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Optical Low Pass Filter
Name: Eric Y.
Status: Other
Age: 40s
Location: N/A
Country: N/A
Date: August 2004
Question:
Some digital camera have "optical low
pass filter" in their lens. What is it (I mean the construction, it looks
like three pieces of glass sandwiched together.) and the physics behind it?
Replies:
The image sensor in a digital camera has a rectangular array of sensing
elements or pixels. You can see these "pixels" when enlarging a digital
image. They form a square grid.
Suppose you are taking a photograph of a wire screen or fence. If the lines
in the screen or fence happen to line up closely (but not perfectly) with
some of the lines of pixels (especially pixels representing different
colors), then a moiré pattern will occur. This shows up as strange
patterns, or a jumble of colors. A moiré pattern (on larger scale) can be
made by placing one window screen on top of another and turning it slightly.
When the wires in one screen almost line up with the wires in the other
screen, you get strange patterns. Same for the camera, when the fence rails
almost line up with the pixels you get strange patterns.
Another effect is aliasing. Aliasing is a seen when a thin straight line is
slightly tilted with respect to the array of pixels. The line will be on
one line of pixels, then jump to the next then to the next. It produces a
"stairstep" effect. [You can easily see the "stairstep" effect on the
monitor screen on the back of most digital cameras. The monitor screen is
not very sophisticated and moiré and aliasing can occur when viewing lines,
bricks and fences. The actual digital picture, however, will be clear.]
The way to avoid these errors is to use an optical low pass filter. An
optical low pass filter is a fancy name for a thin transparent sheet that
causes a special kind of blurring. The filter blurs the image, but only a
pixel or two, and not more than that. This eliminates aliasing and moiré,
yet hardly degrades the image. The amount of spreading is chosen
specifically for the type of camera.
Electrical engineers use "low pass" filters too, but these let the low
electrical frequencies past, but block out the high frequencies. The optical
idea is similar. The highly detailed, "high frequency" image components are
blocked.
Conventional photographic film contains "grains" in a random pattern. That
means aliasing or moire cannot occur in regular film cameras.
Some digital cameras use a birefringent plate to precisely blur the image.
A thin sheet of quartz is used to split or deflect the light according to
the polarization. One spot will be split into two spots by the birefringent
plate. Then a second plate splits those spots again into a total of 4
spots, all of them a pixel or two apart. The sheet is placed in front of
the image sensor. This makes the picture a little blurry and is enough to
eliminate aliasing and moiré. Other digital cameras use a clear plastic
plate that has dots molded into it to blur the image.
Bob Erck
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Update: June 2012
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