Technical Tid-Bit #1

     By Aaron Clevenson

 

I have 8 color filters for my telescope eyepieces.  They are numbered, but the numbers seem to have no basis in reality.  What exactly is the relationship between the colors and the numbers on my color filters?

 

Well, the truth is… they really do have nothing to do with reality!  If you call Orion and ask them, they will send you to Kodak.  If you call Kodak, they will tell you that you can buy their manual: Kodak publication B-3, Kodak Photographic Filters Handbook.  An alternative is to check out the website of one of their engineers.  He scanned a number of the charts that show the frequency sensitivity of various filters.  You can check this out at: http://www.geocities.com/thombell/curves.html

 

But in a nutshell here’s the scoop. Kodak has a series of gelatin filters used in photography called Wratten Filters.   These filters are numbered, and these numbers have been adopted as an industry standard for optical color filters.  Here are the common filters.  Pale Yellow: 2A, 2B, and 2E.  Yellow: 3, 8, 9, 11, 12, and 15.  Yellow-Orange: 16.  Orange: 21.  Red: 22, 25, 29, and 92.  Magenta: 32 and 33.  Violet: 34A.  Amber: 106.  Blue: 38A, 39, 47, 47B, and 98.  Light Blue-Green: 44 and 44A.  Green: 58, 61, and 99.  Yellow-Green: 102.  The ones I have are in Bold.  The astute will notice that two are missing:  Blue: 82A and 80A.  I have also seen a Red: 25A.  I have not seen the booklet, but I assume there are a lot more described than appear on this short list.

 

So why Wratten Filters you may ask?  Well, Frederick Charles Luther Wratten was born in 1840.  He was a pioneer in photograph, and in the early 1900’s he invented photographic color filters.  In 1912 the Wratten Filters were bought by the Eastman Kodak Company.  Wratten lived until 1926, to the ripe old age of 86.