Re: Low cost image sensor

Hi Paul,
Thanks for the comprehensive explanation. I was hoping to get by with
something simple, hoping that I could find an array type device with
about 200 by 30 pixels, on a 0.1mm pitch. That way I could avoid the
complexities of lenses etc. However after reading your reply, I realise
that I probably will need to look at a lens based system.
Thanks again,
Allen.

Paul Carpenter wrote:
> On 28 Jun, in article
> <1119977891.183815.103810@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>
> allenb_deja@xxxxxxxxxxx wrote:
>
> >Hi Paul,
> >Thanks for the reply.
> >
> >Paul Carpenter wrote:
> >> On 28 Jun, in article
> >> <1119945188.468544.97470@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>
> >> allenb_deja@xxxxxxxxxxx wrote:
> >> >Hi,
> >> >I have been searching the web for a low cost image sensor.
> >> >The application is for reading a type of barcode, approximately 20mm x
> >> >5mm, but I could make do with an area as small as 10mm x 3mm. A
> >> >resolution of 0.1mm would be more than adequate, and I only require
> >>
> >> Are you really reading a 2D barcode as compared to normal 1D barcodes?
> >> In other words is the important factor the 20mm dimension which has to be
> >> read to 0.1mm resolution which actually means a minimum of 200 pixels
> >> realistically 400 pixels would be better to ensure the resolution.
> >
> >This isn't a standard bar code, however the resolution in the Y
>
> So you are using some form of effectively a 2D bar code where each line is
> different. If not and the data is ONLY ONE line of information, then the
> vertical resolution only effects the vertical size of the sensor to be
> less than image size vertically. Thus acquiring images are more likely
> to be within the vertical size of image ensuring not averaging foregrround
> and background as one pixel.
>
> >direction is much less critical, it only needs to be about 0.5mm. For
> >the X direction, 200 pixels would be more than adequate.
>
> For 200 pixels to be sufficient your image only needs 100 actual defined
> points. At 0.5mm resolution vertically you are having about 10 'lines' of
> information. I do hope you don't expect to require 200 pixels image
> resolution of the object and to get that from a 200 pixel device. Most
> people doing measurements using video devices rely on 5 to 10 times the
> pixel resolution required to be sure where the edges ACTUALLY are and
> to reduce mechanical alignment issues.
>
> >> You need a square pixel device for certain (not all are).
> >>
> >> >monochrome. Ideally I would like no moving parts, and no lens. The item
> >> >to be scanned would be placed directly on the sensor, illuminated from
> >> >behind.
> >>
> >> No lens at all or built in lens? There are plenty fo cheap lens manufacturers
> >> who no doubt make something you could use.
> ...
> >The final product, will be a small handheld battery operated device,
> >the item to be scanned is placed on it, a button is pressed, and the
> >result is displayed on an LCD. The reason why I mentioned no lens, is
> >for mechanical simplicity and compactness, if the item is placed
> >directly on the lens, I would imagine that the sensor would need to be
> >some distance away.
>
> Lens focal distance depends on many issues including sensor size. To
> use no lens at all then either you are using
>
> a) pin hole camera with need for LOTS of light for SHORT exposure time
>
> b) Sensor has a 1:1 match that is registered EXACTLY between sensor and
> code being scanned.
>
> Otherwise you WILL get out of focus images. None of this is mechanically
> simple as it requires precise registration of object to sensor for
> orientation, distance and perpendicularity.
>
> Consider even standard Line scan devices like Sony ILX511 have a pixel pitch
> of 14um for a 2048 pixel device, and the Silicon Video Inc Elis-1024 has
>
> Resolution Pixel
> mode Pitch
> 1024 7.8um
> 512 15.6um
> 256 31.2um
> 128 64.4um
>
> These are quite LARGE pixel pitches, without a lens of some sort you will
> find it difficult to get 20mm or 10mm onto a sensor. Very FEW sensors are
> stackable in H or V without special optics or overalaps from prisms and
> optical paths.
>
> At 100um (0.1mm) pitch this is too small for photodiodes to be stacked
> and too large for most line or area sensors.
>
> You will need some form of lens, and the size of sensor and lensing are
> affected by object distance and field of view. With the following you get:-
>
> Object size 20mm 10mm
> Object distance 10mm 5mm
> Imager size 28mm 28mm (Sony ILX511 2048pixel line scan)
>
> gives
> Field of view 90 127 degrees
> Focal length 14mm 7mm (lens centre to imager)
>
> Sit down and do some lens calculations based on what you object size and
> distance are to determine from various imager sizes what the focal length
> is. Then consider your arrangements.
>
> --
> Paul Carpenter | paul@xxxxxxxxxxxxxxxxxxxxxxxxxxx
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