scanner resolution

[hovis]

Hi

can anyone tell me how to find the actual resolution of any given flatbed scanner?  They usually state X number of PPI but don't state how long the CCD is, am I missing something?

[Doug Peterson]

Scan a resolution chart. See where the lines deteriorate.

Usually it's not even as high as the "actual" resolution, and is always far less than any interpolated resolution.

You can use this website as a verification of your visual analysis of the test chart: delt.ae.

Any other answer assumes that manufacturers specs are what they say they are. Sometimes that is the case; often it is not.

Courtesy of our experience with image capture for cultural heritage.

[hovis]

Hi Doug

I don't own one as yet, what I'm really looking for is how to find out the number of pixels on the CCD? With cameras it's usually clearly stated but for some reason scanner manufacturers like to play with words.

[yaya]

You can get an idea of the pixel count of the Truesense linear/ trilinear sensors here

As far as I can remember the longest trilinear CCD used in commercial scanners was a Kodak 14K

Yair

[theguywitha645d]

Hi Doug

I don't own one as yet, what I'm really looking for is how to find out the number of pixels on the CCD? With cameras it's usually clearly stated but for some reason scanner manufacturers like to play with words.

Or perhaps the consumer does not understand the words.

Scanners are not one-shot devices and so do not have a fixed pixel resolution. What you want is the scanning resolution in pip that is not interpolated. That will tell you the pixel resolution of the scan. That number can change in the scanner depending on the document/film size--the Epson V700/750, for example.

If you want the actual resolving power of the scanner, you will have to buy one and scan a test target.

[digitaldog]

can anyone tell me how to find the actual resolution of any given flatbed scanner?  They usually state X number of PPI but don't state how long the CCD is, am I missing something?

IF you see a spec like: 1200x1400, ignore that 2nd value. The optical resolution is 1200x1200. If you selected 1400, the other axis the scanner moves (1200) has to be interpolated to match. Pixels are (usually) square.

[Bart_van_der_Wolf]

I don't own one as yet, what I'm really looking for is how to find out the number of pixels on the CCD? With cameras it's usually clearly stated but for some reason scanner manufacturers like to play with words.

Hi,

There are 2 measures that can be called resolution.

One is the number of sensor elements (sensels), and if known it can be divided by the physical length of the linear sensor array(s) to get the sensel pitch, or by the length of the scanned area divided by the number of sensels to get the pixels per inch (PPI). A universal scanning software product such as VueScan will be able to extract the full resolution from an immense list of different devices, it's compatible with over 2100 different supported scanners (also mentions the maximum PPI, as 'optical DPI'), and will offer the maximum PPI as a preset when connected to the particular scanner.

However, that is only part of the story, because the second aspect of resolution is determined by the optical element(s) involved in most scanners. What really determines the actual opto-physical resolution is the combination of the sampling density and the optical resolution. That's commonly expressed in cycles/millimeter.

Since many scanner manufacturers usually only mention the PPI of a scanner, it only tells part of the full story. It is possible to determine the actual optical resolution of a scanner with a so-called "slanted edge" target. There are also other targets possible but some of them are not really suited for discrete sampling devices.

Cheers,
Bart

[hovis]

IF you see a spec like: 1200x1400, ignore that 2nd value. The optical resolution is 1200x1200. If you selected 1400, the other axis the scanner moves (1200) has to be interpolated to match. Pixels are (usually) square.

Hi Andy

this still doesn't tell you how many pixels the CCD has as the manufacturer hasn't stated how long the sensor is. If they're stating 1200 PPI and the sensor is only half an inch long then there are only 600 pixels. I just had a look at Epson's website they don't actually mention PPI, they say you can 'achieve exceptional scanning at 6400dpi (film) / 4800dpi (reflective) optical resolution.' isn't DPI printing terminology, what exactly does that mean? I don't think it's based on available scanning area either as that would mean an A4 scanner capable of DPI (?) 6400 would have a CCD with almost 53,000 pixels.

Perhaps I need to contact manufacturers directly and ask?

