Expert: Scott Valentine - 2/6/2009

Question
QUESTION: Respected Sir
I am a collector of coins. Many times a coin is minted with
different Die Variations. These variations may be also in
terms of different sizes of Alphabets and different sizes
of images. Whether Photoshop  may eable me to findout
measurement  in terms of mm of the Inscribed Alphabets and
images on the coins.If so, please indicate the method.
regards
som

ANSWER: Thanks for your question.

While Photoshop is capable of measuring these differences, it is not strictly a forensic tool. You would need to use some kind of calibrated digital capture method (say, a scanner perhaps) and have electronic calibration for your Photoshop installation. CS4 allows you to preserve calibration, but it can be tricky to deal with.

If you are using CS4 Extended, I would recommend that you get a flatbed scanner, and scan your coins alongside a calibrated, flat ruler - typically a thin metal or glass standard that has 1/10 mm resolution. This will allow you to set calibration from the image itself, so if you send the images to someone else, they will use the same standard you are.

Describing the individual measurements would take more space than I have here, so you'll have to dig through the help files or ask on the Adobe forums for more detail. However, I believe you should be able to make short work of them once you have seen the instructions. You will need to use the Ruler tool and capture the measurements to a text file.

I hope this helps - if you need further clarification, please feel free to ask!

-Scott

---------- FOLLOW-UP ----------

QUESTION: Respected Sir
I am a novice. Kindly bear with me.
I do not require high precision. Even error of plus minus .40 mm is acceptable.In this light please answer me.
I am using HP 1310 series  scanner and have Photoshop 7.

Whether it is possible to scan the coin and have an image which is of the coin size and then import this image in Photoshop 7 and measure Alphabets and/or images.If so, how to do it.
regards
som

Answer
Hi Som,

I've been trying to get my scanner working, but haven't had any luck, so this is off the top of my head.

As I mentioned above, you will need to scan your coins using the same setting each time. Also, be sure to have some kind of ruler with markings a little smaller than the precision you need, but use a good ruler. I suggest small, metal machinist's ruler that has 1/10 mm markings. You should be able to find them at hardware stores - look for pocket-sized versions with very clean edges on the markings.

Simply scan the coin next to the ruler so your image is 'calibrated' to some degree. Within Photoshop, since you are using 7, you will have to count pixels and do some math yourself, so it might be helpful to keep a spreadsheet open. On your image, use the rectangular marquee tool to find out the distance between several markings on the ruler. You are building up a calibration file in this way.

To accurately measure the distance in pixels between marks on the ruler, you will have to be consistent in where you measure. Always orient your ruler straight horizontally or vertically, and always use the same side of a mark edge. For example, if your ruler is horizontal on your screen, choose the left edge of one mark and start your marquee tool there. Drag to the next mark (or the distance you want to calibrate) and stop also at the left edge. This avoids including the width of the mark itself in your measurement.

Open the Information pallette (check your help files to find it) and look at the dX value (it will look like a little triangle for 'delta', next to an X), which will give you the number of pixels your selection has in the X direction. You would use the Y numbers if your ruler is vertical.

On your spreadsheet, record both the pixel value and the ruler measurement. Your calibration will be the ratio of pixels to units. So, if you measure one millimeter, and it takes 200 pixels, your calibration value is 200:1. When you measure the next element on your coin, you will read the X value in pixels and divide that by 200 to get your measurement in millimeters.

Some things to note: horizontal and vertical scales may not be the same; every image will have to be calibrated in this way unless you somehow control the scanner and your images; consider putting the calibration ratio into your image as a text layer for future reference; this technique can be extended to cover rotated features like letters, but you are better off finding one or two letters that are perfectly vertical or horizontal.

Another method involves using area, but the margin of error goes up dramatically and requires a lot more instruction than I can provide here.

For more detail on imaging forensics, I recommend looking up Goerge Riess' book on Amazon, or asking around the numismatics community for plugins that specialize in making these kinds of measurements. There might be some applications out there that will do exactly what you need, but be aware they may not work with version 7.

Cheers,

-Scott