Coin imaging

[Wizard]

Hi, this is a great forum that offers me potential solution to my coin imaging dillema.

As a complete novice, who just explores this method, I have a couple of questions:

1. Coins are small, relatively flat objects, as everyone knows. How do I set up the scene for imaging to avoid ball shadows on the coin? Should I put my coin on a small stand? Should it be put flat or at an angle? Can more than one coin be photographed in one session?

2. Considering the typical coin diameter of 12-45 mm, what size balls should I use? Majority of my coins will be within 16-26 mm range.

3. Should I reduce the number of exposures?

4. Can I use a strong tungsten light, rather than flash? I am asuming, it will be rather close to the object.

 

Thank you in advance.

[cdschroer]

The first thing I recommend is reading through the guide to highlight image capture - a free download from the CHI website:

http://culturalheritageimaging.org/What_We_Offer/Downloads/Capture/index.html

 

Some of your questions are answered there.  The sphere size is based on the resolution of your camera, and the field of view you have set up with the lens you are using.  The recommendation (found in the guide) is to have a minimum of 200 pixels across the diameter of the sphere.

 

The key to avoiding shadows is sphere placement.  There are some good examples in the guide.

 

The number of images we recommend is 36-48.  This is not a function of the size of the subject, but about collecting a good set of data to aide the software in accurately calculating the reflectance properties and surface normals.

 

Carla

[Wizard]

Thank you, Carla.

How can I assess the number of pixels across the sphere? Not quite sure how to do that.

[leszekp]

Take a photograph of the sphere,and load it into Photoshop. Use the Ruler tool to measure the diameter in pixels (you may need to right-click on the ruler and select "Pixels" as the scale).

[Wizard]

Thanks, Leszekp

[Wizard]

Well, the project is well underway 

I have a couple of spheres that are 12mm in diameter, since I don't have Photoshop, I measured the diameter in GIMP and it comes as 255 pixels, so it should be fine.

 

The guide recommends the light distance to be 2-4 times the object's diagonal. In the case of 25-30mm coin that would be 120mm max. A bit too close for my liking to make it work. 

 

What would be the recommended light to object distance in this case?

[cdschroer]

2-4 times is the minimum distance - you can always work from farther away - as long as you have good even coverage across your subject from the light distance.  For example, we do this under microscopes, and we have the light much farther away, because we are imaging small things.  So, you want a comfortable working distance that is easy for you to set the distance.  For small things it is easy for one person to hold the string, and the flash. We often set the camera on a timer and then just move the light, and have it take images.  There are several ways to use a timer, the simplest is being tethered to a computer.  For example, Canon cameras come with free software - the EOS Utility that allows you to automatically take pictures every X number of seconds.  Also this way the images can be named and numbered and saved to an appropriate location in your computer.

 

Carla