– Offset, independent of the exposure time, being equal to 819 DN, So every dot in Figure 1 is the result of 320 x 240 x 25 = 1,920,000 pixels.įigure 1 : average dark signal as a function of the exposure time, measured at 30 oC.įrom the regression line for the linear part of the curve in Figure 1, the following numbers can be deduced : In this measurement a sensor with 320 x 240 pixels is evaluated. At each exposure time all these images were averaged (to reduce thermal noise) and the averaged image is again averaged over all its pixels (to reduce fixed-pattern noise). Once the data of the dark images is present in the computer, calculation of the average dark signal becomes very simple.įigure 1 shows the outcome of an average dark signal measurement : at various exposure times and at 30 oC, multiple images are being grabbed, e.g. – Grabbing images in dark by means of a frame-grabber and a computer. § N V : total number of pixels in vertical direction (including black reference pixels). § N H : total number of pixels in horizontal direction (including black reference pixels), § N pix : average number of electrons per pixel, The relation between the measured reset drain current I RD and the average number of electrons in one pixel N pix is given by : So CCDs do offer a very easy way of measuring the average output signal, just by measuring the average reset drain current. This is the case for CCD imagers, this is not the case for CMOS imagers. This technique is only applicable if the drain(s) of the reset transistor(s) is/are available through a separate connection. – The measurement of the reset-drain current. – A good old oscilloscope : in this way one can measure the average output voltage of a sensor or camera, These measurements can be done by means of : The simplest way to separate the DC offset from the thermal component is to perform measurements at several exposure or integration times. This part is not just (exponentially) depending on the temperature but has a linear dependency on the exposure or integration time as well (at least if saturation of the sensor is not reached). – A thermal component, also known as the dark current or leakage current. – A fixed DC offset, very often introduced by the analog circuitry on pixel-, on column- and on chip-level, or by an extra black level offset, On the other hand, the average dark signal delivered by a sensor or a camera will be composed out of several components : In addition to a capped lens, the measurement room can be made dark and/or the sensor can be covered with a black, non-transparent cloth. For that reason the camera needs to be 100 % shielded from “surrounding” light as well. Still light can come from the sides or even from the back to the sensor. This seems to be very straight forward, but a dark signal measurement needs to be done in fully dark conditions ! Just capping the lens of a camera to shield the sensor from incoming light is not enough. The very first item that will be discussed in the new serie of blogs “How To Measure … ?” will be the average dark signal.Īctually, what is the dark average dark signal of a sensor or a camera ? By definition it is the output of the image sensor or the camera when the system is put in dark.
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