While DLP spectroscopy is a relatively new field, spectroscopy has been used for many years by measuring the reflectivity (or transmission) of individual colors of light, both visible and invisible. 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. By spreading the various light wavelengths across the surface of a DLP chip, each one can be directed to a detector circuit so that the variation in light intensity versus wavelength can be measured.
Use of a DLP based system, a very cost effective solution over linear array detectors is possible by use of a single detector, especially at longer light wavelengths where cooled sensors are required for accurate measurements. The speed and precision of the DLP chip also allows the use of a variable scanning techniques so that faster OR very precise scans can be selected.
DLP spectroscopy designs may use a dispersive or other 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. This method is employed in the DLP NIRscan EVM kit, produced by Keynote Photonics
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.
In a diffractive approach, light is produced by a broadband light source, such as an incandescent light bulb or a phosphor based LEDs, 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.
A key benefit of DLP spectroscopy is that the light passing through the sample is detected by a single point sensor (that is, not an array of sensors), 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.
Key Features and Benefits
- Fully self-contained MEMS spatial light modulator
Use single point sensor instead of expensive array detectors
Allows design of mechanically robust, rugged spectrometers for field use
No moving components required (mirrors, grating, etc)
- Digitally program “scanned lines”, “matched filtering”, or “coded filtering”
Flexibility to analyze multiple materials with a single spectrometer engine
No “analog drift” over time or temp
- Fast Pattern rates up to 32 kHz for faster scans
- Optically efficient from 365nm to 2500nm Supports UV, VIS and NIR spectrometers