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Spectrometer Parameters - The main differences
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Wavelength Range
Array spectrometers are available for wavelengths from the UV (200 nm) and VIS to IR (...3 µm). Wavelengths ranges of commercial array spectrometers are indicated in the following figure (Scheme of the electromagnetic spectrum and of wavelength ranges of commercial spectrometers):
The size of the spectrometer is determined by the wavelength range in connection with the desired optical resolution (see below), resulting in a certain focal length. getSpec.com´s series of instruments currently cover the range of 184 ... 2200 nm.
Resolution
Several definitions are used for the resolution of a spectrometer. One has to distinguish clearly between the optical or spectral and the digital or pixel resolution. The optical resolution is defined by the wavelength difference of two peaks close together in one spectrum and of same intensity, which can be separated. The dip between the peaks has to reach a minimum of at least 19%, related to the maximum intensity. This definition is called the Rayleigh criterion.
Another more practical definition is related to the measured width of a narrow spectral line. Its measured bandwidth DlFWHM gives information about the broadening of the line. This bandwith amounts to about 4/5 of the resolution according to the Rayleigh criterion. The optical resolution is determined by the width of the input slit, the focal length of the optical system and the dispersion of the grating. Dl FWHM is inversely proportional to the linear dispersion dl /dl and directly proportional to the output slit width w':
This equation can easisly be used for estimation of a spectrometer resolution, especially with 1:1 imaging, where the width of the input slite equals to the width of the output slit (w= w'). The smaller the slit width, the higher is the resolution. However, reduced slit width reduces the optical energy, entering the spectrometer, too. This can cause sensitivity problemsfor a spectrometric system. Common resolution values of miniaturized VIS spectrometer say in the region of 5 .. 12 nm, values even below 1 nm are possible. The core diameter of the input fiber influences the optical resolution, if it acts as the input slit. A separate parameter is the pixel or digital resolution. It is the spectral bandwith, which is detected by one pixel of the array and is determined by the width of the pixel and the dispersion of the spectra. Common values are 2 ... 10 nm/pixel. The getSpec.com spectrometers are available with 0,1 to 12 nm/pixel. The digital resolution is related to the spectral resolution via the width of the input slit and the imaging properties of the spectrometer. The ratio of digital spectral resolution should be > 3 to detect savely a peak in the spectrum.
Stray Light
Stray light is radiation of false wavelengths which strikes a pixel.It is caused by imperfections of the grating, dust, reflexion of the spectrometer housing or errors of other optical elements. This parameter influences the precision of a spectroscopic measuring system in a decisive way. It gets measured by a broad band illuminating of the inplut slit through a color filter with long pass characteristics. A filter commonly used therefore is the Schott glass GG 495. The stray light S is the ratio of the transmission in the blocked wavelength region below the filter edge (t(420 nm)) to the transmission in the transmissive region (t(600 nm)).
It shows the influence of the light of longer wavelengths, passing the filter, on the unwanted intensity in the blocked region. A measured spectrum is shown in the following figure.
In case of a low stray light level (as in the figure shown) an additional neutral density filter is used for the measurement in the non blocked region, which will be removed for the stray light measurement.
A standard measurement method is described in the ASTM standard test method E 387 (available from www.astm.org). Detailed information about modifications of the measurement procedure and their influence on the result can be obtained from the Agilent technical note "Measuring the stray light performance of UV-visible spectrophotometers" (available at: www.chem.agilent.com/scripts/literratureSearch.asp) Another often used stray light measuring method uses a monochromatic light source (e.g. a HeNe laser). The intensities at the laser wavelength and at another wavelength, several 10 nm away from the laser wavelength are measured. The ratio of the latter to the former is a measure for the stray light of the system. This measuring method delivers much better values, but is more distant to the main practical applications, where a broadband illumination is used. Therefore, the method, described first, is recommended. The stray light causes a non-linearity of the signal at lower power levels and thus limits the measuring range of the system. For example: the signal in the blue region of a VIS spectrometer depends on the incident intensity in the other regions of the spectrum. This effect cannot be compensated precisely by mathematical calculations in the software. Every specification of stray light has to be given in connection with the used measuring conditions. One has to keep in mind, that the the higher the band width of the light entering the spectrometer, the higher the measured stray light ratio. The value, measured with a long pass filter with an edge in the red region of the spectra is not comparable with a value, measured with a yellow edge filter.
