Detector Spectral responsivity

We assume that the output from both detectors is proportional to the spectral irra-diance times the detector spectral responsivity ... 

Light sources can either be broadband, such as a Globar, a Nemst glower, an incandescent wire or mercury arc lamp or they can be tunable, such as a laser or optical parametric oscillator (OPO). In the fomier case, a monocln-omator is needed to achieve spectral resolution. In the case of a tunable light source, the spectral resolution is detemiined by the linewidth of the source itself In either case, the spectral coverage of the light source imposes limits on the vibrational frequencies that can be measured. Of course, limitations on the dispersing element and detector also affect the overall spectral response of the spectrometer. 

There are important figures of merit (5) that describe the performance of a photodetector. These are responsivity, noise, noise equivalent power, detectivity, and response time (2,6). However, there are several related parameters of measurement, eg, temperature of operation, bias power, spectral response, background photon flux, noise spectra, impedance, and linearity. Operational concerns include detector-element size, uniformity of response, array density, reflabiUty, cooling time, radiation tolerance, vibration and shock resistance, shelf life, availabiUty of arrays, and cost. 

This diagram shows the energy spectrum of a given source, coupled with a filter of defined transmittance, which is established by a detector of known spectral response, as modified by a standard source and modified to that of a Standard Observer. Once an instrument has been set up properly with the proper optical... 

A medically reliable UV sensor should have a spectral responsivity that closely follows the erythemal curve between 390 nm and 290 nm. So far, a photodiode with this specific sensitivity has not been available. Modeling the erythema spectrum with the help of filters also delivered only poor results. In fact, most available sunburn detectors vary in their spectral responsivity and may therefore only be used as an indicator for the actual UV charge. 

The phenomenon of fluorescence has been synonymous with ultraviolet (UV) and visible spectroscopy rather than near-infrared (near-IR) spectroscopy from the beginning of the subject. This fact is evidenced in definitive texts which also provide useful background information for this volume (see, e.g., Refs. 1-6). Consequently, our understanding of the many molecular phenomena which can be studied with fluorescence techniques, e.g., excimer formation, energy transfer, diffusion, and rotation, is based on measurements made in the UV/visible. Historically, this emphasis was undoubtedly due to the spectral response of the eye and the availability of suitable sources and detectors for the UV/visible in contrast to the lack of equivalent instrumentation for the IR. Nevertheless, there are a few notable exceptions to the prevalence of UV/visible techniques in fluorescence such as the near-IR study of chlorophyll(7) and singlet oxygen,<8) which have been ongoing for some years. 

The first measurement we make when starting a fluorescence study is not usually a fluorescence measurement at all but the determination of the sample s absorption spectrum. Dual-beam differential spectrophotometers which can record up to 3 absorbance units with a spectral range of 200-1100 nm are now readily available at low cost in comparison to fluorimeters. The wide spectral response of silicon photodiode detectors has made them preeminent over photomultipliers in this area with scan speeds of a few tens of seconds over the whole spectral range being achieved, even without the use of diode array detection. 

The use of a linear detector array in the image plane of a polychromator in place of the fluorescence monochromator in Figure 12.1 enables the parallel data accumulation of complete fluorescence spectra. Silicon photodiode arrays, operated in a CCD mode(34) are the most widely used detector elements. The spectral response of the diodes enables fluorescence to be detected from the near-UV up to ca. 1100 nm with a peak response in the near-IR. Up to 8192 elements are now available commercially in a single linear array at low cost. However, the small length of each element (ca. 10 [im) presently limits sensitivity and hence cylindrical lens demagnification is often necessary. 

We have shown that the radiant flux spectrum, as recorded by the spectrometer, is given by the convolution of the true radiant flux spectrum (as it would be recorded by a perfect instrument) with the spectrometer response function. In absorption spectroscopy, absorption lines typically appear superimposed upon a spectral background that is determined by the emission spectrum of the source, the spectral response of the detector, and other effects. Because we are interested in the properties of the absorbing molecules, it is necessary to correct for this background, or baseline as it is sometimes called. Furthermore, we shall see that the valuable physical-realizability constraints presented in Chapter 4 are easiest to apply when the data have this form. 

Solid-state light detectors are best in the near IR and IR regions. Here also the spectral response depends on the nature of the material, i.e. the semiconductor. In the very far IR photons cannot be detected directly and the only answer is to convert light into heat. The pyroelectric detectors work on this principle. 

Figure 1. Relative spectral responses of different broadband detectors in the UV spectral region. The dashed line corresponds to the CIE action spectrum. The numbers in the legend correspond to the weighted integral (Warm 2) of a standard solar spectrum (30° SZA, 330 DU).

In principle, there are two ways to achieve the radiometric calibration of an instrument measuring solar radiation. The first is by comparison to a standard radiation source of known output and the second by comparison to a prototype standard instrument that is capable in measuring the same radiometric quantity. The fist can be applied to broadband detectors only if their spectral response over the whole range of the radiation source is known with sufficient accuracy. The second method requires that the standard instrument has exactly the same spectral response, which is rather unlikely to occur. 

From its principle of operation, a broadband detector provides a signal proportional to the integral of solar radiation 1(1), weighted by its relative spectral responsivity w(X), over its entire sensitivity range. Therefore, the eiythemal dose rate E given in units of W ffm 2 can be than given from (1) ... 

As for the second condition, usually the spectral response, which is provided by the instrument manufacturers, is used. For the erythemal broadband detectors, it has been demonstrated that regular testing of their spectral sensitivity is needed, in addition and prior to their absolute calibration. This is because their spectral sensitivity is determined fiom a series of optical filters and other components (e.g. the phosphor layer) that may degrade with time or with environmental conditions (e.g. humidity), changing therefore its characteristics. 

It seems that the PDA is not fully utilized. When quantitative measurements are performed, most of the spectral information is often ignored, since detector s response is locked on a single wavelength (or two). Multicomponent analysis uses multiple wavelengths to deconvolute the unresolved peaks and provides accurate quantitation independent of peak resolution and peak shape. 

Knowledge of the optical properties of materials in relation to the solar spectrum is also important in measuring broadband solar radiation. For instance, a pyranome ter used to monitor total solar radiation for a renewable energy system has a spectral response (due to the special glass dome protecting the detector) that does not respond to the thermal infrared radiation of the sky beyond 3000 nm, as shown in Fig. 15. Flowever, there will be thermal infrared radiation exchanged between the radiometer and the sky dome, which will influence the measurement performance of the pyranometer.9... 

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