Canon EOS 20D Service Manual page 75

5) Saving power
To minimize the increase in power consumption due to the high-speed, 4-
channel reading, the output amp's power consumption is kept to an absolute
minimum.
Also, as with the EOS 10D and EOS-1Ds during long exposures, power to the
output amp is cut off. And as with the EOS-1D Mark II, the circuit-driving
standard current is also cut off. Less power is thereby consumed.
6) Infrared-blocking, low-pass filter
The EOS 20D's infrared-blocking, low-pass
filter has the same construction as the EOS
10D's filter with infrared-absorbing glass and
three crystal plates having a different optical
axis.
The infrared-blocking, low-pass filter has a
dichroic mirror made of a dielectric, multi-layer
coating (in front of the low-pass filter) that
deflects light which has a wide infrared
wavelength. It also has an infrared-absorbing
glass layer to absorb near-infrared wavelengths
in the visible spectrum. This hybrid
construction consisting of deflecting and
absorbing layers reduces the red ghosting
caused by sensor reflections and image fogging.
The low-pass filter consists of two crystal
monocrystal plates that separate the subject
image into four horizontal and vertical
directions. In-between these two plates is a
phase plate (crystal monocrystal plate) that
converts the linear polarized light into circular
polarized light. The first crystalline plate
separates the image into two images horizontally. The light flux that becomes
the orthogonal linear polarized light is converted into circular polarized light by
the phase plate. The second crystal plate separates the images vertically,
resulting in four images square. The image separation in the horizontal and
vertical directions is optimized for the sensor pitch so that false colors caused by
minute horizontal and vertical lines are reduced.
Dichroic mirror
(Reflects infrared rays)
Infrared-absorbing glass
Low-pass filter
(Separates image in
vertical direction)
Phaser layer
(Converts linear polarized
light into circular polarized
light)
Low-pass filter
(Separates image in horizontal
direction)
Fig. 004 Cross section of infrared cut, low-
pass filter
3
Technical Information
CMOS
sensor