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Wednesday, August 3, 2016

Spectrophotometers: Bandpass & Resolution

The smaller the bandpass the better the resolution. Current ISO 2469 conforming instruments are all 10 nm bandpass. However, the new Color Touch™ X has 10 nm, 5 nm and 2 nm bandpass capability.

Excerpt from: 
  • Workman Jr., Jerry & Art Springsteen, "Applied Spectroscopy: A Compact Reference for Practitioners", p. 19-21, 1997.
The terms bandpass and resolution are used to express the capability of a spectrometer to separate spectral bands or lines that are separated by some finite distance.  For an instrument that disperses energy over a prespecified spectral region of the electromagnetic spectrum, the bandpass of a spectrometer is used to describe which portion of the spectrum can actually be isolated by the spectrometer in a "pure" wavelength form.  The spectrometer bandpass is dependent on the dispersion of the grating and the entrance and exit slit width.  An example is often used to illustrate the problem associated with  measuring monochromatic light using conventional spectrometers.  If the ideal spectrometer was used to measure a bright line emission spectrum at a single wavelength (Fig. 8) the spectrum would appear as a single line.  What really occurs when such a spectrum is measured using a conventional spectrometer is a broadband spectrum as shown in Fig 9.  The spectrum assumes a Gaussian-like (or bell-shaped) curve.  This characteristic
broadening of a line spectrum through the spectrometer is an illustration of the spectrometer bandpass.  The actual bandpass for any instrument is assigned a value by determining the FWHM height of the bell-shaped spectrum.  Thus, for the band in Fig. 9, the FWHM could be empirically determined by finding the wavelength at which maximum intensity occurs and measuring the peak height at this position.  This height measurement is divided in half and the bandwidth is measured at this half-height on the band as illustrated in Fig. 10.

To summarize, bandpass and resolution are identical in practice.  Only for the resolution specification of a spectrometer is the expression of bandpass under the specified measuring conditions of an instrument dependent on the slit width settings.