High Performance, Low Cost…

Spectroscopy : Monochromators and Spectrometer Configurations

By adding a wavelength dispersing system to the CCD processing engine, a spectrometer can be constructed. Spectrometers have many uses - they can record a sample’s absorption spectrum, or be used to characterize the emission spectra of LED’s, discharge lamps and other emitting sources. Atoms and molecules have unique spectral fingerprints in absorption/emission, making spectroscopy a powerful tool for identifying chemical substances.

A monochromator is an optical device that transmits a single wavelength of light chosen from a wider range of wavelengths presented at the input. The dispersing element in most monochromators is a diffraction grating. A diffraction grating is an array of fine, parallel, equally-spaced grooves on a substrate that either reflects light off the grating or is transparent, allowing light to pass through the grating.

High performance monochromators with photomultiplier detectors behind their exit slit are expensive devices that also require an accurate motor scanning system to rotate the grating in order to scan the wavelengths being recorded. The advantage of a CCD-based system is that it can capture a range of wavelengths simultaneously without needing any moving parts.

Examples of the types of spectrometers that can incorporate my CCD processing engine and a suitable CCD detector chip include :

• a transmission grating spectrometer (CCD-TGS)
• a concave grating spectrometer (CCD-CGS)
• coupling can be to a third party system, such as the Sol Instruments SL100M lens spectrograph

These spectrometers can be built at very low cost and have many attractive features, including

• Direct connection to a PC’s USB port (no external power is required).
• Use of a fiber optic input port simplifies coupling of light into the spectrometer from external sources.
• Good spectral response using low cost CCD's from <400 nm out to beyond 1 micron.

Several LabVIEW stand-alone executables are available to give a GUI for control of CCD parameters such as exposure time, number of scans and spectra averaging. Additional vi’s facilitate wavelength calibration, real-time logging of spectra etc.

(1) Transmission Grating Spectrometer (CCD-TGS)

Using a piece of transmission grating film, and inexpensive plastic lenses, a spectrometer can be built that has very good performance yet at extremely low cost. The light to be spectrally analysed is first delivered to the transmission grating via a fiber optic cable or alternatively, can be passed through a slit. A collimating lens then ensures that a parallel beam falls on the grating surface. The dispersed light then passes through a second lens in order to image it onto the row of pixels in the CCD detector.
To realise good performance, the CCD must be accurately positioned in the focal plane of the lens system (the line at position 1) so that each peak in the spectrum is as bright and as sharp as possible.

The above image is an optical simulation (using OpticsLab software) of the beam path in a transmission grating spectrometer and the corresponding "spot diagram" when the detector has been correctly placed. As can be seen, if the detector is not located precisely in the focal plane, the quality of the spectrum would rapidly deteriorate the further it is moved (either forward or back) from its correct position.

(2) Concave Grating Spectrometer (CCD-CGS)

Concave gratings are ruled onto the surface of a spherical concave mirror. A light beam incident on a concave grating is then both dispersed and focussed, thereby simplifying the design and fabrication of many optical instruments.

One type of concave grating configuration is the constant-deviation monochromator, which allows wavelength scanning to be performed with fixed entrance and exit slits simply by rotating the grating. When used with a CCD detector, no exit slit is needed as the detector is simply placed at the image plane.

CCD-CGS Implementation

The image to the right shows a small spectrometer (overall dimensions (mm) L(130) x W(130) x H(53)) we have constructed using a Shimadzu model #20-046 aberration-corrected concave grating to image light onto an ILX511 linear CCD array. This grating has 800 grooves per mm and a dispersion D of 14.7 nm/mm.

The CCD detector PCB (at lower left) is connected via a short ribbon cable to the CCD processing engine PCB.

The position of the delrin carrier mounting the CCD detector needs to be carefully adjusted to achieve the desired wavelength coverage and also to yield the narrowest/most symmetrical spectral peak shapes.

Typical spectra obtained using this instrument can be found on the Calibration and Emission pages of this website.
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Image Sensor Breakout PCB

The photo at left shows the small carrier board that is used to mount the CCD image sensor in the CCD-CGS.

A cylindrical lens can be glued to the sensor’s window to reduce image height and concentrate more light onto the sensors 200 um tall pixels.
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(3) Sol Instruments Lens Spectrograph

The Sol Instruments Model SL100M is a lens spectrograph designed for imaging applications, with an optical path using objective lenses that makes for a very compact monochromator design.

Once again, a spectrum is obtained by placing the CCD detector at the focal plane and making adjustments to obtain spectral peaks that are as intense, narrow and symmetrical as possible.

In an optical spectrometer using the SL100M there are three key adjustments:

• Entrance slit width and height
• Grating centre position (sets centre wavelength)
• Positioning the CCD detector exactly in the focal plane

A photo showing the SL100M in use in an area array spectroscopic imaging system can be found here.