original spectroscope design in the early 19th century, light entered a slit and a
collimating lens transformed the light into a thin beam of parallel rays. The light was then passed through a prism (in hand-held spectroscopes, usually an
Amici prism) that
refracted the beam into a spectrum because different wavelengths were refracted different amounts due to
dispersion. This image was then viewed through a tube with a scale that was transposed upon the spectral image, enabling its direct measurement.
With the development of
photographic film, the more accurate
spectrograph was created. It was based on the same principle as the spectroscope, but it had a camera in place of the viewing tube. In recent years the electronic circuits built around the
photomultiplier tube have replaced the camera, allowing real-time spectrographic analysis with far greater accuracy. Arrays of photosensors are also used in place of film in spectrographic systems. Such spectral analysis, or spectroscopy, has become an important scientific tool for analyzing the composition of unknown material and for studying astronomical phenomena and testing astronomical theories. The wavelengths are measured with the spectrometer.
Spectrographs
A
spectrograph is an instrument that transforms an incoming time-domain
waveform into a
frequency spectrum, or generally a sequence of such spectra. There are several kinds of machines referred to as
spectrographs, depending on the precise nature of the waves. The first spectrographs used
photographic paper as the detector. The star
spectral classification and discovery of the
main sequence,
Hubble's law and the
Hubble sequence were all made with
spectrographs that used photographic paper. The plant pigment
phytochrome was discovered using a spectrograph that used living plants as the detector. More recent spectrographs use electronic detectors, such as
CCDs which can be used for both visible and
UV light. The exact choice of detector depends on the wavelengths of light to be recorded.
The forthcoming
James Webb Space Telescope will contain both a near-infrared spectrograph (NIRSpec) and a mid-infrared spectrometer (MIRI).
An
echelle spectrograph uses two
diffraction gratings, rotated 90 degrees with respect to each other and placed close to one another. Therefore an entrance point and not a slit is used and a 2d CCD-chip records the spectrum. Usually one would guess to retrieve a spectrum on the diagonal, but when both grating have a wide spacing and one is blazed so that only the first order is visible and the other is blazed that a lot of higher orders are visible, one gets a very fine spectrum nicely folded onto a small common CCD-chip. The small chip also means that the collimating optics need not to be optimized for coma or astigmatism, but the
spherical aberration can be set to zero.
A spectrograph is sometimes called
polychromator, as an analogy to
monochromator.
Further Information
Get more info on 'Spectroscope'.
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