There are two ways you can do CL imaging with the SEM system.
The first is using the VP detector, which is light-sensitive. It’s OK for brightly-luminescent samples and broad areas. Remember that it’s off to one side, so you will see shadowing effects. Also, areas farther from the detector will be darker, though higher magnification, multiple images, and stitching software can help with that problem, as will lowering the stage.
The other method is to use PMT1. It uses the RPM to gather light from the luminescing sample. It can obtain panchromatic images (all colors, no color filter), or with the color wheel filters (controlled by the LabSpec software).
Turn on and optimize the SEM using the SE detector, turn on the CL hardware and software, and center the RPM.
The PMT of interest is in the little white box on the right. Rotate the thumbwheel so that it clicks with the ON label visible. That moves a mirror into the light path, directing light to the PMT.
Note: Remember to move the thumbwheel to OFF when you’re finished. You can’t collect spectra or center the mirror with it on.
The SEM should already be running and focused, and should be on a brightly and evenly luminescing material.
The SEM brightness and contrast controls don’t work with PMT1, so you use this controller box instead. You DO use the SEM scan and image acquisition controls, however, NOT the LabSpec software. For PMT1 imaging, the LabSpec software is just used to center the mirror, and to change filter wheel positions.
Press the STAND BY button. That turns the box on.
Press the HV button. That turns on the PMT1 high voltage.
Press the Load button on the screen to turn on the control interface. This screen shows bar graphs for contrast (left) and brightness (right). The two buttons at the top of the screen top give you coarse or fine control for the contrast and brightness control knobs.
In the SEM software, change the detector to AUX1, which is PMT1. Adjust the control box brightness and contrast to see an image.
The image you see will probably look like nothing at first. Remember that you are looking at a homogeneously luminescing surface, so there’s no detail.
Also remember that CL light comes from a volume 1-2 microns across, not the much smaller electron beam spot. You can’t get the resolution you might expect from an SE image. You can get better resolution with lower beam voltages, though you may need higher current, too, to get the brightness you need.
And another thing. The luminescence electronic transitions can have relatively long lifetimes (phosphorescence), so images tend to streak out to the right. Slower scan speeds may be needed.
Slowly raise the stage using the Z-axis controls, maybe 0.2-0.3 mm from the RPM focus position. Your CL image should blurr, and the bright region should move to the right.
On LabSpec go to: Menu Maintenance / RPM adjustments / RPM_X / Offset. Change the X value to move the bright region of the image back to the left.
Play around with the stage Z-axis position, and the LabSpec X-axis position, until the image field is more or less evenly illuminated, like this. Because of the focal characteristics of the parabolic mirror, perfection is probably not possible.
Using the SEM SE detector, re-focus the SEM using the keyboard focus controls.
Move the stage to a sample you want to image, and switch back to the PMT1 detector.
Because slow scans may be necessary, setting the best contrast and brightness for the PM1 detector is not easy using the whole image frame. Instead, on the SEM scannng tab, select the Line scan check box. That gives you a horizontal line across the field, which you can move up and down to cross dark and bright areas. Adjust contrast and brightness (Horiba control box) so the red signal intensity line intersects the graph top and bottom (maximum contrast). The slower the scan speed, the better this works. Remember that the electron beam is rastering across a single line, not the whole field, so even a slow scan speed is fast.
Once you have adjusted contrast and brightness, un-check the Line scan box and acquire your image in the normal SEM way. If it looks good, now you can make a color image.
In LabSpec, go to: Menu Acquisition / Instrument setup / Filter wheel, and select the blue filter.
Adjust contrast and brightness using the SEM line scan feature, and acquire a blue image.
Change to the green filter.
Adjust contrast and brightness using the SEM line scan feature, and acquire a green image.
Change to the red filter.
Adjust contrast and brightness using the SEM line scan feature, and acquire a red image.
Use third-party software to do any additional image processing, and combine the images to make a color RGB image.
Here, the microcline host luminesces blue, and the albite lamella luminesces red.
