Wavelength Selection Devices

1. Absorption Filters

2. Interference Filters

3. Prism Monochromators

4. Grating Monochromators

5. Interferometers

1. Absorption Filters

Optical material containing an absorber that permits transmission only at certain wavelengths

2. Interference Filter

A “sandwich” of at least two parallel optical flats that transmit only rays that undergo constructive interference.

3. Prism Monochromator

Wavelength selection device containing a movable prism.

Snell’s Law of Refraction

n1sin Θ1 = n2sin Θ2

4. Grating Monochromator

Wavelength selection device based on the constructive interference of light rays that have traveled different differences to reach the same point.

Work via:

(1) transmission, or

(2) reflection

Reflection Grating:

Linear DispersionDl

Dl = f × Da = dx/dλ

where:

f = focal length(of monochromator)

x = distance(along focal plane)

Spectral Bandpass(s or Δλ)

s = RLD × w

where:

w = slit width (mm)

s in nm

Reciprocal LinearDispersion (RLD)

RLD = Dl-1

=dλ/dx

nm/mm

Echelle

prism

ICP

Camera

Rowland Circle

Limitations??

5. Interferometer

Allows multiple wavelengths to fall on the detector simultaneously.

Corrects for interference patterns to calculate the signal at different wavelengths.

Fabrey-Perot Interferometer

Michelson Interferometer

Interferometer Advantages

1. Fellget’s Advantage (multiplex)

2. Jacquinot’s Advantage (throughput)

3. Wavelength Accuracy

Interferometer Disadvantages

1. Fellget’s Disadvantage (multiplex)

2. Low wavelengths

3. $$$ for high resolution