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The diameter of the contrast aperture sets the angular acceptance of a PEEM. Increasingly smaller contrast apertures cut off-axis beams, reducing blurring by spherical and chromatic aberrations. Improving the lateral resolution thus implies decreasing the tramsmission, so that in practice one has to look for a compromise. Furthermore, in XPEEM, one has to choose the photon energy so that the kinetic energy of photoelectrons is far from the tail of secondary emission, but sufficiently low to ensure reasonable transmission of the microscope.
The transmission of the SPELEEM as a function of kinetic energy in the case of contrast apertures of 20, 30 and 100 µm is illustrated below.
| K.E. | C.A. 20µm | C.A. 30µm | C.A. 100µm |
| 1 | 10 | 22.5 | 100 |
| 1.2 | 8.2 | 18.45 | 100 |
| 2 | 4.8 | 10.8 | 100 |
| 2.47 | 4 | 9 | 100 |
| 3 | 3.28 | 7.38 | 82.2 |
| 5 | 1.96 | 4.41 | 49.3 |
| 7.5 | 1.32 | 2.97 | 32.9 |
| 10 | 1 | 2.25 | 24.6 |
| 15 | 0.64 | 1.44 | 16.4 |
| 20 | 0.48 | 1.08 | 12.3 |
| 30 | 0.32 | 0.72 | 8.22 |
| 40 | 0.24 | 0.54 | 6.16 |
| 50 | 0.2 | 0.45 | 4.93 |
| 75 | 0.132 | 0.297 | 3.3 |
| 100 | 0.1 | 0.225 | 2.46 |
| 200 | 0.048 | 0.108 | 1.23 |
| 500 | 0.02 | 0.045 | 0.493 |
| 1000 | 0.01 | 0.022 | 0.246 |
[data courtesy of Elmitec]
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