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**Wavelength Selection Devices**

1. Absorption Filters 2. Interference Filters 3. Prism Monochromators 4. Grating Monochromators 5. Interferometers

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1. Absorption Filters Optical material containing an absorber that permits transmission only at certain wavelengths

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

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3. Prism Monochromator Wavelength selection device containing a movable prism.

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**Snell’s Law of Refraction**

n1sin Θ1 = n2sin Θ2

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**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

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Reflection Grating:

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Linear Dispersion Dl Dl = f × Da = dx/dλ where: f = focal length (of monochromator) x = distance (along focal plane)

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Spectral Bandpass (s or Δλ) s = RLD × w where: w = slit width (mm) s in nm Reciprocal Linear Dispersion (RLD) RLD = Dl-1 =dλ/dx nm/mm

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Camera Echelle prism ICP

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Rowland Circle

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Limitations??

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5. Interferometer Allows multiple wavelengths to fall on the detector simultaneously. Corrects for interference patterns to calculate the signal at different wavelengths.

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**Fabrey-Perot Interferometer**

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**Michelson Interferometer**

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**Interferometer Advantages**

1. Fellget’s Advantage (multiplex) 2. Jacquinot’s Advantage (throughput) 3. Wavelength Accuracy

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**Interferometer Disadvantages**

1. Fellget’s Disadvantage (multiplex) 2. Low wavelengths 3. $$$ for high resolution

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