Setting Exposure and Gain
The exposure of an image is a
function of the exposure and
gain registers. Exposure sets the
length of time each pixel inte-
grates the light (shutter speed).
Gain settings allow pixel values
to be amplified. Gain values from
1x to 40x are allowed, but higher
gain settings amplify noise (much
like higher ISO film speeds are
grainier). It is best to use the
lower gain settings for better
images. Gains from 1x to 10x are
generally recommended.
Note in Table 2, there are two
green gain registers, one for the
odd number row green pixels and
one for the even number row
green pixels. The green color
filters can be slightly different
between rows and this allows
fine-tuning. Using the same gain
setting for both green registers is
usually enough. Since the blue
channel is not as sensitive, using
blue gains approximately double
that of red and green will allow
the A/D full range on all three
channels.
Using a MacBeth Color Checker is
a good way to judge exposure and
color balance. A good raw image
will have a good grey scale (the
bottom patches on the chart).
Gain settings should be adjusted
so the red, green, and blue values
are equal on any one grey patch.
After setting gain, the exposure
registers should be adjusted for a
good exposure. There are three
exposure registers (see Table 3).
Table 2. PGA Gain Register Settings.
Register Name
PGA Gain Register: Green
PGA Gain Register: Red
PGA Gain Register: Blue
PGA Gain Register: Green
Mnemonic
ERECPGA
EROCPGA
ORECPGA
OROCPGA
Address (hex)
0x0F
0x10
0x11
0x12
Table 3. Row Exposure Register Settings.
Register Name
Row Exposure Low Register
Row Exposure High Register
Sub-Row Exposure Register
Mnemonic
ROWEXPL
ROWEXPH
SROWEXP
Address (hex)
0x13
0x14
0x15
The row exposure high register
(upper 8 bits) and row exposure
low register (lower 8 bits) act as
a single 16 bit register. This
16 bit register sets the integra-
tion time (shutter speed) of the
sensor. The sub-row exposure
register is used for very small
changes to exposure and allow
fine-tuning for exact shutter
speeds.
Proper exposure will result in
black values near 0x00 and white
values near 0xFF (assuming
8 bits). All six grey patches on the
MacBeth chart should have
different average intensity values
in the image. If the two brightest
patches both appear white then
the exposure is too long. If the
two darkest patches both appear
black then the exposure is too
short. Remember that the raw
image does not have gamma
correction applied yet. The final
grey scale image needs to be
evaluated after gamma correction.
Image Processing
The raw data from the sensor
requires image processing before
a digital image is ready for
viewing. Some standard steps of
image processing are as follows:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Defective pixel correction
Lens flare subtraction
Auto-exposure
Auto-white balance
Color filter array interpolation
(demosaic)
Color correction (3x3 matrix)
Gamma correction
Color space correction (3x3
matrix)
Data compression
Image processing is not part of
the sensor and must be supplied
separately. Image processors
that are compatible with these
sensors are available from Agilent
Technologies and selected Agilent
partners.
4
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