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Comparison between a low-light-level visible channel and an IR channel for spaceborne night imaging
Abstract
Aiming at night time spaceborne imaging, we compare the expected performances of a low-light-level visible sensor with a conventional IR sensor. The low-light-level visible sensor, an electron multiplier CCD (EMCCD), is a close to ideal photon counting device, with possibly negligible dark current noise and negligible readout noise. This fact, along with the significant improvement of diffraction (about an order of magnitude), suggests an interesting competition between the two technologies. In essence, this is a tradeoff between noise and optical performances (favoring the visible channel) and basic target radiance (favoring IR). Other factors such as reliability and cost can also play an important role. While we consider two different spectral ranges with different imaging content, we are able to conduct a cautious theoretical comparison based on standard targets in various lighting conditions. We show that for a given set of system parameters, even when lighting conditions are favorable, i.e. a night with a full moon, the low-light-level visible channel performances are inferior to those of an IR channel. We also comment on the significance of the system working point regarding performances under varying condition.
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A novel CCD has been commercially produced by Marconi Applied Technology, UK under the trade name of L3Vision, and by Texas Instruments, USA under the trade name Impactron, both of which incorporate an all solid-state electron multiplying structure based on the Impact Ionisation phenomenon in silicon. This technology combines the single photon detection sensitivity of ICCDs with the inherent advantages of CCDs. Here we compare the electron multiplying CCD (EMCCD) with scientific ICCDs. In particular we look at the effect of the Excess Noise Factors on the respective S/N performances. We compare QEs, spatial resolution, darksignal, EBI and Clock Induced Charge (CIC), with the latter two as the ultimate limitations on sensitivity. We conclude that the electron multiplying CCD is a credible alternative to ICCDs in all non-gated applications.
Electron multiplying charge-coupled devices (EMCCDs) enable imaging with subelectron noise up to video frame rates and beyond, providing the multiplication gain is sufficiently high. The ultra-low noise, high resolution, high-quantum efficiency, and robustness to over exposure make these sensors ideally suited to applications traditionally served by image intensifiers. One important performance parameter of such low-light imaging systems is the noise introduced by the gain process. This work investigates the noise introduced by the electron multiplication within the EMCCD. The theory and measurements of the excess noise factor are presented. The measurement technique for determining the excess noise factor is described in detail. The results show that the noise performance matches that of the ideal staircase avalanche photodiode. A Monte Carlo method for simulating the low-light level images is demonstrated and the results compared with practical experience.
High quality imaging is a key parameter in many scientific applications. CCD and ICCD cameras have proven to be powerful tools and are consequently used in a wide range of fields such as engineering research and physical or biological sciences. The very new Electron Multiplying CCD technology seems now to provide the most sensitive detection capabilities. Here we compare analytically the signal-to-noise performance of the three systems and identify the most influencing parameters. The SNR provided by CCDs is strongly influenced by the readout noise and is also a significant function of the pixel rate. ICCD cameras are practically not at all affected by the CCD chip temperature and are shown to be mostly shot-noise-limited because readout and dark current noises are negligible. Therefore no cooling is needed for ICCDs. Although EMCCDs unite the quantum efficiency of CCDs and the gain of ICCDs, their performance is constricted by charge transfer and dark current noises which will be multiplied up along with the signal by the gain register. Therefore, EMCCDs must be strongly cooled (down to -70°C) and slowly read out in order to get rid of any unwanted "pseudo signal". In addition, their properties limit exposure times to milliseconds time scales and longer. We conclude that ICCD cameras remain the most efficient systems in all gated experiments and perform very well in extreme low light situations. They still keep great advantages over standard CCDs and the new incoming generation of EMCCDs.
Infrared Detectors and Systems , Wiley-Interscience publication
- Dereniak
- Boreman
Dereniak & Boreman, Infrared Detectors and Systems , Wiley-Interscience publication, 1996. Proc. of SPIE Vol. 6940 69401H-11