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Livingstone's Letter from Bambarre

A Multispectral Critical Edition

All letter images and text published by permission of Peter and Nejma Beard. Licensed for use under the Creative Commons Attribution-Noncommercial 3.0 Unported License. ©2010

*Beta Edition, 2010
*First Edition, 2011

Note on the Images
The imaging team produced twenty-three raw and twenty-one processed multispectral images as well as one natural light image (for comparative purposes) of each of the two sides of the two leaves of Livingstone’s letter to Waller. Only a small selection of the images appears in this critical edition, but the full set will eventually be published. The processed images included here demonstrate some of the possibilities for enhancing text, topography, and other material features of Livingstone's letter and for highlighting the relationship of these three elements.
The wavelengths, or colors, of the raw images included one ultraviolet, two blue, three green, two red, and five infrared wavelengths. In addition, four images were taken with raking light, i.e., light shining on the surface of the paper at such a low grazing angle that the variations in the surface are emphasized. These four images were taken from the left and right sides of the paper, in both blue and infrared light. All of the raw images were taken sequentially without moving the paper in between exposures. As a result, every image is in perfect registration with every other, allowing several images to be combined so as to enhance different parts of the document. 
Most of the processed images included in this critical edition combine two images together. To create a "pseudocolor" image, two images are locally adjusted in brightness and contrast over a sliding window (about three text lines high) to make the images uniform in appearance. The two images are then combined into one color image by putting one image in the red separation and the other image into the green and blue separations. Where the two images are of comparable value, the pseudocolor image appears neutral, i.e. without color, or black and white. Where the two images are different, the pseudocolor image appears red or cyan, depending on which image is brighter. If the word "ratio" is included in the name, then the two images are divided by the 940 nm infrared image before being combined (for more on multispectral imaging and processing, see here and here). The images included are as follows:
Natural Light
This is not a processed image, but an image "stack" created by combining red (630 nm), green (535 nm), and blue (450 nm) images.
Sharpieratio 0505-0780
The green image (505 nm) and the infrared image (780 nm) are locally adjusted for brightness and contrast over a sliding window and then the difference of the two images is linearly stretched to enhance the low-contrast details. Enhances undertext, as especially evident on 1v and 2v.
The raking image taken in blue light for the right side is divided by the right side raking image taken under infrared illumination. The result is linearly stretched so that the mean of the image is at mid-gray and the full variation of the image content varies from black to white. Enhances text and overtext.
Each of three separations (450nm, 592nm, and 850nm) is divided by the 940nm separation. Because the printed text changes very little over this wavelength range, the division suppresses the printed text and reveals Livingstone's writing. These three separations are then put into a pseudocolor image, the 450nm in the red, the 592nm in the green and the 850nm in the blue. The three divided separations reveal Livingstone's writing best in different regions, so combining the three in color reveals all of the writing in one image. Enhances text and overtext.
Pseudo_0780 and Pseudo_0780_by_940
Each of these two images uses both sides of the given page to present the handwriting on the surface and the bleed through from the other side in different colors. In addition, the second image has been first divided by the corresponding 940 nm image to suppress the printed text at the center of the page, a method that works even on the side where the printed text is only show through. However, this latter processing method also makes the edges of the page and those places where Livingstone’s writing appears in the folds of the page more difficult to read. As a result, both these processed images are made available here. Enhances text and overtext.
Pseudoratio 0505-0780
The green image (505 nm) and the infrared image (780 nm) are combined in a pseudocolor image. Both images are locally adjusted for brightness and contrast over a sliding window before being combined in the pseudocolor image. Enhances text and overtext.
The blue and infrared raking images for the left side are combined in a pseudocolor image. Highlights relationship of text to topography.
The left and right raking images taken under infrared illumination are combined in a pseudocolor image. These images are combined in the same manner as the raking images taken in blue light were combined. Enhances topography.
The two raking images (left and right) taken under infrared illumination are subtracted from each other and the result is linearly stretched to enhance the detail in the difference. Enhances topography.
(Image notes provided by Keith Knox)
Spectral Imaging: A Brief Introduction