ADC Resolution
The analog intensity distribution of the optical spectrum on the detector array has to be converted pixelwise to a digital signal by an analog digital converter (ADC). Usual electronic resolutions are 12 ... 16 bit (4095 ... 65 535 counts full scale). These numbers are finally reduced by the dark spectrum of the line array and the driving of the converter. The ADC resolution should be in a suitable ratio to the stray light of the spectrometer and to the dynamics of the detector array. The dynamics D of the entire system is determined by the ADC resolution RADC, divided by the noise signal F.
A 12 bit spectrometer with a noise signal of 4 counts offers a dynamics of about 1000.
Integration Time
The light intensity coupled into the spectrometer and illuminating a pixel, results in a proportional signal from the AD converter. The exposure time of the light to the pixel is called integration time and is a main parameter to adjust the ADC signal. A level between 2/3 and full scale is suited for best measuring results, using the full dynamics of the system. Off scale peaks or regions in the spectrum have to be avoided. Common values for the integration time of array spectrometers are 20 ... 5000 ms. Several instruments, as the getSpec.com "High performance" devices, are equipped with an automatic integration time adaption .
Spectral Sensitivity
The sensitivity of a spectroscopic system E(l is another important issue in many applications, especially in fluoroscence detection, and is a main criterion for the choice of spectrometer. It is measured in counts/ Ws and means the ADC signal r [counts] divided by the optical energy P(l ) ×tint[Ws], entering the spectrometer optical input.
The spectral sensitivity has to be specified for a given wavelength mainly because of the spectral dependencies of the grating reflectivity and the receiver sensitivity. The measurement can be done with bandpass filtered white light illumination or a monochromatic light source, from which the radiant flux P (l ) is known. Furthermore, sensitivity data have to be related to the ADC resolution of the read out electronics. The main uncertainity in sensitivity measurements is caused by the measuring error of P (l ), coupled into the spectrometer. The spectral sensitivity can be adapted in certain limits to the application by a proper selection of the spectrometer's detector array of the spectrometer.
Wavelength and Intensity Calibration
The AD converter delivers the spectral signal in counts for each pixel. The pixel are numbered and these numbers have to be transformed into the corresponding wavelength. This can be done by a multiorder polynom as follows :
where n is the pixel number, k0 [nm] the wavelength of the first pixel, k1 [nm/pixel] the pixel resolution and k2, k3, .., ki are the higher order coefficients. This approximation into a polynom gives a wavelength precision, which has to be taken into consideration in the application. The choice for the order of the polynom depends on the non-linear behavior of the spectrometer.The calibration can be done by relating the peaks of a suited spectral lamp (e.g. Hg in the VIS range) to the corresponding pixel and subsequent calculation of the k-parameters by regression (e.g. in Excel). getSpec.com offers a special fit program for this purpose. This program can be used to measure the spectra, indicate the peaks, calculate the k-parameters and check the results. A spline fit is used to get a subpixelprecision. A simple way to check the calibration of a spectrometer for somebody who has no special spectral lamp is to use a common fluorescent lamp (F11, possibly on top of the lab), which has a spectrum as shown in the following diagram. The intense peaks of 546 and 614 nm can be used for a roughly test of the calibration.
Something else, sometimes mixed with the wavelength calibration, is the calibration of the intensity axis. A non-calibrated spectrometer as the SentroSpec2048 shows an intensity axis (y-axis) in counts or percent. Therefore, only relative measurements of light sources, e.g. the spectral distribution of LED, are possible. One has to keep in mind, that the spectrum is weighted with the instrument function (grating efficieny, transmittance, detector sensitivity). For radio and photometric measurements of light sources, an absolute calibration of the intensity axis in W/nm or W/sr m2 nm is necessary. In this case the individual influence of the optical parts in the spectrometer ray path is eliminated and calculations as of luminance and color temperature are possible.
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Last change 08/15/2007 02:52 PM
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