What is a Spectral Detector (SP Detector)?

Borlinghaus, Rolf T.

March 10, 2019

What is a Spectral Detector (SP Detector)?

Technology of SP8 confocal microscope

Prism-based tunable multiband spectral detection. The SP detector from Leica Microsystems denotes a compound detection unit for point scanning microscopes, in particular confocal microscopes. The SP detector splits light into up to 5 spectral bands. The bands are independent and continuously tunable within the full visible spectrum. The light in each band is detected by a light sensor: a photomultiplier tube (PMT) or a Hybrid Detector (HyD).

Spectral dispersion

The spectral separation of the emitted light from the sample occurs in two steps. The first step is the dispersion of the emitted light spatially into a spectrum, like in a classical spectrometer (1). A prism (Fig 1) is chosen as it is much more efficient in terms of transmission compared to gratings (2).

Prism and motorized spectrometer slit. — Fig 1: Dispersion of light by a prism into a color spectrum and selection of a fraction of that spectrum by a motorized spectrometer slit as a building block for the Leica SP-Detector.

Figure 1 shows the principle of a prism spectrometer, here explained by a simple prism based on a triangle. The SP-Detector employs a prism with a special geometry, a “Pellin-Broca-Prism” (3). This geometry ensures minimal losses, simplifies adjustments, and enhances mechanical stability of the whole setup.
In a classical slit-spectrometer, the desired fraction of the color spectrum is selected by a pair of blackened barriers that are individually movable and, therefore, allow any desired band to be selected from the whole spectrum.

Separation of spectral bands

The second step in the SP-detector applies the slit concept with modifications. Instead of blackened metal, the barriers are coated with a highly reflective layer on the incident side (4).
They are also arranged in an angle. The hereby reflected light can then enter a further arrangement of mirror sliders and sensors. Cascading such arrangements allows the entire spectrum to be split up into a series of spectral bands. The center frequency and bandwidth of each band are continuously tunable.

Tunable Multiband Spectral Detection System SP Detector — Fig 2: Principle of tunable multiband detection system Leica SP-Detector. The spectrum generated from a prism is separated into up to 5 individual fractions by means of motorized mirrors. The bandwidth and center frequency of each band is tunable independently.

The arrangement of prism and mirror-slit cascaded sensors is virtually polarization-independent and highly transparent. Therefore, it features a very high photon efficiency which is needed to collect as much as possible of the precious fluorescence photons by applying illumination with as low an intensity as possible.

Individual gain

The number of sensors may be chosen according to needs of the application. They can be classical photomultiplier tubes (PMT) or Hybrid Detectors (HyD) that come with a highly efficient GaAsP-photocathode. As the gain of each sensor is controlled independently, the dynamic range in each channel can be adapted appropriately (see fig 3) to enhance the S/N ratio. That capability is an advance compared to array-detectors that operate with only one single gain setting. Numerical adjustments offered with array detectors as a means to adapt a number of channels to various intensity ranges do not improve the signal quality.

Performance of individual point detectors and array detectors

Individual spectral band and detector settings like in the SP-detector allow the sensing performance for each fluorescence channel to be fine-tuned individually (5). That function ensures the best signal/noise ratio for all fluorescence channels and allows illumination at the lowest intensity possible.

See reference 5 for a more detailed discussion.