Choosing an OEM Spectrometer
Understanding the Application Landscape and Your Customer’s Needs
Our partnership process starts with understanding the big picture of your business model and its focus on the customer, and drills down further into the specific product needs to deliver a complete solution. While the need to balance price and performance is obvious, what is not always clear to OEM customers is the importance of linking the spectrometer to the application. While ultimately this is distilled into a numerical product specification, the route to that specification begins at the sample and the measurement application.
Advances in optical and spectroscopy technology make measurements that were once performed by very sophisticated laboratory instruments accessible to everyone. Specifically, miniature spectrometers make a world that once required returning the sample to the laboratory obsolete. Now, spectroscopy can be carried out near the sample: in the manufacturing process, beside the patient’s hospital bed or inside an erupting volcano. Liberating the measurement from the laboratory takes optical sensing to the next level - unlocking numerous applications and creating new and expanding markets.
The ability to customize a miniature spectrometer enables systems that solve specific application problems. Whether measuring absorbance, reflectance or the Raman spectrum, these tools give information that deliver a targeted answer. In order to deliver a properly specified component or system, the information that the optical system will deliver and the problem that your product helps the customer overcome must be well understood. A complete understanding of the
measurement application throughout the process drives to the proper final product specifications.
Lastly, the product you produce must meet or exceed customer expectations. This is not purely a question of addressing a level of technical competence; it also includes the overall customer experience and the proper fit of the product into the customer’s workflow. From the viewpoint of the OEM, this means complete understanding of the product’s form, fit and function. The risk of poor market reception can be mitigated by having a comprehensive understanding of the product’s critical performance needs and keeping these requirements top of mind through the development process.
Developing and Configuring a Solution
The selection of a spectrometer involves a series of tradeoffs among different optical parameters. In order to understand these tradeoffs we must first understand the components of the optical bench and their role in segregating and measuring the spectrum. A typical cross Czerny-Turner monochromator is shown in Figure 1. The optical bench comprises an entrance slit (A), a folding mirror (B), the grating (C) and the detector (D). The entrance slit, grating and detector combine to determine the wavelength range and resolution of the spectrometer.
Figure 1: Typical cross Czerny-Turner monochromator
More specifically, the wavelength range and the resolution are influenced by (1) the size of the entrance slit, (2) the groove density of the grating and (3) the number of active elements (pixels) in the array detector. Let’s examine these parameters individually starting with the entrance slit. The optical bench transfers an image of the entrance slit onto the detector such that monochromatic light will fall across all the pixels illuminated by the slit image. The optical resolution is directly related to the slit width. For example, in one bench configuration a 100 micron slit will have an optical resolution of 14.0 pixels FWHM. Decreasing the slit width to 50 microns will improve the optical resolution to 7.4 pixels FWHM, but the throughput of light will be reduced by approximately 50%. As the slit width decreases it approaches the limit of diffraction and the improvement in resolution begins to diminish. Moving from a 50 micron slit to a 5 micron slit only improves resolution by a factor of 3.7, instead of the anticipated factor of 10.
Also, the choice of grating and wavelength range must take into account how gratings function. Light striking the grating is diffracted into multiple angles, see Figure 2. The angles are whole number multiples or orders that vary with the incident wavelength. For example, incident 200 nm light will be diffracted at angle #1, and also at angle #2, which is exactly twice as large as #1. Angle #2 also corresponds to the first order diffraction of light at exactly twice the wavelength, or 400 nm. If the application requires measurements at 400 nm, and 200 nm light enters the spectrometer, light from both the second order diffraction of 200 nm and the first order diffraction from 400 nm will reside at the same pixel. Therefore, the measured intensity at that pixel will include contributions from both wavelengths.
Figure 2: Dispersion of monochromatic light from the grating illustrating higher order diffraction.
Second order light is removed by using filters. Array spectrometers intrinsically lend themselves to the use of linear variable filters designed to match the dispersed spectra and provide the appropriate blocking at each pixel in an array. Because these linear variable filters are designed and fabricated for a particular bench, grating and starting wavelength, the customer loses the option to adjust the spectral range. Alternatively, light from higher order diffraction can be excluded by filters before entering the spectrometers, preserving the flexibility of the optical bench.
Gratings can be optimized for particular wavelength ranges. This is accomplished by tilting the grooves, called the blaze angle. The efficiency of the first order diffraction is enhanced in one wavelength region, with the tradeoff that it decreases in another region. For example, a 600 line/mm grating blazed for optimum efficiency at 300 nm is <30% efficient at wavelengths greater than 575 nm. The same grooves tilted at an angle to accentuate the NIR region are >30% efficient from 530-1100 nm but are low efficiency in the UV. Groove density also influences the available blaze angles. Special composite gratings, like the Ocean Optics HC-1 grating, have regions with different blaze angles. This yields a grating with good efficiency over a wider wavelength range (200-1050 nm) than is possible with traditional gratings.
For example, in our “S” bench, a 600 line/mm grating will cast a 650 nm band of light across the active area of the detector. By rotating the grating, the spectral range illuminating the detector can be varied. This facilitates the measurement of a spectrum from 200-850 nm, or from 300-950 nm, and so on. Substituting a 1200 line/mm grating causes the light to be diffracted at twice the angle, thus the detector will intercept half the wavelength range. Because the same amount of light is spread out over twice the angle the signal will be decreased by a factor of two. With everything else held constant, the optical resolution would be twice as fine.
