Various types of optical spectrometers are in use for such purposes as atomic emission spectroscopy, atomic absorption spectroscopy and astronomy. A complete system generally consists of a source of radiation, a spectrometer for separating and detecting individual spectral components, and a data station for processing the information from the spectrometer. The radiation source, for example, may be a system for injecting a test sample into an inductively coupled plasma where the atomic species in the sample are excited to radiate characteristic atomic emission. As another example, a sample is evaporated in a graphite furnance where the gaseous sample absorbs certain frequencies of the incident radiation to provide atomic absorption lines. Similarly, astronomical sources provide atomic emission and absorption lines for spectrographic analysis.
Spectrometers generally are based on dispersion of radiation by diffraction gratings, prisms and combinations of the two. Electronic detection devices are taking over from photographic film for accurate and timely measurements of the emission or absorption lines.
There are several types of detectors used for reading the spectrum. The conventional type has been one or more photomultiplier tubes or devices which receive directed radiation to produce free electrons which are multiplied in number to provide a real-time signal output proportional to the radiation intensity. A newer type is based on the principles of charge generation upon the incidence of radiation on a surface such as silicon. To provide resolution of spectral lines (or, more broadly, image resolution) such a surface on a semi-conductor chip is divided into pixel areas. The accumulation and handling of signals from the pixels is effected through the transfer of charges in the chip from the pixels. One type is a charge coupled device (CCD). A related approach for such detectors is charge injection device (CID) technology.
A particular form of solid state detector is disclosed in Barnard U.S. Pat. No. 4,820,048 of the present assignee, and Becker-Ross et al U.S. Pat. No. 4,940,325. A solid state chip has on the front surface thereof a two dimensional array of photosensitive pixel sensors receptive of radiation of selected spectral lines and proximate background radiation. The pixels are arranged in a plurality of subarrays with each subarray consisting of at least one of the pixels. The subarrays are positioned at a projection location on the front surface of at least one of the selected spectral lines. Electronic components formed on the chip among the subarrays are operatively connected to the pixels for producing readout signals correlating with intensities of the spectral lines.
Photomultiplier tubes have quite a wide dynamic range, i.e. the practical range of radiation intensity over which detection may be effected. Solid state sensors do not have such a range, typically having a maximum range of about four orders of magnitude. Also solid state sensors have finite readout times which may may result in contamination by spillover by subsequent radiation-induced electrons, particularly from very intense radiation on the pixels.
An object of the field of the present invention is to provide a novel means and apparatus for extending the effective dynamic range of a multiple-sensor detector in a spectrophotometer. Another object is to provide for improved organization of data runs into several groups in spectrophotometer operation with such a detector, so as to achieve simultaneous data acquisition within the groups. A further object is to provide for improved operational efficiency of a spectrophotometer operation with such a detector, and with improved signal-to-noise efficiency. Yet another object is to provide for spectrophotometer operation with such a detector without sensor readout contamination.