Conventional commercial spectrometers or spectrophotometers are usually able to measure optical spectrum from a specified surface area at one point. This is done either with one detector scanning the spectrum in narrow wavelength bands or with an array detector, in which case all the spectral components are acquired at once. If one desires to measure the spectrum at several spatial locations of the specified surface, the target under examination or the measuring instrument has to be mechanically scanned.

The Spectrograph System samples the moving web along a thin line. The reflected light of each point is dispersed along the spectral axis. The dotted line points to the row where the spectral information of the marked point is projected. The captured spectrum is then analyzed and the absolute color values computed by the smart camera.

Technically, it is not possible to measure, simultaneously, spectral information across a two-dimensional surface matrix, because this would lead to a four-dimensional information space (X,Y coordinates, wavelength, and intensity). This is obviously impossible to realize with standard two-dimensional detectors, which can register only position and intensity of radiation at a time. This leads to the idea of measuring the spatial information across a line only, and the spectral information (wavelength and intensity) for each point in this line. A three-dimensional information space results that can be measured with an area (matrix) detector array connected to a dispersive, stationary spectrograph module.

An imaging spectrograph system for industrial machine vision applications uses a color machine vision sensor that features a built-in spectrograph for enhanced color recognition and measurement. The spectrograph module employs a direct-sight (on-axis) optical configuration and a volume-type holographic transmission grating. This grating is used in a prism-grating-prism construction (PGP-element), which provides high diffraction efficiency and good spectral linearity. It is nearly free of geometrical aberrations due to the on-axis operation and independence from incoming light polarization due to the use of transmission optics only.

A collimated light beam is dispersed at the PGP so that the central wavelength passes symmetrically through the grating and prisms (so that it stays at the optical axis), and the shorter and longer wavelengths are dispersed left and right relative to the central wavelength. This results in a minimum deviation from the ideal on-axis condition and minimizes geometrical aberrations both in the spatial and spectral axis. This provides a significant advantage when using this component as an imaging spectrograph.

The spectrograph module samples a line region with specified length and small, but finite, width. It then disperses the reflected light of each point over the spectral domain and projects the light onto the X-axis of the CCD (see figure). In this way, the image projected onto the CDD contains spatial information on the Y-axis (480 pixels) and spectral information on the X-axis (640 pixels). Using a smart camera means that the captured image is immediately accessible to the firmware on the camera. All processing is done on the camera and the desired information is computed and reported.

Some advantages of the system include reduced measurement time and simultaneous measurement across a line area. In comparison to point color spectrophotometers, this system provides improved spatial resolution and simultaneous measurement across a large number of points. And, because it uses fewer components than on-line spectrophotometers, it can prove more economical. The system offers high color resolution, full compensation for light source intensity and color temperature changes, and full spectral information with flexible wavelength range selection by the firmware.

This article was written by Dr. Ali Zadeh, Sr. R&D Engineer at DVT Corporation. For more information, call (770) 814-7920, or visit www.dvtsensors.com.