[tig] Spectral wedge for camera/scanner testing

Tom Tcimpidis tgt
Wed Sep 3 17:50:15 BST 2003


I can't remember who asked me for this info, or in what group, so I 
apologize in advance to any who have no interest in the subject.

I was asked how to make the calibrated Spectral Display Generator that I 
use to test the colorimetry and optical response characteristics of 
camera systems (and which can also be used on film scanners since it is 
a transmissive device).  Here is the simple procedure to construct and 
use one:

I used a Bausch & Lomb 2nd Order Spectrum Glass Wedge Interference 
Filter (#33-80-02) which I acquired from Edmund Scientific.  It is about 
1 inch high and 3 inches long. The filter consists of a uniformly wedged 
transparent spacer film between two semi-transparent silver films, which 
are then deposited on a glass microscope slide and covered with a glass 
plate cemented in place. The action of the filter is based on the 
interference of light and the wedged spacer film is a full wave length 
thick for green light at a position halfway along the slide. The 
spectrum produced by the filter is of high purity and spectral lines of 
about 10 millicrons across can be resolved. The transmittance of the 
filter is about 35 percent.

You will also need a #12 yellow Wratten gel filter or the equivalent. 
This  creates the calibration points on the scope and, more importantly, 
eliminates overlapping blue light in the filter's far red region arising 
from 3rd order interference.

By cutting the #12 filter and properly placing it over the "Wedge", you 
  get calibration marks on the scope display allowing you to accurately 
determine the response of the cameras system across the spectrum from IR 
to UV.  It looks like this:

----------------------------------------------------------
|                                                        |
|          #12 yellow filter                             |
|                                                        |
|                                             ------------
|        This is over the Red                 | Put at Grn/Blu
|     and cuts out the third order Blue       |
|                                             |
|                                             | 

|                                             |
|                                             |
|                                             |
----------------------------------------------
                                        Put at Grn/Red

Since the wedge filter is linear in response, it can be accurately 
calibrated by using a known spectral source and marking the positions at 
which the lines of said source appear. From this, the wavelengths on any 
other portion of the filter may be read. An excellent calibration source 
is a fluorescent desk lamp which emits, on a continuous background, 
blue, green and yellow mercury lines at 435, 546, and 578 millicrons 
respectively.

For actual camera testing purposes after calibration, a 24 volt halogen 
light, with the UV glass filter plate removed, makes an excellent broad 
spectrum light source. The filter can also be placed in the light path 
of an open gate film scanner for testing such a device.

    -Tom






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