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.

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.

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