Justin Charles Dunlap

Exposure Time Dependence of Dark Current in CCD Imagers

In this paper, we present a systematic study on the rate of dark current generation of two scientific charge-coupled device imagers. The dark current in both imagers was measured for exposure times from 5 to 7200 s at a constant temperature. As one would expect, the majority of pixels show a linear increase in dark count with exposure time. However, we found distinct groups of pixels that show a nonlinear dark current dependence versus exposure time well below saturation. Since the dark count is often assumed to scale linearly with exposure time, these pixels can pose a problem during dark current correction. We also discuss what could cause some pixels to produce a dark count that is linear versus exposure time whereas others do not.

Dark current measurements in a CMOS imager

We present data for the dark current of a commercially available CMOS image sensor for different gain settings and bias offsets over the temperature range of 295 to 340 K and exposure times of 0 to 500 ms. The analysis of hot pixels shows two different sources of dark current. One source results in hot pixels with high but constant count for exposure times smaller than the frame time. Other hot pixels exhibit a linear increase with exposure time. We discuss how these hot pixels can be used to calculate the dark current for all pixels. Finally, we show that for low bias settings with universally zero counts for the dark frame one still needs to correct for dark current. The correction of thermal noise can therefore result in dark frames with negative pixel values. We show how one can calculate dark frames with negative pixel count.

Modeling Nonlinear Dark Current Behavior in CCDs

A model explaining nonlinear time dependence of pixels in charge-coupled device (CCD) pixels is presented. The model describes the movement of the pixels depletion edge based on varying quantities of signal charge. Dynamic depletion edges will affect dark current collected by pixels with impurities located in the region of movement of the edge. The model attempts to address nonlinear behavior of dark current with respect to exposure time well below the saturation level seen in some CCD imagers. Modeling an imager by giving pixels varying number of impurities and depths of those impurities, assuming a uniform distribution, leads to characteristic behavior observed in the imagers.

An Easily Assembled Laboratory Exercise in Computed Tomography.

In this paper, we present a laboratory activity in computed tomography (CT) primarily composed of a photogate and a rotary motion sensor that can be assembled quickly and partially automates data collection and analysis. We use an enclosure made with a light filter that is largely opaque in the visible spectrum but mostly transparent to the near IR light of the photogate (880 nm) to scan objects hidden from the human eye. This experiment effectively conveys how an image is formed during a CT scan and highlights the important physical and imaging concepts behind CT such as electromagnetic radiation, the interaction of light and matter, artefacts and windowing. Like our setup, previous undergraduate level laboratory activities which teach the basics of CT have also utilized light sources rather than x-rays; however, they required a more extensive setup and used devices not always easily found in undergraduate laboratories. Our setup is easily implemented with equipment found in many teaching laboratories.

Correction of dark current in consumer cameras

A study of dark current in digital imagers in digital single- lens reflex (DSLR) and compact consumer-grade digital cameras is presented. Dark current is shown to vary with temperature, expo- sure time, and ISO setting. Further, dark current is shown to in- crease in successive images during a series of images. DSLR and compact consumer cameras are often designed such that they are contained within a densely packed camera body, and therefore the digital imagers within the camera frame are prone to heat generated by the sensor as well as nearby elements within the camera body. It is the scope of this work to characterize the dark current in such cameras and to show that the dark current, in part due to heat gen- erated by the camera itself, can be corrected by using hot pixels on the imager. This method generates computed dark frames based on the dark current indicator value of the hottest pixels on the chip. We compare this method to standard methods of dark current correction.

Dark current behavior in DSLR cameras

Digital single-lens reflex (DSLR) cameras are examined and their dark current behavior is presented. We examine the influence of varying temperature, exposure time, and gain setting on dark current. Dark current behavior unique to sensors within such cameras is observed. In particular, heat is trapped within the camera body resulting in higher internal temperatures and an increase in dark current after successive images. We look at the possibility of correcting for the dark current, based on previous work done for scientific grade imagers, where hot pixels are used as indicators for the entire chips dark current behavior. Standard methods of dark current correction are compared to computed dark frames. Dark current is a concern for DSLR cameras as optimum conditions for limiting dark current, such as cooling the imager, are not easily obtained in the typical use of such imagers.

Influence of illumination on dark current in charge-coupled device imagers

Thermal excitation of electrons is a major source of noise in charge-coupled-device (CCD) imagers. Those electrons are gen- erated even in the absence of light, hence, the name dark current. Dark current is particularly important for long exposure times and elevated temperatures. The standard procedure to correct for dark current is to take several pictures under the same condition as the real image, except with the shutter closed. The resulting dark frame is later subtracted from the exposed image. We address the ques- tion of whether the dark current produced in an image taken with a closed shutter is identical to the dark current produced in an expo- sure in the presence of light. In our investigation, we illuminated two different CCD chips with different intensities of light and measured the dark current generation. A surprising result of this study is that some pixels produce a different amount of dark current under illumi- nation. Finally, we discuss the implication of this finding for dark frame image correction.

Measurements of dark current in a CCD imager during light exposures

Thermal excitation of electrons is a major source of noise in Charge-Coupled Device (CCD) imagers. Those electrons are generated even in the absence of light, hence the name dark current. Dark current is particularly important for long exposure times and elevated temperatures. The standard procedure to correct for dark current is to take several pictures under the same condition as the real image, except with the shutter closed. The resulting dark frame is later subtracted from the exposed image. We address the question of whether the dark current produced in an image taken with a closed shutter is identical to the dark current produced in an exposure in the presence of light. In our investigation, we illuminated two different CCD chips to different intensities of light and measured the dark current generation. A surprising conclusion of this study is that some pixels produce a different amount of dark current under illumination. Finally, we discuss the implications that this has for dark frame image correction.

Pulse Oximetry in the Physics Lab: A Colorful Alternative to Traditional Optics Curricula

We designed a physics laboratory exercise around pulse oximetry, a noninvasive medical technique used to assess a patients blood oxygen saturation. An alternative to a traditional optics and light lab, this exercise teaches the principles of light absorption, spectroscopy, and the properties of light, while simultaneously studying a common medical device. Pulse oximeters are ubiquitous in clinical environments; many people who have undergone surgery or visited a hospital environment have experienced the use of this device, making it a good candidate for an investigative lab. The experiment elicits the creative process of device development from students as they conduct measurements using a blood analog that reconstructs the principles of pulse oximetry.

Dark current modeling with a moving depletion edge

Abstract. Within a pixel in a digital imager, generally either a charge-coupled device or complementary metal oxide semiconductor device, doping of the semiconductor substrate and application of gate voltages create a region free of mobile carriers called the depletion region. This region fills with charge after incoming photons or thermal energy raise the charges from the valence to the conduction energy band. As the signal charge fills the depletion region, the electric field generating the region is altered, and the size of the region is reduced. We present a model that describes how this dynamic depletion region, along with the location of impurities, will result in pixels that produce less dark current after being exposed to light and additionally show nonlinear production rates with respect to exposure time. These types of effects have been observed in digital imagers, allowing us to compare empirical data with the modeled data.