Imaging Systems in Spectroscopy

The term imaging commonly comprises everything, which has to do with a digital image-making process. As in spectroscopy the sensors are also used for the two-dimensional recording of brightness gradients, clear definition of our core competence is useful.

The focus of A. S. & Co. are divided into the following segments

  • imaging for documentation in spectroscopy applications
  • imaging as a tool to accelerate spectroscopic measurements and
  • imaging as an independent measuring method

Imaging for Documentation in Spectroscopy Applications

In quality control or during the production process the examination of manufacturing tolerances requires in addition to the assignment of limit values also the image documentation about their localization. By measurement techniques it is sufficient to provide simple CMOS or CCD cameras, which incorporate their images in the reporting.

An advantage of A. S. & Co. is the ability to freely combine camera and spectrometer with the requirement that the camera allows external control. Cameras with different sensors, various resolutions and speeds will be selected and integrated by A.S. & Co.. In addition, we also integrate consumer cameras, because with their image processors they are designed to create good photo quality, which is conducive to documentation.

PDA or PMT Systems using Cameras Process Acceleration

Regarding the working ergonomics, a video image in live mode, in which measurements such as brightness distributions can be accessed along a freehand line drawn through a mouse click, has a certain charm. However, metrological it is to question whether a CCD TV camera can provide comparable performance to photomultipliers or spectrometers. This can be clarified by a comparison of the detectable optical densities as a result of the sensor dynamics. In principle, sensor characteristics are generally S-shaped, and the most interesting range is close to the inflection point. It should have a long linear progression, which can be proven for example by measuring the transmission of gray filters with different densities. The comparison table shows the differences of dynamics and demonstrates that cameras have a much lower range compared to the other sensors. This could cause problems, if the brightness is used for example as a criterion for a concentration measurement.

Comparison of dynamic ranges of different electronic sensors
12 BIT one Chip BW o Color Cameraca. 1,5 OD
16 BIT Photo-Diode Array Module (PDA)ca. 2,5 OD
16 BIT PMT Measuring Head with post-connected Amplifier Module≥ 3,0 OD

Therefore it is obvious to use a PDA or a photomultiplier as a sensor for critical measurements of optically dense materials, and to combine this with an imaging system and a motorized stage to speed up the experimental procedure and for reporting.

The SpectraVison mapping of A. S. & Co. targets the camera scanning stage synchronization, where the stage coordinates are aligned during the initialization process. By a joystick interesting positions within the sample can be observed on the monitor and through the stage their location can be marked. In the following the measurement is carried out automatically and the spectra of the PDAs or the brightness values of the PMT can be summarized and displayed in clear diagrams.

Layer Thickness Measurement

In this case, the camera has only observation functions. Therefore, in this configuration also ordinary, inexpensive CMOS sensors can also be used. In addition, it is possible to interpret the camera image through a variety of image analysis functions. For this purpose, we assemble according to individual needs

  • a selection of compact and SLR cameras
  • CMOS or CCD industrial cameras with UV or NIR sensitivity or
  • Microscope special cameras with high linear dynamic range
  • with motorized stages of different resolutions and position repeatability
  • with PDA or CCD spectral sensors or
  • PMT-monochromator solutions
  • and documentation and archiving software
  • Methods of gray value-based image analysis
  • Databases according to individual customer requirements
Deconvolution image of a metal etching
Spectra of different penetration depths

Real Color Measurement with Camera-Monochromator-Systems

Specially designed black and white cameras often draw single images from very limited excitation ranges with wide linear dynamic range. This method, which is typically applied in fluorescence investigations, can be extended by utilization of a monochromator so that image sequences are collected as a function of wavelength alteration to X / Y / λ-image cubes. A. S. & Co assembles the modules for this direct detection of spectral components and synchronizes their coordination by our SpectraVision software.

Cooled CCD camera mounted on microscope

For this we use a variety of CCD cameras

  • with different resolutions
  • spectral sensitivity ranges from UV to NIR and
  • different signal/noise ratio by use of Peltier cooling in the combination
  • with galvanometer- or stepper monochromators.

From the X / Y / λ image cubes spectra with the resolution of the monochromator step width can be displayed at any point in the image. Advantages of this spectral mapping are

  • the measurement of color changes in closely adjacent areas
  • the evaluation of diffusion through color determination along predefined lines
  • micro-homogeneity evaluations with statistically relevant patterns of analysis
  • color analysis of particle distributions and pigment mixtures

This method of direct colorimetry is significantly different from the approach to calculate the spectral distribution using mathematical algorithms via a RGB camera. By the black-and-white sensor a much higher resolution is achieved in comparison to the 1-chip color camera. Also in comparison with 3-chip CCDs the benefits of efficient light guiding are predominant. The resolution depends in principle also on used lenses, and may be quite less than the range of a micrometer.