Re: Basic newbie question on cmos sensor and optics


Antonio Pasini wrote:

> Sensor A (MT9V111):
>
> Resolution: 640x480
> Pixel size: 5.6um x 5.6um
> Sensitivity: 1.9 Volts / lux*sec at 550 nm
>
> Sensor B (MT9M001):
>
> Resolution: 1280x1024, region of interest really used: 1024x768
> Pixel size: 5.2um x 5.2um
> Sensitivity: 1.8 Volts / lux*sec at 550 nm
>

> Obviously using two different custom designed optics.
>
> But now I'm asking myself.... if the light I drop on the target is
the same,
> the light that comes back must be spread on a much wider area on
sensor B,
> so the single pixels will receive a much smaller light flux... Used,

> "illuminated" area of B would be 21.25 / 9.63 = 2.2 times bigger
>
> What signal I'll get from B ?
>
> Just 1.8 / 1.9 = 0.947 times the signal of A as stated from
sensitivity ?
> This would be okay...
>
> Or will B output 0.947 times * (1 / 2.2) = 0.430 times the signal of
A ?
>
> In other words... if the two different optics are made to fit each
one
> active area, and target size, distance, light will be the same, do I
need to
> factor in also the bigger area of B ?
>
> It's counter-intuitive to me why a much bigger, more costly sensor,
with
> similar pixel size, given the same target image and illumination,
would give
> me back a more noisy image...
>

It depends on the two different custom designed optics you mention.
The bigger, more costly sensor, with more pixels of a similar size,
needs MORE LIGHT to illuminate those pixels, so your optics must be
designed with a larger field of view, in order to provide that light.
If you take the same optics, and just increase the magnification, so
that the real field of view is the same, but your resolution just went
up -- your effective sensitivity will go down in proportion as the
sensor area goes up, just as you are worried about. So, if you really
need that sensitivity, either use the higher resolution sensor to
image a larger (real) field of view, or use a bigger lens to collect
more light at the higher magnification, so that you can view the same
real field of view, with the same sensitivity, at higher resolution,
with (incidentally) reduced depth of field (inversely proportional
to your lens diameter).

Have you ever noticed how those little cheap point-and-shoot digital
cameras with lots and lots of megapixels, take really crappy pictures
in dim light situations? It's because what they really need is a
bigger diameter lens to collect enough light to fill all of those
pixels.

.

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