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Here are two responses from my question:

1) I suspect it may have to due with how well the refractive indices of the
base materials match the coatings.  Triacetate base has a refractive index
of 1.48.  Polyester base is birefringent, with an index of about
1.64-1.66.  Gelatin emulsion has a refractive index of about 1.50, and the
binders for magnetic coatings vary.  Typically, a close match in
refractive indices would allow light to more easily pass from one material
to another.  A significant mismatch in refractive indices will have some
reflection or light scatter at the interface.

Depending on the product, the base itself may have some dye added to
reduce "light piping".

John P. Pytlak
Senior Technical Specialist
Eastman Kodak Company

_____________________________________________________________________________________________________________________________

2) I discussed your question with Charlie Mayn, Chief of our NARA Dynamic Media Lab.  Here is his answer with a few comments of my own mixed in:

The "piped" light test [i.e. holding the entire tape pack up to a light source] is done looking at the edge of the base material that, generally, is at right angles to the image/sound carrying surface.  This edge-view is accomplished on rolled strands of base material looking at the side of the rolled strand pack.  The amount of light "piped" by a particular base material varies with the type of material and how it has been formulated.  

Quarter-inch audio tape is formulated such that polyester base material does not pipe much light through the strand pack and acetate base material pipes relatively a lot more.  It isn't really "opaque" and "translucent", just "very little" and "quite a bit" with a sufficient difference to make the test reliable for audio tape.

Motion picture film is formulated such that acetate does not pipe much light and polyester pipes somewhat more.  The amount of light piped varies with the particular polymer orientation in the base material, the nature of the coatings applied to the base material, etc.  Motion picture film has considerable variation in the amount of light piped among different film stocks (camera, lab, print, reversal, etc.) and light-piping characteristics are therefore not quite as reliable a test for motion picture film stocks.  The test can indicate that the film in question is likely to be acetate (little light piped) or polyester (somewhat more light piped than acetate) but is not as definitive a test as it is for magnetic recording tape.

A better test for identifying base material is using crossed-polarizing-filters which works for motion pictures as well as still picture images.  This test is done when viewing the base material [frame] from a position perpendicular to the surface of the base material that carries the image, not when viewing the edge of the base material as in the "piped" light test.  The filters test can be used to determine, with a polyester frame showing through light as blue/green color striations , but acetate/nitrate frames are clear to the light through the crossed-polarized filters.

Les Waffen
Special Media Archives Services Division
National Archives
 
Lance Watsky
Preservation & Media Specialist
The Georgia Archives
5800 Jonesboro Road
Morrow, GA 30260
678-364-3764 (phone)
678-364-3860 (fax)
[log in to unmask]
www.GeorgiaArchives.org




 



 

 




-----Original Message-----
From: Association for Recorded Sound Discussion List
[mailto:[log in to unmask]]On Behalf Of Watsky, Lance
Sent: Wednesday, March 02, 2005 8:51 AM
To: [log in to unmask]
Subject: Re: [ARSCLIST] Identifying Acetate Tapes


The strange part is the fact that with film  - light is more translucent with a polyester base and less so thru the acetate base. Anyone have any ideas, or guesses why? No one has been able to answer this yet!! It is a very strange phenomenon. I still think that it has to do with the tapepack, but I could be wrong.

Lance Watsky
Preservation & Media Specialist
The Georgia Archives
5800 Jonesboro Road
Morrow, GA 30260
678-364-3764 (phone)
678-364-3860 (fax)
[log in to unmask]
www.GeorgiaArchives.org




-----Original Message-----
From: Association for Recorded Sound Discussion List
[mailto:[log in to unmask]]On Behalf Of Edward A. Falk
Sent: Wednesday, March 02, 2005 2:05 AM
To: [log in to unmask]
Subject: Re: [ARSCLIST] Identifying Acetate Tapes


On 2005.03.01 07:25, Watsky, Lance wrote:
> I was hoping that someone can tell me if the following statement is true or incorrect:
> 
> A colleague was telling me that you can tell the difference between an audiotape that was recorded on an acetate tape, versus one that was created on a polyester base, by holding the tape up to a light source. The colleague went on to say that light will pass thru the acetate tape, whereas it will not pass thru polyester tape.  

They're right.  You can also tell the difference by pulling on it until it
breaks; the acetate snaps off cleanly while the polyester stretches first.