IMG 9831

Order Sorting Window in holder

Stray-light is radiation of undesired wavelengths that activates a signal at a detector element. Sources of stray-light can be:

  • Ambient light
  • Scattering light from imperfect optical components, or reflections of non-optical components
  • Order overlap

Avantes symmetrical Czerny-Turner optical bench designs favor stray-light rejection relative to crossed designs. Additionally, Avantes Ultra-Low Stray-light (AvaSpec-ULS) spectrometers have a number of internal measures to reduce stray-light from zero order and backscattering.

When working at the detection limit of the spectrometer system, the stray-light level from the optical bench, grating and focusing mirrors will determine the ultimate limit of detection. Most gratings used are holographic gratings, known for their low level of stray-light. Stray-light measurements are conducted using a halogen light source and long-pass or band-pass filters.

Typical stray-light performance for the AvaSpec-ULS and a B-type grating is <0.04% at 250-500 nm. Second order effects, which can play an important role for gratings with low groove frequency and therefore a wide wavelength range, are usually caused by the 2nd order diffracted beam of the grating. The effects of these higher orders can often be ignored, but sometimes need to be addressed using filtering. The strategy is to limit the light to the region of the spectra, where order overlap is not possible.

Second order effects can be filtered out, using a permanently installed long-pass optical filter in the SMA entrance connector or an order-sorting coating on a window in front of the detector. The order-sorting coatings on the window typically have one long-pass filter (600 nm) or 2 long-pass filters (350 nm and 600 nm), depending on the type and range of the selected grating.

In the table below a wide range of optical filters for installation in the optical bench can be found. The filter types that are 3 mm thick give much better 2nd order reduction than the 1 mm filters. The use of following long-pass filters is recommended: OSF-475-3 for grating NB and NC, OSF-515-3/550-3 for grating NB and OSF-600-3 for grating IB. For backthinned detectors, such as the 2048XL and 1024x58/122 we recommend an OSF-305 Filter, when the starting wavelength is 300 nm and higher.

In addition to the order-sorting coatings, we apply partial DUV coatings on the Sony 2048 detectors to avoid second-order effects from UV response and to enhance sensitivity and decrease noise in the visible range.

This partial DUV coating is done automatically for the following grating types:

  • UA for 200-1100 nm, DUV400, only first 400 pixels coated
  • UB for 200-700 nm, DUV800, only first 800 pixels coated

 Filters installed in AvaSpec spectrometer series

OSF-305-3

Permanently installed 3 mm order sorting filter @ 305 nm

OSF-385-3

Permanently installed 3 mm order sorting filter @ 385 nm

OSF-475-3

Permanently installed 3 mm order sorting filter @ 475 nm

OSF-515-3

Permanently installed 3 mm order sorting filter @ 515 nm

OSF-550-3

Permanently installed 3 mm order sorting filter @ 550 nm

OSF-600-3

Permanently installed 3 mm order sorting filter @ 600 nm

OSF-850-3

Permanently installed 3 mm order sorting filter @ 850 nm

OSC

Order sorting coating with 600 nm long pass filter for VA, BB (>350nm) and VB gratings in AvaSpec-ULS2048(L)/3648/2048x16/64/XL

OSC-UA

Order sorting coating with 350 and 600 nm long pass filter for UA gratings in AvaSpec-ULS2048(L)/3648/2048x16/64/XL

OSC-UB

Order sorting coating with 350 and 600 nm long pass filter for UB or BB (<350nm) gratings in AvaSpec-ULS2048(L)/3648/2048x16/64/XL

OSC-HS500

Order-sorting coating with 350 and 600 nm long-pass filter for HS500 gratings in AvaSpec-HS

OSC-HS900

Order-sorting coating with 600 nm long-pass filter for HS900 gratings in AvaSpec-HS

OSC-HS1000

Order-sorting coating with 350 nm long-pass filter for HS1000 gratings in AvaSpec-HS

OSC-NIR Order-sorting coating with 1400 nm long-pass filter for NIR100-2.5 and NIR150-2.0 gratings in AvaSpec-NIR256/512-2.2/2.5TEC