3D Time-of-flight distance measurement with custom - Universität ...

72 CHAPTER 3
A
s =
sf
⋅ q
C
⎡ V ⎤
⎢ ⎥
⎣electron⎦
Equation 3.19
A small conversion capacitance C therefore leads to both low noise and high
sensitivity.
The responsivity r additionally takes into account the quantum efficiency of the
optical detection mechanism. It is expressed in terms of volts per photon and
depends on the wavelength of the light:
r = QE
3.1.6 Optical fill factor
A
() λ ⋅ s = QE()
λ ⋅
sf
⋅ q
C
⎢ ⎥
⎣photon⎦
⎡
V
⎤
Equation 3.20
The last important measure we would like to mention for solid-state photo-sensing
is the optical fill factor. This is the ratio of light sensitive pixel area to the total pixel
area. In APS sensors a certain amount of pixel area is occupied by the in-pixel
amplifier. Photons impinging on these areas do not in general contribute to the
optically generated signal. They are either reflected or absorbed by opaque metal
regions or they generate electron hole pairs, which are directly drained to a
diffusion. CMOS APS sensors have typical optical fill factors between 30% and
50%. In CCDs the optically generated charge is sometimes - depending on the
CCD architecture - stored in an opaque region of the pixel, also decreasing the
optical fill factor. CCDs usually have optical fill-factors between 50% and 100%.
For our TOF application a large optical fill factor is of essential importance since we
use active illumination. The optical power of the modulated illumination source is
both expensive and limited by eye-safety regulations. This requires the best
possible optical fill factor for an efficient use of the optical power and hence a high
measurement resolution.