3D Time-of-flight distance measurement with custom - Universität ...
3D Time-of-flight distance measurement with custom - Universität ...
3D Time-of-flight distance measurement with custom - Universität ...
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DEMODULATION PIXELS IN CMOS/CCD 147<br />
5.3 Outlook: Two-photosite demodulation pixel<br />
The 1-tap pixel, introduced and characterized in the previous sections, suffers from<br />
one major drawback compared to pixel realizations <strong>with</strong> more storage sites: Since<br />
only one storage site is available per pixel, the sampling points have to be acquired<br />
sequentially rather than in parallel and therefore the pixel has problems in<br />
measuring fast changing scenes that require long integration times. In such scenes,<br />
each <strong>of</strong> the sequentially acquired sampling points per pixel could possibly belong to<br />
another region <strong>of</strong> the scene, assuming that the scene has changed (moved)<br />
between the acquisition <strong>of</strong> the sampling points.<br />
This problem can be overcome by a slightly modified pixel structure, the twophotosite<br />
demodulation pixel, which we suggest in this section for a future<br />
realization. The following things are changed <strong>with</strong> respect to the 1-tap device:<br />
(1) Since we use an integration time <strong>of</strong> half the modulation period, the<br />
photoelectrons that are dumped during integration <strong>of</strong> a sampling point directly<br />
correspond to the 180° delayed sampling point. Instead <strong>of</strong> draining this charge<br />
to a dump diffusion it will now be integrated in a second storage site <strong>with</strong>in the<br />
pixel.<br />
(2) The two-photosite demodulation pixel will consist <strong>of</strong> two equal structures<br />
according to the description under (1), which are operated in quadrature. In this<br />
way the first site acquires the sampling points A0 and A2 (phase 0° and 180°<br />
respectively) and the second site acquires the sampling points A1 and A3 (phase<br />
90° and 270°), so that all sampling points are acquired in parallel.<br />
(3) In order to achieve this, both photosites <strong>with</strong>in the pixel are illuminated <strong>with</strong> the<br />
same optical scene information. A special microlens array is suggested, which<br />
(a) makes an optical averaging <strong>of</strong> the complete optical information imaged to<br />
one pixel and (b) focuses the spatially averaged pixel information in equal<br />
proportions to both light sensitive photosites. In addition to the generation <strong>of</strong> two<br />
optically identical photosites, such a microlens structure can increase the optical<br />
fill factor up to 100%, a factor <strong>of</strong> 2 compared to the realization <strong>with</strong>out microlens<br />
array, and even a factor <strong>of</strong> 5 compared to our current 1-tap pixel realization.<br />
The microlens structure is relatively simple to realize and mount, since the chip<br />
structure only varies in one dimension.
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