4 Wavelength Selection

The transmission or blocking of certain wavelengths is an important requirement for optical spectroscopy and imaging. Many analytical methods require (approximately) monochromatic light for measurements, require a user to be able to select an arbitrary wavelength or scan across an arbitrary wavelength range, and/or require that the light emitted or scattered from a sample is detected without interference from the light used to stimulate the sample. Depending on the application, such wavelength selection is achieved via optical filters, gratings, and prisms.

Wavelength Transmission and Blocking

Interference Filters. These filters are the most versatile class of optical filter. They are designed to transmit certain wavelengths of light and reflect others. Bandpass filters transmit a defined range of wavelengths, longpass and shortpass filters transmit wavelengths longer and shorter than a particular cutoff wavelength, and a notch filter transmits all wavelengths except for a narrow range. Dichroic mirrors are interference filters that have been designed for controlled reflection of selected wavelengths of light and transmission of others (i.e. used as a wavelength-selective mirror).

Absorptive Filters. These filters, which are often coloured glass or polymers, absorb light rather than reflect it, and are also available in longpass, shortpass, and bandpass varieties. Absorptive filters cost less than interference filters but offer less precise control (e.g. less sharp wavelength cutoffs) over transmitted wavelengths.

Wavelength Dispersion and Selection

Gratings. Reflection gratings consist of a metalized series of grooves with spacing similar to the wavelength of light. Each groove acts as source of diffracted light. The rays of diffracted light from adjacent grooves interfere with one another and constructive interference is observed for different wavelengths at different angles. White light is thus diffracted into its rainbow-like spectrum.

Prisms. Although less common than gratings, glass prisms are sometimes used for wavelength dispersion and selection, including as components of monochromators. Prisms take advantage of the wavelength dependency of refractive index. Each wavelength of light is refracted by the prism differently and emerges from the other side of the prism at a different angle. White light is thereby dispersed into its rainbow-like spectrum.

Monochromators. Most monochromators consist of an entrance slit, grating, exit slit, and mirrors (as necessary) to direct light between these components. The entrance slit forms a narrow beam of incoming white light that the grating diffracts into its spectrum. The exit slit selects a small portion of this spectrum for use. The entrance and exit slit widths determine how narrow a range of wavelengths is selected (typically a few nanometers to a few tenths of nanometers). The nominal output wavelength is varied by rotating the grating. Monochromators offer flexible wavelength selection, but are less efficient, much larger, and more expensive than optical filters.

Polychromators. When the exit slit of a monochromator is removed, the result is a polychromator. This format is useful when a single photodetector is replaced with a closely-spaced array of many (typically small) photodetectors (e.g. linear photodiode array, CCD camera).

Connections

Wavelength selection is important in instruments for optical spectroscopy:

• Optical filters use absorption or interference (Ch. 2) to block and transmit certain wavelengths of light.
• Bandpass and longpass filters are commonly used for measurements of fluorescence (Ch. 9).
• Notch filters are commonly used for measurements of Raman scattering (Ch. 15).
• Monochromators and polychromators are essential components of UV-visible spectrophotometers (Ch. 7),  spectrofluorimeters (Ch. 9), atomic absorption and emission spectrometers (Ch. 11), and Raman spectrometers (Ch. 15).

Post-Reading Questions

1. List the three main components of most monochromators?
2. List two ways in which the output wavelength(s) of a monochromator are controlled?
3. Identify on what behaviour of light from Ch. 2 is the wavelength dispersion of gratings based. What about for prisms?
4. Based on the provided descriptions, sketch diagrams of transmission versus wavelength for bandpass, longpass, shortpass, and notch filters. For simplicity, assume transmission is either 100% or 0%, depending on the wavelength.

Topic Learning Objectives

The chapter is a primer for the following learning objectives, which will be covered in lecture and/or with additional assigned reading:

• Qualitatively explain the operating principles of a grating-based monochromator.
• Use the quantitative description of a reflection grating to calculate diffraction angles for a given wavelength.
• Draw diagrams that illustrate the practical operation of grating monochromators and filters
• Predict how orders of diffraction might appear in a spectroscopic measurement. Explain how they may be eliminated.
• Explain the role for slits in wavelength selection and when the entrance and/or exit slit is not required.
• Assign transmission diagrams to specific types of optical filters.
• Select appropriate types of optical filters, monochromators, and polychromators for specific light sources and measurement requirements.