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Home > Frequently Asked Questions > Products > Spectrometers > How does my choice of spectrometer slit size affect optical resolution?

How does my choice of spectrometer slit size affect optical resolution?

Optical resolution of a monochromatic source — — measured as Full Width Half Maximum (FWHM) — depends on the groove density (mm-1) of the grating and the diameter of the entrance optics (optical fiber or slit). In configuring your spectrometer, consider two important trade-offs:

  1. Resolution increases with an increase in the groove density of the grating, but at the expense of spectral range and signal strength; and
  2. Resolution increases as the slit width or fiber diameter decreases, but at the expense of signal strength.

How to Calculate the Approximate Optical Resolution in nm (FWHM)

1. Determine the Spectral Range of the Grating. You can find the Spectral Range of Gratings at:

2. Divide the Spectral Range of the Grating by the Number of Detector Elements. The resulting value is the Dispersion.

Dispersion (nm/pixel) = Spectral Range of the Grating/Number of Detector Elements

The table below lists the Detector Elements for various spectrometer benches and models.

Spectrometer Number of Detector Elements
Ocean FX 2048
Flame-S Spectrometer 2048
Flame-T Spectrometer 3648
Jaz Spectrometer 2048
USB2000+ Spectrometer 2048
USB4000 Spectrometer 3648
HR2000+ Spectrometer 2048
HR4000 Spectrometer 3648
Maya2000 Pro Spectrometer 2048 x 64 (active)
Maya LSL 2048 x 64 (active)
QE Pro Spectrometer 1044 x 64 (total pixels)
NIRQuest 256 Series 256
NIRQuest 512 Series 512
Torus 2048

3. Determine the Pixel Resolution

The pixel resolution is listed below for various sizes of slits (or the optical fiber diameter if the fiber is the limiting factor). The width of the entrance aperture slits differs; the Height of all slits are the same (1000 microns).

Resolution in nm according to available slit size
Spectrometer 5 µ  10 µ 25 µ 50 µ 100 µ 200 µ
Ocean FX ~3.0 pixels ~3.2 pixels ~4.2 pixels ~6.5 pixels ~12.0 pixels ~24.0 pixels
Flame-S Spectrometer ~3.0 pixels ~3.2 pixels ~4.2 pixels ~6.5 pixels ~12.0 pixels ~24.0 pixels
Flame-T Spectrometer ~5.3 pixels ~5.7 pixels ~7.5 pixels ~11.6 pixels ~21.0 pixels ~42.0 pixels
Jaz Spectrometer ~3.0 pixels ~3.2 pixels ~4.2 pixels ~6.5 pixels ~12.0 pixels ~24.0 pixels
USB2000+ Spectrometer ~3.0 pixels ~3.2 pixels ~4.2 pixels ~6.5 pixels ~12.0 pixels ~24.0 pixels
USB4000 Spectrometer ~5.3 pixels ~5.7 pixels ~7.5 pixels ~11.6 pixels ~21.0 pixels ~42.0 pixels
HR2000+ Spectrometer ~1.5 pixels ~2.0 pixels ~2.5 pixels ~4.2 pixels ~8.0 pixels ~15.3 pixels
HR4000 Spectrometer ~2.0 pixels ~3.7 pixels ~4.4 pixels ~7.4 pixels ~14.0 pixels ~26.8 pixels
NIRQuest
512 Series
N/A ~1.5 pixels ~1.7 pixels ~2.8 pixels ~4.3 pixels ~7.5 pixels
NIRQuest
256 Series
N/A ~1.0 pixels ~1.5 pixels ~1.6 pixels ~2.2 pixels ~4.0 pixels
Maya2000 Pro ~1.5 pixels ~2.0 pixels ~2.5 pixels ~4.2 pixels ~8.0 pixels ~15.3 pixels
Maya LSL 0.5 0.7 0.7 1.2 2.2 NA
Torus 0.5 0.7 0.7 1.2 2.2 NA
QE65000 Spectrometer ~2.0 pixels ~2.2 pixels ~2.6 pixels ~3.3 pixels ~4.7 pixels ~8.9 pixels

 

4. Calculate the Optical Resolution (in nm)

Dispersion (Step 2) x Pixel Resolution (Step 3)

Example: Determine the Optical Resolution of a USB4000 Spectrometer with Grating #3, 10-micron Slit

650 nm (spectral range of Grating #3)/3648 (detector elements in USB4000) = 0.18 nm/pixel x 5.7 pixels = 1.0 nm (FWHM)

Note: Values rounded up to nearest tenth.

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NIRQuest256-2.1

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Small-footprint Spectrometer for Near-Infrared Measurements
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Application-ready Spectrometer for the UV-VIS
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Application-ready Spectrometer for the Visible and near-IR
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Extended Range Spectrometer for UV-NIR applications with Enhanced Sensitivity
USB4000 (Custom)

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USB4000-FL

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USB4000-XR1-ES

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Extended Range Spectrometer for UV-NIR applications with Enhanced Sensitivity
USB4000-XR1

USB4000-XR1

Extended Range Spectrometer for UV-NIR applications
USB4000-VIS-NIR-ES

USB4000-VIS-NIR-ES

Application-ready Spectrometer for the visible and near-IR with Enhanced Sensitivity
USB4000-VIS-NIR

USB4000-VIS-NIR

Application-ready Spectrometer for the Visible and near-IR
USB4000-UV-VIS-ES

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Application-ready Spectrometer for the UV-VIS with Enhanced Sensitivity
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Application-ready Spectrometer for the UV-VIS
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HR4000 (Custom)

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High Resolution Spectrometer for Laser Characterization