Spectroscopy is a powerful technique for recognizing and characterizing physical materials using the principle of the varying absorption (or emission) of different wavelengths of light by samples of the physical materials. The samples may be material in any of the various physical phases: solid, liquid, gas, or plasma, and may be light emitting or light absorbing. The light used in spectroscopy may lie within the range of wavelengths visible to the human eye, or may be in the infrared or ultraviolet regions of the electromagnetic spectrum. Spectroscopy requires the spreading out of light into a rainbow of wavelengths, so that the variation in light intensity versus wavelength can be measured (and usually also recorded).
Spectroscopy employs a dispersive optical element to spread the spectrum of light into spatially separated wavelengths. Prisms are sometimes used, but diffraction gratings are more commonly employed, because of their higher dispersion, and their ability to be optimized for a wide range of optical wavelengths. There are several optical and physical arrangements used in spectroscopy.
The spectroscopy application illustrated in the diagram is used to identify or characterize a prepared sample of some material, which must be homogeneous and translucent. It can be solid, semisolid (gel), powder, or liquid, depending on the method of holding the sample. The diagram shows schematically a sample of material spread out on a glass slide, as an example.
The light is produced by a broadband light source (perhaps an incandescent light bulb), then collected and collimated, and passed through a narrow slit. The slit provides a geometrically sharply defined source of light which is then shines on a diffraction grating. The diffraction grating reflects each of the wavelengths of light at precisely different angles, thereby spreading the dispersed spectrum of light across the mirror array of a DLP® Digital Micromirror Device (DMD).
The embedded processor commands the DMD controller to turn on only the precise columns of mirrors which are illuminated by the specific wavelength of light that is desired at each instant of time. Over a short period of time, the entire spectrum is sequentially scanned, and used to illuminate the sample.
The light passing through the sample is detected by a single point sensor (that is, not an array), and the signal is processed by the embedded processor. The result of the completed measurement is shown in the graph of light intensity vs. wavelength. The distinctive shape of this curve constitutes a spectral signature of the material being examined. By comparing the spectral signature of the sample to stored reference signatures, it is possible to ascertain the physical and chemical composition of the sample.
The diagram shows a DLP® chipset, which includes the DMD, and a DMD Controller chip, plus a DMD Analog Control chip (depending on the specific DLP® chipset). Various DLP® chipsets are available, with different DMD sizes, resolutions, and other specifications. The best choice for a DLP® chipset will depend on the spectroscopy system’s specifications, such as the range of wavelengths to be measured, the resolution of wavelength desired, the speed of acquisition of a spectrum measurement, etc.
The choice of sensor will depend, again, on the range of wavelengths to be measured. Other considerations for the sensor include the required sensitivity, speed of acquisition, noise, temperature range, interface requirements, cost, and other factors.
The system control and signal processing is accomplished by the Embedded Processor (Such as TI OMAP®). Power is provided by TI Power devices. The details of the optical layout and components are not shown in the diagram. The diagram is intended to convey as simply as possible the overall functionality of a DLP-based spectroscopy application. An actual product will require additional optical components and optical design in order to achieve full functionality.
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