Apparent violation of the radiant exposure reciprocity law in interline CCDs

Abstract

We analyze in detail the apparent violation of the reciprocity law that occurs for an interline CCD array owing to its responsivity that varies with exposure time, and we investigate other parameters having an influence on this variation, such as the numerical aperture of the system or the illumination wavelength. The variation can be explained from a qualitative point of view by the charge leakage that occurs from the sense region to the register region during readout time. This change of the responsivity can be avoided either by using a synchronized external shutter or by limiting the numerical aperture while keeping relatively high exposure times. A detailed characterization for each particular CCD model must be carried out if it is to be employed in low-uncertainty radiometric measurements.

Characterizing integration time and gray-level-related nonlinearities in a NMOS sensor

“…Nonetheless, results proved that responsivity was also affected by the integration-time set. This phenomenon had been previously reported in a CCD imager sensor [3]. In both cases, it was concluded that the phenomenon was due to the leakage of photo-generated electrons in the photodiode during the readout phase; these electrons would be added to those generated during the integration time.…”

“…On the other hand, in pixel 32 any fitting that took into account the different error bars would led to an almost constant L norm curve within the whole integration-time range. A relationship between responsivity and the integration time had been previously reported [3] in CCD detectors. Since light is not blocked during the readout phase, the phenomenon could be explained by the leakage of a significant amount of electrons during this phase due to pixel irradiation.…”

“…Two functions were defined and fit to correct each type of nonlinearity. On the one hand, ℜ IT t int is the nonlinearity correction factor related to the integration time, and it was defined as a hyperbolic function as in [3]. On the other hand, …”

“…A radiometric detector is said to be linear if its response-once the dark signal has been subtracted-is proportional to the number of collected photons. Linearity is usually affected by various factors, such as charge leakage during the readout phase [3], supraresponsivity [4], saturation and antiblooming switches [5], or the electronic transformations required to convert photoelectrons to digital numbers (DN) [6]. During the nonlinearity characterization and correction, the signal generated by the sensor in the absence of light (dark signal) must be subtracted.…”

EUROSPEC: at the interface between remote-sensing and ecosystem CO<sub>2</sub> flux measurements in Europe

“…In turn, non-linearities related with the integration time affected also the characterization of other instrumental artifacts . This dependence, previously reported in cameras (Ferrero et al, 2006) but not in field spectroradiometers, is significant when the integration time is close to the readout time of the sensor (i.e. a photodiode continues to collect photons during the time when the signal is being processed (the readout time), producing an extra signal that is added to that obtained during the integration time).…”