Re: Camera for recognition system


"Perro Flaco" <fabioarbelo@xxxxxxxxxxx> wrote in message
news:1124364203.033657.300610@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> It is not so simple. I just couldn't "block the sun" because the
> application has to run in every light condition, including sunny
> weather (it's a control access). The problem with the sun is that it
> covers the full range of radiation, so when the sun appears, we can't
> take good photos, even with the filters.
>
> I've been reading about some similars problems, and I hear something
> about using a quite narrow filter, in order to reduce as much as we can
> the radiation from the sun (a guy from other group suggest me this
> possibillity).
>
> I also read something about using a pulsed IR diode. IR
> diodes can have a high output energy when they are pulsed. If I use a
> camera with an electronic shutter, when I press the shutter, in this
> short time only the short pulse from the IR diode flash has an effect,
> and the light from around won't disturbe. Has anyone
> done anything similar? Could you tell me the type of camera,
> diodes,...? If not, any other idea to solve this problem?
>
> Thank you for your help!
>

If your camera has sufficient dynamic range (it probably doesn't!) you can
take two images. First with no IR & second with IR. Subtract 1 from 2 to get
IR only image (assuming linear detector response - gamma=1.0). This removes
shadows. This might be useful for diffuse ambient lighting and cloudy day
shadows. Direct sunlight is VERY bright and you don't have much chance of
competing with it. As other contributors suggest, block direct sunlight.

You might also expect subjects to be wearing sunglasses if it is sunny (and
even if it's not). How will you cope with that?

In theory a very bright IR source imaged through a matched narrow band
filter can give a good result but you have to output perhaps >=10 times the
sun's illumination level. That could be difficult to achieve! There might
also be health concerns if you are shining such bright lights into peoples
eyes. Just because they can't see the IR doesn't mean it does no harm and
there is no blink reflex to protect the eye.

There are some cameras that do multiple exposures at pixel level on the CCD
to combine very bright and very dark parts of an image to give detail in
both. This technique manages to avoid significant delays between exposures
(read-out delays) by holding both exposures on-chip.

Look for backlight compensation CCTV cameras or high dynamic range cameras.
These might help. Possible manufacturers Baxall, Panasonic, Sony.

If you take multiple shots at varying exposure times you can get all parts
of the face well illuminated, but not all in one frame. If the subject
doesn't move significantly between exposures you might be able to composite
a reasonable image. Alternatively you might be able to make your analysis
work on parts of the face and combine the results. This is not likely to be
too successful if you are trying to measure distances between facial
features!

Graham


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