Image Processing Reference
In-Depth Information
7
Technolog
ies to Improve Image Information Quality
In this topic, it is explained that image information is composed of light intensity, space,
wavelength, and time, and it is the role of imaging to obtain each factor with a sufficient
level of information quality for the goal of the imaging system. Since imaging systems are
used for various applications, the information provided is not always the same. As the
most important information varies according to the purpose of the imaging system, tech-
nologies to improve performance of the important factors have been developed. In this
chapter, some examples that advance the information quality of each factor are described.
7.1 Light Intensity Information
Light intensity is the most important information and contains both sensitivity and dynamic
range (DR).
7.1.1 Sensitivity
Sensors measure the amount of light coming to each built-in coordinate point, and sen-
sitivity is important to their performance. Although sensors have only been talked about
thus far in terms of a sensor chip, they are actually mounted in imaging systems after they
are bonded in packages sealed with transparent glass, as shown in Figure 7.1a.
Therefore, losses are incurred at the light phase before arriving at a silicon surface and
at the stage of light and signal charge after penetration into the silicon. This is explained
in stages. (1) Incident light decreases by about 5%-10% by reflection at both surfaces of
a sealing glass, as shown in Figure 7.1a. However, sometimes antireflection coatings are
used to recover the loss depending on the application, but these are generally not used
due to their cost. (2) Absorption in the sealing glass is negligible except in the ultraviolet
(UV) region. (3) Arriving at the sensors, part of the light is reflected at the surface of an
on-chip lens (OCL). Sometimes, OCLs are covered with material having a low refractive
index to suppress the loss. (4) Further, an OCL and an on-chip color filter (OCF) absorb
part of the light. Since the OCF has the role of restricting the wavelength region of light
that passes through it, and the spectral response is directly related to color performance,
it is necessary for the OCF to absorb light that should not be transmitted through the filter
of the color. (5) Absorption and reflection by a passivation film and an interlayer isolation
film follow. (6) Arriving at the photodiode (PD), there is reflection at the silicon surface.
A mirror-polished silicon wafer surface shows gross* reflectance in the visual region of
30%-40%, similar to metal. As this impact is never low, an antireflection (AR) film often
forms, as mentioned in Section 5.1.2. (7) Before then, light that comes to the area outside
the OCL and is not led to the aperture area is also lost.
* However, the reflection is not due to a free carrier.
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