Emission Patterns

Angle-resolved cathodoluminescence (ARCL) can be useful to capture the emission pattern of the light emitted from a specimen (angular distribution, θ, and Φ in polar coordinates). You can collect individual emission patterns from a few well-chosen points—typically giving the highest quality patterns. Or you can map and analyze a complete emission pattern at each point (pixel) of an image—analogous to spectrum imaging.

The workflow is as follows:

  1. Locate the region of interest within the sample.

  2. Set the desired SEM conditions.

  3. Ensure the alignment of the CL system to the SEM and sample is correct.

  4. Define the position of the electron beam.

  5. Acquire an emission pattern.

  6. Optimize the emission pattern (optional).

In the following section, we will focus on capturing an emission pattern and optimizing the system to maximize angular (and spectral) information and signal-to-noise ratio.

Click for the related spectrum imaging experiment.

Alignment for Emission Patterns

Alignment of the electron beam, cathodoluminescence system, and sample are critical to capturing emission patterns with meaningful information, plus the highest signal-to-noise ratio and angular resolution. Please refer to the alignment discussion to discover why this step is critical and the steps to take to capture the highest quality emission patterns.

Go from the quality on the left to the resolution of the right.

Collecting Emission Patterns

Acquire an emission pattern

There are two methods for collecting a cathodoluminescence (CL) emission pattern: Angle-resolved (ARCL), and wavelength- and angle-resolved (WARCL) acquisition. During ARCL, each pixel in the captured image corresponds to a unique emission direction from the specimen (unless an optical filter is inserted into the light path). In contrast, WARCL collects a 3D data cube (λ,θ,Φ) and can be thought of as a stack of wavelength-filtered emission patterns or a (wavelength) spectrum recorded in every emission direction.


Within Gatan Microscopy Suite® software, an emission pattern may be captured using the ARCL Imaging technique:

  1. Position the beam on the sample region of interest using the Scan palette.
    • Spot – Sets the beam at a stationary location on (or to avoid damage, off of) the specimen
    • Focus – Scans a sub-region of the field of view

Ensure that the refresh rate of the scan is shorter than the exposure time of the emission pattern.

  1. Capture an emission pattern.


Within the CL palette, set the mode to ARCL, then press View or Capture to collect an ARCL emission pattern. This will open two image displays within the View workspace that show the raw image projected from the back surface of the collection mirror and an emission pattern image that is transformed into polar coordinates.

  • View – The image continually updates per the exposure time selected
  • Capture – Records a single exposure, then opens the data and includes the SEM's electron beam location in a new workspace
  • Optimization – Determines the read-out configuration of the detector
    • SNR – High signal-to-noise
    • Res. – High resolution
    • User – User-defined binning and dark correction

The Monarc Spectrometer State palette also permits the insertion of an optical filter into the beam path (optional item).

For more information on collecting spatial- and angle-resolved emission patterns (4D data), see the Spectrum Imaging section.

Wavelength- and angle-resolved

In the ARCL mode, the emission pattern does not contain (or only limited) wavelength information. In that case, it measures the entire angular range in parallel without recourse to dispersing the light by wavelength.

In contrast, the WARCL mode selects a subset of the angular range, then records the dispersed light by wavelength. It then serially scans the angular range to collect all angular and wavelength information available.

WARCL uses the same CL acquisition palette as ARCL. 

  1. Position the beam on the sample region of interest using the Scan palette.
    • Spot – Sets the beam at a stationary location on (or to avoid damage, off of) the specimen
    • Focus – Scans a sub-region of the field of view

Ensure that the refresh rate of the scan is shorter than the exposure time of the spectrum.

  1. Capture wavelength-resolved emission patterns.

Within the CL palette, set the mode to WARCL, then press View or Capture to collect WARCL emission patterns.

The Monarc system collects a 2D image stack whose x-axis corresponds to wavelength—centered around the value defined in the WL (nm) entry—and y-axis angle-space θ’,Φ’. The angle-space in the raw image cannot be interpreted easily due to the complex geometry of the projected image and constitutes only a fraction of the total angular range. It records a series of angle-space (θ’,Φ’)-wavelength images, with the total angle-space captured serially. You can transform the final 3D (λ,θ’,Φ’) data cube to reconstruct wavelength-resolved emission patterns using the Remap WARCL Image item from the Monarc Remap menu. Standard tools such as the spectrum picker and slice tools are useful to display the emission pattern within a specified spectral range or to extract a spectrum from a specific emission direction.

Optimizing Emission Patterns

Alignment is key

High-resolution, angle-resolved cathodoluminescence (ARCL) requires each pixel in the projected image to directly relate back to an emission direction. Misalignment(s) in the optical system can easily destroy the pixel-direction relationship unless you use the specially-designed transfer optics of the Monarc™ Pro detector.

To learn more about the alignment of the Monarc, please click here.

The alignment of the optical system remains critical. With the Monarc Pro, the system simplifies this process using an automated alignment algorithm combined with the largest field of view in the x-y plane and z-axis.  Here an angle-resolved spectrum image with a field of view in excess of 10,000 µm2 is shown, more than 400x larger than with other systems.

Transformation into polar coordinates

The software stores information about where the projected image last fell on the camera and uses this to perform the transformation of the raw images into polar coordinates. In rare cases (such as exchange of the collection mirror), it may be necessary to update the software with a new image location. To do this, open the CL Spectrum Setup from the Setup icon in the CL palette, then select Show to see the ARCL or WARCL Bounding box for the appropriate data. Reposition the region of interest so that the projected image falls precisely within the bounding box and click Save.