Frequency-Domain Instrumentation

There is significant debate about the relative merits of frequency and time domain. In principle, they are related via the Fourier transformation and have been experimentally verified to be equivalent [9], For some applications, frequency domain instrumentation is easier to implement since ultrashort light pulses are not required, nor is deconvolution of the instrument response function, however, signal to noise ratio has recently been shown to be theoretically higher for time domain. The key advantage of time domain is that multiple decay components can, at least in principle, be extracted with ease from the decay profile by fitting with a multiexponential function, using relatively simple mathematical methods. 

Figure 3. An example of frequency domain instrument. ISS, Inc. s oximeter model 96208.

An ingenious multichannel instrument for tissue imaging was developed at the University of Illinois [162], The thrust of the research was to develop a frequency-domain instrument for noninvasive, real-time NIR optical tomography of tissue in... 

Optical tomography techniques for human medicine are currently at the stage of clinical tests [204, 225, 489, 490]. Frequency domain instruments using modulation techniques are competing with time-domain instruments using TCSPC. 

A comprehensive overview of frequency-domain DOT techniques is given in [88]. Particular instraments are described in [166, 347, 410]. It is commonly believed that modulation techniques are less expensive and achieve shorter acquisition times, whereas TCSPC delivers a better absolute accuracy of optical tissue properties. It must be doubted that this general statement is correct for any particular instrument. Certainly, relatively inexpensive frequency-domain instruments can be built by using sine-wave-modulated LEDs, standard avalanche photodiodes, and radio or cellphone receiver chips. Instruments of this type usually have a considerable amplitude-phase crosstalk". Amplitude-phase crosstalk is a dependence of the measured phase on the amplitude of the signal. It results from nonlinearity in the detectors, amplifiers, and mixers, and from synchronous signal pickup [6]. This makes it difficult to obtain absolute optical tissue properties. A carefully designed system [382] reached a systematic phase error of 0.5° at 100 MHz. A system that compensates the amplitude-phase crosstalk via a reference channel reached an RMS phase error of 0.2° at 100 MHz [370]. These phase errors correspond to a time shift of 14 ps and 5.5 ps RMS, respectively. 

The count-rate-dependent shift of the first moment found for some typieal detectors is summarised in the table 7.2. For eomparison, in the last eolumn the shift of Ml was converted into the equivalent amplitude-phase-erosstalk of a frequency-domain instrument. 

T.H. Pham, O. Coquoz, J.B. Fishkin, E. Anderson, B.J. Tromberg, Broad bandwidth frequency domain instrument for quantitative tissue optical spectroscopy, Rev. Sci. Instrum. 71, 2500-2513 (2000)... 

One of our most carefully characterized intensity decays is for 2,S-diphenyl-l,3,4-oxadiazole (PPD) in ethanol at 20 °C, in equilibrium with air. The frequency response was measured with a gigahertz frequency-domain instrument. No deviations from a single-exponential decay were detected over the entire range of firequencies (Figure II.2). 

The frequency-domain methods are very suitable for measuring the lifetimes of most luminescent lanthanide compounds. The reason is the range of rate constants of decay processes, which fits well in the frequency range of the conventional inexpensive lock-in amplifiers. Together with the easily modulated light-emitting diodes or laser diodes as the excitation sources, frequency-domain instruments for the lifetime measurements can be constructed in any workshop with rather minimal resources and expertise. 

Vinogradov SA, Femandez-Searra MA, Dugan BW, Wilson DF (2001) Frequency domain instrument for measuring phosphorescence lifetime distributions in heterogeneous samples. Rev Sci Instrum 72 3396-3406... 

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