Bayer mosaic filter array question

Discussion in 'Digital Photography' started by ronviers, Aug 25, 2006.

  1. ronviers

    ronviers Guest

    The figures in most documentation, that depicts the Bayer filter array,
    show the green filter elements as green, the red as red and the blue as
    blue. Is that true in reality? Are the green ones really green? For
    the purposes of this question let's say they it's true.

    The role of the individual elements of the Bayer pattern is to filter
    (turn to heat?) their associated color - e.g. the green elements turn
    green light to heat and pass red and blue. Is that correct?

    The upshot of the GRGB Bayer mosaic is the transmission (pass to the
    sensor) of a third more blue/red light than green. Is that correct?

    ronviers, Aug 25, 2006
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  2. ronviers

    Steve Wolfe Guest

    The figures in most documentation, that depicts the Bayer filter array,
    It is.
    No, a green element would pass green light to the sensor, and convert
    other wavelengths to heat.
    Yes. That's because the human eye is much more sensitive to green than to
    red or blue, so by doubling up the green sensors, the data recorded is
    somewhat closer to what our brains expect.

    Steve Wolfe, Aug 25, 2006
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  3. ronviers

    Mxsmanic Guest

    No, it's the other way around: green elements pass green light and
    turn red and blue to heat.
    No. The Bayer pattern transmits mostly green. Green is half the
    overall light transmitted, and twice the level of blue or red. The
    reason for this is that the human eye is much more sensitive to green
    light than it is to red or blue light. The Bayer pattern simply
    profits from this idiosyncrasy, improving resolution in green at the
    expense of resolution in blue and red.

    Some sensors have used secondary primaries (cyan, yellow, and magenta)
    to increase sensitivity to light. This increases low-light
    performance and luminance resolution at the expense of color accuracy
    and resolution.
    Mxsmanic, Aug 25, 2006
  4. ronviers

    ronviers Guest

    Hi Steve,

    What is still throwing me off is, why would the Bayer filter pass more
    green if that is what humans are most sensitive to? Doesn't it make
    more sense to pass the light we are less sensitive to and transmit more
    of the parts of the visible spectrum we are less sensitive to? It
    seems like to pass more of the light we are most sensitive to would be
    to compound an imbalance.

    ronviers, Aug 25, 2006
  5. ronviers

    ronviers Guest


    You mention that green is used because it gives better resolution but
    wouldn't blue be better at improving resolution since it has a
    shorter wavelength? Is it because blue would require longer exposures?

    Thanks for the reply,
    ronviers, Aug 25, 2006
  6. ronviers

    stauffer Guest

    Depends on the type of filter. Some filters reflect the light that the
    filter does not transmit, others absorb it. My impression is the ones
    used in the mosaic is the type that reflects the unwanted light. There
    is a terminology issue here- I would call the green filter the one that
    TRANSMITS the green. Same with the other two. A filter of the
    intererence/reflecting type appears purple in appearance if you view it
    from the front, green if you view it from the back.
    stauffer, Aug 25, 2006
  7. ronviers

    ronviers Guest

    Leave it to me to complicate and confuse something that should be so
    easy. I think that when I see diagrams like figure three at this link:
    When I see the arrow and the reference to the green filter I will think
    to myself, green 'pass' filter. Like the 'ew' in New Orleans, you
    never say the 'ew' just say N'orleans and think 'ew'.

    ronviers, Aug 25, 2006
  8. ronviers

    Mxsmanic Guest

    Because you need high resolution in the green, and that in turn is
    because human beings can resolve detail best in that color. A lack of
    resolution in green is much more obvious than a lack of resolution in
    blue or red.
    Mxsmanic, Aug 25, 2006
  9. ronviers

    Mxsmanic Guest

    The wavelengths of these different frequencies of light are far
    smaller than the retinal structures that capture them, so the shorter
    wavelength of blue light doesn't have any real effect on the resolving
    power of the eye, which is very coarse by comparison with wavelengths.

    Red light has a wavelength of about 564 nm, whereas the spacing
    between cone cells is around 8000 nm, or about 14 times the wavelength
    of red (and 19 times the wavelength of blue).
    Mxsmanic, Aug 25, 2006
  10. ronviers

    Mxsmanic Guest

    I think not. That would cause unwanted artifacts.

    Dichroic filters (the kind that reflect the unwanted light) are used
    in the prisms of 3-CCD video cameras, so that no light is wasted. But
    that wouldn't help in a mosaic filter placed directly over the CCD.
    Mxsmanic, Aug 25, 2006
  11. ronviers

    ronviers Guest

    Finally I get it! Thanks for sticking with me.

    ronviers, Aug 25, 2006
  12. ronviers

    Jim Guest

    There are very tiny filters in front of the sensors which pass the displayed
    color. The sensors have no color. It is just the filters that are colored.
    Jim, Aug 25, 2006
  13. ronviers

    Jim Guest

    The reason that these sensors are shown in a particular color is that that
    is the color which they measure. Thus, there are twice as many green
    sensitive sensors as either red or blue for the reason that another poster
    Jim, Aug 25, 2006
  14. ronviers

    stauffer Guest

    There are several kinds of "interference" filters- ones that use
    physical optics. There are high pass and low pass ones, many of which
    can be used as dichroics. There is also the bandpass. I believe the
    filters on most mosaics are bandpass interference filters. I am not
    sure which artifacts you mean. Yes, the reflected energy can cause
    ghosts and flares if internal baffling is not good. But absorption
    filters also reflect light, so baffling must be good in either case.
    stauffer, Aug 26, 2006
  15. I don't think they are interference filters. The bandpass profiles
    look more like simple absorption filters. Also, interference filters
    also include absorption filters to block higher orders. Interference
    filters are typically used in narrower bandpass applications than
    RGB color.

    Roger N. Clark (change username to rnclark), Aug 26, 2006
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