Physically increasing the size of the optical bench has the net effect of achieving higher resolution at the expense of a narrower wavelength range. The “S” bench is a 42 mm focal length design. The HR bench is a 102 mm focal length design. In the HR bench, the same 600 line/mm grating would yield a range of about 430 nm and resolution that is 66% finer. Of course, the signal is lower because of the higher resolution. The other tradeoffs with the longer focal length are size and weight.
Lastly, the choice of detector pixel density drives the optical performance specifications. All other things being equal, the density of the detector elements across the dispersed light determines the resolving power of the spectrometer. If a 1024 element array is replaced with a 2048 element array, the resolution of the spectrometer is improved by a factor of about 2, providing a higher data density over the same operating range. The size of the pixels in the array also imposes a limit on the resolution that can be obtained with a given bench. In general, our detectors have pixel spacing from 8μm to 14μm. These detectors are small enough to achieve the maximum resolution possible with our smallest slits.
Detector Material and Optimization
Detectors are chosen for several design features. Semiconducting detectors are used to capture photons and convert them into electrons. The absorption band gap of silicon allows for good sensitivity over the ultraviolet (UV), visible and shortwave NIR region (from around 160 nm to nearly 1100 nm). Indium Gallium Arsenide (InGaAs) detectors generally work well above 900 nm to about 1700 nm. Special dopants placed in InGaAs detectors can be used to extend the range to 2100-2500 nm. For example, HgCdTe can extend the measurement range to the mid-IR.
Also, the effective spectral range and resolution for detectors depend on architecture. The simplest design is called a photodiode array (PDA). In PDA detectors, each diode in the array is connected to readout circuitry that occupies a portion of the array next to the detector. The diodes are supplied with a bias voltage, and photons that are absorbed by the Si generate a current. The sampling circuitry features a sample and hold scheme with the ability to strobe through the row of detectors to acquire the voltage signal from each pixel. The stream of analog signals is amplified and converted to digital spectral data by low-noise external circuitry.
The need to produce images, like those in the first photocopiers, fax machines and digital cameras led to the invention of the charge coupled detector or CCD. Here the photodiode is covered by a transparent capacitor that accumulates the signal during a length of time called an integration period. The advantage of this architecture is that there are no dead spots occupied by readout circuitry and images can be captured. A linear CCD array is the same architecture but consists
of a single line of CCD devices instead of the bare photodiodes.
CCDs have a great advantage over photodiodes in that they have very low levels of readout noise. Their main disadvantage is that the polysilicon gates or capacitors absorb UV light, therefore CCDs generally do not respond to light much below 350 nm. There are two remedies to this problem. The CCD array can be coated with a phosphor, which absorbs UV light and emits visible light. This renders a detector with adequate UV response for many applications. A more
expensive solution is to make the device very thin and to turn it around so it is illuminated from the backside. This exposes the photodiodes to the UV light and the resulting device is significantly more sensitive to UV than the phosphor-coated types.
Some applications require detectors with specific features. For example, time resolved kinetics spectroscopy requires a detector that can be gated. A light source excites the sample at time zero. The detector must be triggered to start collecting the light emitted by the cooling plasma after a microsecond delay. Here is it critical for the detector and therefore, the spectrometer, to communicate externally both with incoming triggers and outgoing data.
Spectrometer Communication
The connection of the spectrometer to the outside world is very important. The ability to communicate effectively with input and outputs is vital to the proper integration of your device. This communication includes receiving input triggers and providing the proper data, in the correct format. We have designed our spectrometers, firmware and software to support your communication requirements.
Input signals can come from a variety of different sources. However, the two most common are a contact closure and TTL pulse. Both can be used to trigger GPIOs on the spectrometer. Typically, the interface for communication with triggers is a serial port; however, this can be configured to your specifications as appropriate. More important is how the spectrometer data is transferred to your computer systems. We offer a high degree of flexibility with data acquisition and process through our OmniDriver, SeaBreeze and SPAM tools. Our OceanView product allows you to graphically configure your own dataflow and spectral transformations visually from an intuitive schematic view. Alternately, our multi-platform SPAM library sports a comprehensive collection of validated spectral math functions and CIE tables, allowing you to quickly develop your own custom GUI in the language of your choice. Written with compatibility in mind, these software tools allow you to take the data stream from the spectrometer and turn it from a waveform into an answer, converting data to a format that works for your algorithms.
Finally, the spectrometer firmware can also be customized to allow onboard processing. This changes the output of the spectrometer to more specifically match your data requirements. As regulatory and data security rules change, having a customizable output is becoming more important than ever. Also, more companies are migrating to whole-house LIMS systems where calculations are performed, streamlining data analysis and demystifying results obtained across multiple different instruments.
Engineering Support, Production Capability and Vertical Integration
Throughout the process of developing a solution for your product, Ocean Optics has a team of dedicated engineering resources standing by to assist your development. We know time to market is often one of the most important drivers for your business and we are ready to assist and help you accelerate your process. Whether your needs are mechanical or electrical, software or hardware, we have the resources to help turn your product dream into a reality.
