Design Considerations
DLP: Multispectral Imaging – Overview
Multispectral Imaging is a technique for recognizing and characterizing physical properties of materials using the principle of the varying absorption (or emission) of different wavelengths of light by the objects. This technique has been applied in areas of science such as medicine, forensics, geology, and meteorology.
The wavelengths of light used in multispectral imaging usually lie within the Infrared (IR) and Near Infrared (NIR) ranges. In contrast to hyperspectral imaging, which characterizes materials by measuring the variation in light intensity over continuous ranges of wavelengths, multispectral imaging utilizes a relatively small set of specific wavelengths. The desired wavelengths may be selected by a set of dichroic interference filters of specific wavelength and pass-band. The variation of light intensity vs. wavelength is measured by means of appropriate sensor techniques for each point in the scene being imaged.
The Multispectral Imaging application illustrated in the diagram shows an external scene which passes through a selected dichroic filter before being imaged by a lens onto a DLP® Digital Micromirror Device (DMD). The filter wheel rotates to each filter in turn, allowing a scan of the scene at each selected wavelength. The DMD is used to decompose the image by turning on a single mirror at a time in a raster (or other) pattern, until the entire image has been recorded by the sensor. The output signal from the single element sensor then passes through an amplifier to the input of an embedded processor.
The embedded processor has two functions. The first is to send the commands to the DMD controller to turn on only the precise mirror at each instant to scan (or decompose) the image. The second function of the embedded processor is to collect the data from the sensor and produce a set of images of the scene corresponding to each filter. This data can be viewed as a graph of the light intensity of each wavelength for each pixel, creating a signature of the scene or material that can later be analyzed and compared to those of similar signatures to determine the composition or state of the material which has been examined.
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 Multispectral Imaging system’s specifications, such as the selected wavelengths to be measured, the resolution of the image needed, and the speed of acquisition of a filtered images, etc.
The choice of single element sensor device will depend, again, on the selected wavelengths 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 Multispectral Imaging application. An actual product will require additional optical components and optical design in order to achieve full functionality.
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