Yair Talmi

Self-scanned photodiode array: a multichannel spectrometric detector.

The performance characteristics of a 1024-element self-scanned photodiode array, related to its use as a multichannel spectrometric detector are described. The parameters discussed include, spectral and temporal response, blooming, geometric accuracy, noise sources, dynamic range, signal integration and spectral as well as spatial resolution.

Spectrophotometry and Spectrofluorometry with the Self-scanned Photodiode Array

A state-of-the-art self-scanned photodiode array, utilized as a spectrometric multichannel detector, was studied for its applicability to molecular absorption and molecular fluorescence spectrometry. Molecular absorption spectrophotometry requires detectors with high UV to near IR response, high dynamic range, linear response, low noise, and temporal as well as thermal stability. Molecular fluorescence spectrometry requires, in addition, sensitivity to low-light level signals. These requirements were satisfied by the systems under study. Other parameters that govern the overall performance of diode array-based spectrophotometers and spectrofluorometers include: photometric range, stray energy radiation and its effect on photometric accuracy, photometric precision, spectral resolution sensitivity, and signal-to-noise considerations. Various readout methods for the array detectors were examined including: real time, in-memory signal integration, on-target signal integration, variable integration time (VIT), diode grouping, and diode fast access (a pseudorandom access readout). Performance demonstrations of these multichannel spectrometers were made with a few typical chemical systems. The spectrophotometric performance of the diode array system was excellent, practically equal to that of conventional single-channel systems. Exceptions were higher amenability to the adverse effects of stray radiation energy and a dependency of the geometric (wavelength) accuracy on spatial fluctuations (wander) in the light source. Potential long-term UV degradation of diode arrays may present an inherent problem, but was not discussed here because data accumulated up-to-date was considered inconsequential. As a spectrofluorometer multichannel detector, the diode array showed an “unexpected” signal-to-noise (detectability) performance, closely matching that of conventional high gain detectors. However, this performance was achieved only if sufficiently long signal integration times (20 to 160 s) were feasible. Overall performance of the diode array has demonstrated that theoretical multiplex advantages are achievable, resulting in either a considerable improvement in signal-to-noise or a corresponding shortening in observation time.

The Use of a Near-Infrared Array Detector for Raman Spectroscopy beyond One Micron:

The performance characteristics of a multielement Ge array detector have been measured. The effects of read noise and dark signal on signal integration have been evaluated. The performance of this array for Raman spectroscopy with excitation at one micron has been compared with FT-Raman spectroscopy. It is an excellent detector for narrow-spectral-bandwidth Raman studies, and the sensitivity compares favorably with that for FT-Raman techniques.

Pyroelectric vidicon: a new multichannel spectrometric infrared (1.0–30-μm) detector

A pyroelectric (triglycine sulfate) vidicon tube has been used in conjunction with an optical multichannel analyzer (OMA) to obtain ir (1-30-microm) spectral information. The system can detect continuous as well as pulsed ir signals. Improvement of SNR through accumulation of data in memory has been demonstrated. Various parameters that affect the performance of the system include variation of sensitivity across the target, thermal diffusion, discharge lag, thermal lag, and noise. The applicability of the system to ir absorption and laser (cw and pulsed emission) spectrometry has been demonstrated.

CCDs for spectroscopy

CCDs have been used in spectroscopy for a number of years and for all the obvious reasons. Unfortunately most scientific CCDs are square arrays and are not ideally formatted for spectroscopic applications. This paper discuses the design and fabrication of two CCD arrays specifically intended for use in spectroscopic applications. The devices have 1100 X 330 and 1752 X 532 pixel formats, and are fabricated using the three phase, overlapping polysilicon gate technology and they are available in both front-illuminated as well as back- illuminated versions. In addition, devices with enhanced UV sensitivity are being fabricated. Characterization data are presented. The architecture of these devices leads to some interesting applications which we discuss briefly.

Guidelines For The Selection Of Raman Multichannel Detectors

The performance characteristics of seven Raman multichannel detectors are discussed. The detectors are proximity-focused MCP image-intensified diode array (P/IPDA), electrostatically-focused MCP image-intensified diode array (E/IPDA), double MCP image intensified diode (D/IPDA), dual, MCP image-intensified diode array (DIDA), charge couple device (CCD), intensified charge couple device (ICCD) and intensified resistive anode (IRA): also known as MEPSICRON. The main objective of this paper is to provide Raman spectroscopists with a comparative performance information that should facilitate the selection of the optimal detector for a particular application.