colour purple

Started Mar 12, 2005 | Discussions thread
omr Senior Member • Posts: 1,118
why violet comes out blue

sylvia3 wrote:

Why are Sony and Canon digital cameras unable to photograph purple.
I am sure they know about the problem but keep saying I am the only
one they have had a complaint from. Looking around the net they had
complaints back in 2002.

You deserve a decent response from someone
with an IQ far higher than mine, but here goes ...

Right or wrong, here's one of the possible explanations that have
been suggested. Consider definitions of "purple" and "violet":

Define "purple" as a mix of Red and Blue.

But then consider the spectrum of visible light; ranging from
"reds" at the low-frequency end, to "violet" at the opposite,
high-frequency end. (Call that "spectral violet", just to clearly
distinguish it versus the previous definition of "purple".)

There are NO "red-light" frequency components in such spectral
"violet" (here we are speaking only in terms of frequencies of light,
not in terms of our visual perception).

Now consider our perception of "violet".
We perceive that spectral "violet" looks something like "purple".
(Even though a pure spectral-violet light does not contain any
'red' frequencies of light, still we 'perceive it as though it did').

Speaking loosely, spectral violet stimulates our eyes' red and blue
receptors -- in other words, our eyes' response to "spectral violet" light
is somewhat similar to our eyes' response to "purple as a mix of
red and blue light".

Unfortunately a camera's sensor does not respond
to spectral violet light in the same way as a human eye:

A typical digital camera's 'Bayer type' sensor array contains pixel-sensors
individually filtered to respond to 'Red', 'Green', or 'Blue' frequency ranges.

[Note: We can ignore the additional so-called "Emerald" or cyan-like
value that Sony used in the sensor that appeared in the DSC-F828.
It doesn't matter much for this discussion. (Also, obviously
I am not including Foveon sensors in this discussion.)]

When the camera constructs the final image, those pixel sensors'
output values contribute respectively to the color-interpolated R/G/B
components of the pixel-values in the camera's final representation
of the image.

The camera's 'red receptors' (the pixel-sensor photosites which are
filtered to respond to red light) essentially don't respond to spectral
violet. Only the camera's 'blue receptors' (the pixel-sensor photosites
which are filtered for blue response) will respond strongly to spectral violet.

Result: Your photo of a "violet" object (a violet flower, for example)
may turn out too "blue", and not "purple" enough. The camera sensor
didn't 'see' much red in that mostly-violet light reflected from the flower.

The camera sensor has no straightforward way to recognize
spectral violet as anything other than 'blue' -- since only the 'blue'
pixel-sensors responded strongly to the violet light.

Those 'blue' sensors are filtered for a 'blue' range of light-frequencies
(a range including the spectral violet), AND designated to represent
a BLUE component of the final output R/G/B pixel-values.

Since only those 'blue' pixel-sensors (and none of the 'red' ones)
responded strongly to the violet light, the violet flower
unfortunately appears blue in your photo.

Also, the camera cannot (automatically) determine that some red value
should be 'artificially added' to make the result look 'more purple'.
(You can try alternative white-balance settings as noted below.)

Yet SOME "purple" flower photos do turn out OK -- perhaps because
some "purple" flowers have just-enough "redness" in their pigments,
to get an adequate response from your camera's red-responsive

Someone who really knows this stuff can argue for or against
such an explanation. (I'm ignoring some other popular arguments,
e.g. about "ultraviolet", or about monitors or other things involved
in the process of producing the image you see, etc.).

Right or wrong, the explanation above doesn't address
the sometimes-discussed supposition that "Sony sensors"
(used in various brands of cameras, not only in Sony cameras) may be
particularly susceptible to the problem. But I don't know about that.
(I have not found information to convince me to 'blame Sony'.)

In any case ... you may be able to partly compensate for the problem:

Use "manual" white-balance (if available -- see your camera manual).
Or try another preset white-balance setting (e.g., try 'Flash' WB).

If your camera supports RAW capture, try that.

Or adjust hues in Photoshop.

Or try out a new camera, and see if results turn out any better.

Good luck. You're right, you are certainly not the first person
to complain to a camera manufacturer about this issue.
Nor are you the first to be told that nobody else ever complained,
or that the manufacturer has never heard of the problem.

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