Motorised pinholes are fitted in the Hyperion microscope, controlled with a QEGui program. While the pinhole sizes are fixed, the projected aperture (i.e. size of beam hitting sample and detector) will be dependent on the technique and the objective being used.
The calculation is simple enough:
For transmission:
Spot size (μm) = pinhole size (μm) / magnification
For ATR experiments, the refractive index (n) of the crystal being used needs to be factor in:
Spot size (μm) = pinhole size (μm) / magnification / n
Below is what the QEGui looks like. Note the selected objective. The selected objective is the “standard” for the technique, but we do have other configurations. Please contact a Beamline Scientist to discuss your project further.
The final considerations when selecting pinhole size are resolution and signal intensity. Please refer to the appropriate sections on diffraction limited resolution and experimentally determined spatial resolution for further discussion of these topics. A “typical” signal count for each experimental technique can be found below along with the effect of pinhole size on an example spectrum.
Figure: Signal counts for different acquisition modes. Shown are values measured using the standard “narrowband” detector and the “wideband” detector. Note, values are an indication only and may vary over time.
Example spectra from a range of pinhole sizes using the narrowband and wideband detector
Figure [left]: Effect of pinhole size on an example spectrum of a salt crystal extracted from a mature gouda cheese
NARROWBAND detector
Diamond infrared window
32 co-added scans per spectrum
40kHz, 4cm-1 resolution
5 spectra averaged
Spectra max-min normalised on band at 1100cm-1
Figure [right]: Effect of pinhole size on an example spectrum of a salt crystal extracted from a mature gouda cheese
WIDEBAND detector
Diamond infrared window
256 co-added scans per spectrum*
40kHz, 4cm-1 resolution
3 spectra averaged
Spectra max-min normalised on band at 1100cm-1
*The wideband detector has ~10× less detectivity (sensitivity) compared with the narrowband detector, therefore more co-added scans are required to achieve similar signal-to-noise. Example spectra collected using 32 and 256 scans are shown HERE.