[digitaldog]

this still doesn't tell you how many pixels the CCD has as the manufacturer hasn't stated how long the sensor is. If they're stating 1200 PPI and the sensor is only half an inch long then there are only 600 pixels.

It's telling you per some unit, usually inch here in the US (i.e. 1200x2400 PPI, Pixels per CM, Pixels pre MM etc).

they say you can 'achieve exceptional scanning at 6400dpi (film) / 4800dpi (reflective) optical resolution.'

That's quite clear to me. Optical (not interpolated) max resolution is 6400ppi (or dpi if you want to specify that scanners make dots, not pixels).

[Bart_van_der_Wolf]

I just had a look at Epson's website they don't actually mention PPI, they say you can 'achieve exceptional scanning at 6400dpi (film) / 4800dpi (reflective) optical resolution.' isn't DPI printing terminology, what exactly does that mean?

Hi,

They mean the number of pixels per scanned surface, expressed as PPI (although they call it DPI).

I don't think it's based on available scanning area either as that would mean an A4 scanner capable of DPI (?) 6400 would have a CCD with almost 53,000 pixels.

The mentioned numbers are scanned area (width), and they use different lenses to scan that width (and a matching scanning step distance for the other dimension).

Cheers,
Bart

[hovis]

It's telling you per some unit, usually inch here in the US (i.e. 1200x2400 PPI, Pixels per CM, Pixels pre MM etc).
That's quite clear to me. Optical (not interpolated) max resolution is 6400ppi (or dpi if you want to specify that scanners make dots, not pixels).

Hi Andy

that's ok, I'm not sure you've understood what's been said so far. I think Bart's explanation of using different lenses to scan the width of a sensor then alter the steps sounds right. I did think that the specified number was probably the number of pixels across the sensor width, don't know why they can't just say it.

[digitaldog]

I'm not sure you've understood what's been said so far. I think Bart's explanation of using different lenses to scan the width of a sensor then alter the steps sounds right. I did think that the specified number was probably the number of pixels across the sensor width, don't know why they can't just say it.

Some scanners do work with lens (anyone recall the Duoscan from Agfa)? Each will provide a different DPI (can we call it PPI because that's what is being produced?). But the manufacturer should state the optical resolution for each. They may state something like: for 35mm, 6400dpi (so that's what you'll get), for 120, 3200ppi etc.

[hovis]

Some scanners do work with lens (anyone recall the Duoscan from Agfa)? Each will provide a different DPI (can we call it PPI because that's what is being produced?). But the manufacturer should state the optical resolution for each. They may state something like: for 35mm, 6400dpi (so that's what you'll get), for 120, 3200ppi etc.

Sorry Andy, wasn't meaning to be rude, I did make the point earlier that DPI/PPI doesn't mean much without the distance being covered.

[digitaldog]

Sorry Andy, wasn't meaning to be rude, I did make the point earlier that DPI/PPI doesn't mean much without the distance being covered.

How so? You want to scan something right? You know the size of the original (say it's 35mm). IF the optical resolution is say 3200ppi, what more do you need to know? If you have a 4x5 print and you want 5000 pixels over the long axis, you know it's 5 inches and at 1000PPI, you're all set. I'm not sure what else needs to be known, can you explain what you're tying to get here?

What isn't clear is whether you're getting true optical resolution and in the example I gave, once given a scanner spec, ignore the 2nd figure which is the max but interpolated data.

[Bart_van_der_Wolf]

... once given a scanner spec, ignore the 2nd figure which is the max but interpolated data.

Hi Andrew,

Actually it might(!) be real sampling resolution, and not necessarily just mathematical interpolation.

Scanners, like several Epson flatbed models, can use so-called staggered sensel arrangements, i.e. a kind of interlaced dual (half-sensel offset) tri-linear sensors. That will require similar half sensel pitch (slow-scan direction) stepping intervals to achieve square sensel resolution.

Cheers,
Bart

[theguywitha645d]

Sorry Andy, wasn't meaning to be rude, I did make the point earlier that DPI/PPI doesn't mean much without the distance being covered.

The dpi/ppi is the distance being covered. It is an inch at the document plane, not at the sensor plane. That should be what you care about.