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.

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 differ; the Height of all slits are the same (1000 microns).

	5 micron Slit	10 micron Slit	25 micron Slit	50 micron Slit	100 micron Slit	200 micron Slit
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
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
NIR256-2.1 Spectrometer	N/A	~1.2 pixels	~1.2 pixels	~1.5 pixels	~2.2 pixels	~4.0 pixels
NIR256-2.5 Spectrometer	N/A	~1.2 pixels	~1.2 pixels	~1.5 pixels	~2.2 pixels	~4.0 pixels
NIR512 Spectrometer	N/A	~2.4 pixels	~2.4 pixels	~2.9 pixels	~4.4 pixels	~7.9 pixels
QE65000 Spectrometer	~2.0 pixels	~2.2 pixels	~2.6 pixels	~3.3 pixels	~4.7 pixels	~8.9 pixels

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 5.6 pixels = 1.0 nm (FWHM)