Photodiode arrays (PDAs) and charge coupled devices (CCDs) are multichannel detectors. They vary in the signal handling and readout mechanisms, and in sizes of sensitive elements or pixels. Another aspect is the linear versus two dimensional value of the arrays.
Silicon is used as a detector in the LineSpec™ family and most of the InstaSpec™ line of multichannel detectors from Oriel Instruments. Hence, these detectors are responsive in the spectral range of 200-1100nm. InGaAs PDAs offered by Oriel Instruments are responsive from 800 to 1700nm.
The self-cleaning type is the most useful and one-of-its-kind arrays in the Oriel catalog. Here, the incoming photon flux is integrated for some time by the individual detector elements and their related circuitry, which are then read out sequentially. This differs greatly from the real time operation of single element detector systems.
The dynamic range of individual photodiodes is limited by the self-scanning mode of operation, from nine or ten orders of magnitude to a more realistic three to five orders of magnitude, depending on devices. The capacity of the charge storing structures is limited by the high density packing of the small photosensitive pixels to reduce non-photoresponsive areas and deliver high resolution.
Requiring only a single data channel to read the data from all the spectral or imaging channels is the key advantage of this charge storage capability. All of the arrays contribute to the major advantage of multichannel or multiplexing capability. They allow for a nearly immediate acquisition of one or more complete spectroradiometric data sets, when integrated to a standard or imaging spectrograph. The conventional method would involve mechanical scanning of one or more monochromators to bring different wavelengths of light in a serial fashion to the single detector location.
Photodiode Arrays (PDAs)
Oriel PDAs are of linear format, with an aspect ratio of individual pixels of 50µm wide by 200µm high for the InGaAs models, and 25µm wide by 25mm high for silicon based models. They are engineered to complement spectrograph function. Narrow width yields high resolution, while considerable height enables more signal acquisition. They run in a photoconductive mode. The individual diode elements generate photocurrents in response to the incident photon flux. This, in turn, charges individual storage capacitors, which are read and released by on-chip circuitry, and the data is fed out in a timed sequence.
Integration time is a key factor in making measurements with the arrays. Dark current negatively affects these storing detectors, due to prolonged integration times that lead to reaching full capacity of all the storage elements. This, in turn, wipes out all the valuable data. This phenomenon is described using a concept of saturation time. The characteristic temperature dependence of saturation time for PDAs is depicted in Figure 1.
Figure 1. PDA saturation time vs temperature
Oriel Instruments offers a variety of PDA based systems, which are economical and have higher light exposure capability when compared to CCDs. The drawback of PDA based systems involves higher readout noise, and hundreds of thousands of electrons per channel, limiting their ultimate detectivity.
LineSpec™ and InstaSpec™ CCDs
LineSpec™ CCDs are economical linear CCD detectors, allowing high speed data acquisition at a lower cost. With 14x14µm or 14 x 200µm pixels, they are suitable for intermediate illumination level applications. InstaSpec™ CCDs are 2D arrays offering diverse electronic construction when compared to PDAs. They have very low readout noise, down to a few electrons per pixel, with good separation between signal channels without blooming. However, their cost is higher than PDAs.
Software options facilitate leveraging the 2D nature of CCDs for a spectroscopic form of imaging (Figure 2). The entrance slit of an imaging spectrograph is illuminated by multiple, spatially separated inputs. The CCD is illuminated by the spectrally resolved image of the sources at the output plane of the spectrograph. Multiple spectra of excitation and different responses, for instance, can then be recorded concurrently for comparison and analysis.
About Oriel Instruments
Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.
Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators, that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.
This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.
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