 |
Color SEM-CL |
|
Color SEM-CL |
This is a discussion of how color images are made on a scanned CL device. Conventional (cold-cathode) CL devices typically produce color images, but scanned CL typically produces only panchromatic gray-scale images.
Currently color CL images are produced using an Oxford Instruments MonoCL2 cathodoluminescence detector (185-850 nm sensitivity range) with a PA-3 amplifier attached to a Philips XL30/TMP scanning electron microscope. In normal operating mode, each CL scan produces an 8 bit panchromatic gray-scale CL image. Color images are obtained by filtering this panchromatic CL signal through a red, green, or blue filter (a filter monochromator). With one color filter inserted, each scan produces a gray-scale representation of a particular color range. Three separate scans, one with each color filter, must be made to produce a color image. The color filters were made by Corian Corp. The blue filter transmits light primarily in the range 380-500 nm, the green filter transmits light primarily in the range 520-570 nm, and the red filter transmits light primarily in the range 595-800 nm.
Once gray-scale images are obtained, the files are opened in Adobe Photoshop. In Photoshop the images are adjusted separately for contrast and brightness to bring out the maximum possible detail from each color. Then the images are combined by assigning each as a color channel in a single Photoshop file. This process produces a color image that is probably somewhat different in color than might be obtained from a conventional cathodoluminescence device. However, both the filters and the photomultiplier have variable transmissivity in different wavelengths, so adjusting intensity in Photoshop may merely offset earlier color changes. Even if colors are slightly different than those that would be obtained from a conventional CL device, these scanned CL colors have been adjusted to show maximum detail and are therefore probably more useful for fabric analysis.
One common question is whether the colors in color CL images are "real". The "real"-ness of any color is dependent on a number of factors, especially sensitivity bias of the detector (and your eyes). Reproduction only adds to the problem. In the case of color scanned CL, the "blue" filter admits significant amounts of the UV spectrum into the blue color. The "red" filter admits some of the infrared and makes it "red". However, I consider the colors to be representative of "real" colors, and they certainly illustrate real features. But you can decide for yourself, some comparison shots of cold-cathode and scanned CL are available.
Most of the images on this web site were produced on an earlier system described below. Image processing was similar to that described above. This earlier system was more sensitive in the red range, and this is apparent in the images.
Prior to June of 1999 color images were made in the SEM Lab at The University of Texas Department of Geological Sciences. The equipment used was an Oxford Instruments CL302 cathodoluminescence detector attached to a JEOL JSM T-330A scanning electron microscope. Signal is processed by an Oxford Instruments PA-1 amplifier and then passed to an interface from 4pi Analysis which routes it to a PowerMacintosh 7300 using a plug-in for Adobe Photoshop. The filter monochromator used in this set-up was constructed in the Scientific Instrument Shop at the University of Texas Department of Geological Sciences. Filters are Kodak Wratten gelatin filters. Image acquisition and processing were similar to those described above.
Once in Photoshop the images are typically processed further. In particular, red, green, and blue channels are adjusted separately for contrast and brightness to bring out the maximum possible detail from each channel. This process will produce a color image that is slightly different in color than might be obtained from a standard cathodoluminescence device. However, the filters have different transmissivity in the different wavelengths and photomultipliers typically work more effectively in one color or another, so adjusting intensity either with the amplifier or with Photoshop is necessary. Adjusting the contrast and brightness of the individual channels typically restores detail lost versus the panchromatic image.
Samples are typically polished carbon-coated thin sections, although polished rock chips and polished grain mounts can be used. Samples can also be gold coated but we have found that gold-coated thin sections tend to scratch if you breathe on them. Plus a gold coat acts as something of a blue filter, reducing response in the blue range
