Digital Cameras and Imaging Sensors

Just for the record, the maximum capture area to be "SWGIT compliant" using a Canon EOS 30D is 3.5 inches by 2.25 inches; for the 10D, the area is restricted to 3 inches by 2 inches -- i.e., the resolution of the imaging sensor divided by 1000.

Technically speaking, if the size of the object exceeds the "compliant" area, you should capture the overall image as one single image, which is required for demonstrative purposes to illustrate that the entire image existed as one contiguous image.  Then you may photograph smaller segments of the image.  For example, if you have a large palm print, you must photograph the entire palm print, then you may segment the palm print into multiple photographs.  (Some AFIS systems require you to segment the palm for searching anyway.)

Alternatively, if you are capturing the full palm print with a resolution of 850 PPI, it is "permissible" (albeit not recommended) to resample the image to 1000 PPI. The underlying question is what is the image quality of the image when you capture it as a whole image?  Are you able to distinguish the detail in the image properly?  That ultimately is the really BIG question that only you can answer. 

In addition, please be very careful when you are evaluating digital cameras and image resolution! 

Furthermore, most digital cameras (all digital cameras except those with the Foveon sensor) blur image detail with a built-in anti-alias filter to eliminate some of the artifacts caused by the mosaic color filter array process.  You almost always have to use either Unsharp Mask or Smart Sharpen to refocus the image and clean up some of the blurring artifacts caused by the anti-alias filter.  

All cameras, including cameras with the Foveon sensor, have problems with moir© patterns because of naturally occurring irregularly spaced lines or diagonal lines being captured by perfectly horizontal rows and vertical columns of imaging sensors.  This issue is exacerbated for latent prints when you are capturing fingerprints on clothing or bedding material, which has very fine, irregularly spaced threads as well as fine printed lines on checks or money orders.  That's why capturing images with a high resolution is so critical!

For example, some imaging sensors are very, very small, both in physical dimensions as well as in the total number of photo diodes.  Plus, some camera manufacturers do not tell you the whole truth!  For instance, the Sigma marketing literature says that the Foveon imaging sensor is a 14.1 MP imaging sensor.  It is not!!!  The sensor is physically 4.65 MP €“ measuring 2640 pixels x 1760 pixels.  Since each diode captures three color values, they (Foveon/Sigma) multiply the total number of diodes times three to come up with the Megapixel count.  The bad news is that when you capture an image with this camera and then go into image size to see the resolution, it only contains 2640 pixels by 1760 pixels, so your image resolution would fall below 1000 PPI if you were to capture an area larger than approximately 2.5 inches by 1.75 inches.  Therefore, anything larger than just a single fingerprint would not meet the so-called image quality requirements.  (You are out of luck for large-size, "high" resolution images.)  The good news is that the upsampling of images from this camera is actually quite good; MUCH better than any regular digital camera.