Selecting Components: Optical Resolution

Optical resolution -- measured as Full Width Half Maximum (FWHM) -- of a monochromatic source depends on the groove density (lines/mm) 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.

The approximate optical resolution in nm (FWHM) can be calculated as follows:

1) Dispersion (nm/pixel) = Spectral Range of the Grating (see Choosing a Grating) ÷ Number of Detector Elements (2048 for S2000 or 1024 for TR1000)

2) Resolution (in pixels) = value from slit size/fiber diameter chart (see below)

Typical pixel resolution by slit size/fiber diameter

• 10 micron slit = 3.2 pixels
• 25 micron slit = 4.2 pixels
• 50 micron slit = 6.5 pixels
• 100 micron slit = 12.0 pixels
• 200 micron slit = 24.0 pixels

3) Optical Resolution (in nm) = Dispersion (nm/pixel value from #1) x Resolution (pixels value from #2)

EXAMPLE: S2000 spectrometer with Grating #3, 10-micron slit

    650 nm ÷ 2048 = 0.32 nm/pixel x 3.2 pixels = 1.02 nm (FWHM)

This material taken from the Ocean Optics web site: www.oceanoptics.com