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Dark current dark noise1/30/2024 ![]() Other thermal sources, such as off-sensor high frequency components (i.e. ![]() Dark "current" ultimately gets its name from the fact that friction of electrons flowing through a circuit generates heat, which, in the case of a photodiode system like a sensor, can inadvertently release electrons. Other thermal sources beyond just the ambient temperature can affect dark current, though. Generally speaking, for every 10° ambient temperature change, the contribution of dark current should change by a factor of two. I wouldn't expect this to be much, but I haven't quantified it, so I can't say for sure.ĭark Current should primarily be a thermal factor, however heat can be introduced from a variety of sources. Radiation like this is all around us, and the camera sensor can pick it up just like a Geiger counter can. This can include cosmic rays or maybe the radiation from a luminous dial wrist watch (some of us have these!). One thing to consider is any penetrating radiation. There might be other factors that can account changes. You might see that noise (or noise figure) apparently isn't identical from one measurement to another. This is a statistical measurement, and your results get better if you make the measurement over a longer time. I don't imagine this would be easy for a multi-megapixel sensor, but it is doable. There is plenty of literature on how to quantify noise figure, and I won't go into it here (unless you have a specific question, and then I can try to answer it). Measuring and quantifying noise figure is a little complicated, but it's a common part of designing any electrical circuits that are sensitive to noise (like your cell phone receiver, for example). You should be able to do that without enabling a noise reduction feature - just leave the lens cap on and take a picture. I would take that as an endorsement.Īs I explained, the noise can be measured by taking a dark exposure. I have never tried NR on my camera, but I have seen professional light painters use it. The assumption here is that the noise level measured at each pixel is nearly identical, so subtraction removes much of the noise. Once you have two "exposures" from the sensor, the dark image is subtracted from the real image. It the circuitry waits too long to measure the charge at every pixel, then noise current can continue to add to the charge. Even when the NR dark image is recorded (and the shutter is closed), this sensor processing has to be done. At the end of the exposure, the electrical charge must be measured. At the beginning of an exposure, every pixel must be drained of any electrical charge caused by the previous exposure and any noise since the previous time it was cleared. At the same time, the sensor must be operated. In the description above, and exposure is not just the time the shutter is opened. So some pixels will have more noise than others, even though they are at the same temperature. This is because there may be minute differences in noise figure for each pixel (even though they are manufactured on the same wafer, or chip). This sensor noise can be different for every pixel in the sensor. The amount of charge is proportional to the photonic power plus the noise power that it is sensing. The accumulation results in a measurable electric charge at each pixel of the sensor. If the shutter is open, this accumulation of sensor noise also has an accumulation from the image intensity added to it. The longer the exposure is, the longer this accumulation takes, and the sensor noise from dark current builds up. (You must choose to enable this NR process).įirst, you should understand that what the sensor is recording is an accumulation of the effects of photonic and electrical current over the duration of the exposure. It then subtracts the sensor noise from your real photo. This is done immediately after the real picture is taken, so it is done under the same conditions (same temperature) as the original picture. (I suppose other makers have something similar.) In this NR process, a dark image is recorded from the sensor, with the shutter closed. Nikon uses a noise reduction process that is worth understanding. Analog circuits with a higher noise figure means that the random fluctuations in the noise current has a wider spread (variance, in mathematical terms) than circuits with lower noise figures, given that they are at the same temperature. ![]() There is an intrinsic quality of the camera sensor and analog processing called "noise figure".
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