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

122 CHAPTER 5
from their site of optical generation to the storage area. If they do not reach the
right storage site in time, they will contribute to a wrong sampling point and lower
the demodulation amplitude.
• Fringing fields: The higher the fringing fields between two adjacent electrodes
the higher is the speed of charge transfer and thus the resulting demodulation
efficiency increases, especially at high frequencies.
• Gate length: The smaller the gate length the more quickly the photoelectrons
arrive at the storage sites and the higher becomes the influence of fringing
fields. A small gate length is therefore the key parameter for a fast demodulation
performance.
• Wavelength: The use of long wavelength light leads to a large penetration depth
of the incident light. This again causes long diffusion times and blurred diffusion
directions. The effective charge capturing area grows. This reduces the
sharpness (crosstalk) and leads to a lower demodulation contrast. Long diffusion
times lower the demodulation cut-off frequency, while the blurring effect (also
opaque CCD gates directly collect photogenerated charge) also appears at low
modulation frequencies or even under DC conditions.
Optical measurement setup
Figure 5.11 shows the optical test setup for the measurements described below.
The lock-in sensor chip is mounted on the camera board, which is fixed to an XYtable
under a microscope. This microscope is used to illuminate the chip with a
variable amount of optical power. LEDs of different wavelength are used to
illuminate the pixels. They can be plugged into a fast LED driver module (MV-LED1
[CS1]), which is capable of modulating them up to 20 MHz, depending on their
internal capacitances.
The light of the LED passes through an iris (or pinhole) and the objective of the
microscope. The distance from the objective to the lock-in chip is adjusted such that
the light beam is not in focus on the chip. In this way we get a homogenous light
spot on top of the chip, leading to an equal illumination of all the pixels. By varying
the distance of the objective to the chip we can adjust the size of the light spot and,
hence, the illumination intensity on the chip. By measuring the total power of the
light spot and the diameter of the spot, the total optical power on the photogate can