Years ago, I realized the limitations of simply measuring light intensity within an aquarium and longed for a way to measure spectral quality. Then, the cost of a spectroradiometer equaled that of purchasing a new luxury sedan. Times have certainly changed. If you have deep pockets and a true desire to tinker with and know practically everything about your lighting system and then some, then the relatively low-cost spectrometers and accessories from Ocean Optics just might be for you.
One may wonder why one would want to invest about a thousand dollars in a ‘big boy’s toy.’ After all, dedicated hobbyists, such as Sanjay Joshi, have analyzed the spectrum of hundreds of lamps and have freely distributed this information. Why invest in a spectrometer? While monitoring spectral quality of lamps is likely the primary goal of most hobbyists, the Ocean Optics spectrometers are capable of doing much more (provided necessary accessories and additional software – all at extra cost – are available). Of these, reflectance, transmittance, absorbance, color analysis, relative irradiance and absolute irradiance are of possible interest.
This article will relay some of my personal experiences with two Ocean Optics spectrometers in my lab (a USB-2000FL – a model designed to measure low level fluorescence and a USB-2000 capable of measuring ultraviolet radiation and visible radiation from 200 nm to 850 nm) as well as touch upon some exciting new introductions. The USB-2000 has just recently been replaced by a newer model – the USB-4000 – however, the observations of the older models are generally valid for the 4000 series, as well as the new, low-cost ‘Red Tide’ USB650 spectrometer.
The following article will also describe various options available from Ocean Optics; however, it is not a substitute for consultation with an Applications Scientist. The few minutes required for a phone call to Ocean Optics is time well spent, and will ensure that your spectrometer is correct for your application. There are some things you’ll need to know before calling, and this article can assist you with that.
Ocean Optics revolutionized the spectrometer field when it introduced the diffraction grating-based spectrometer using a charged coupled device (CCD) for light collection. The early models were just a fraction of the cost of spectrometers then on the market. Combined with fiber optic technology, the Ocean Optics equipment was, and continues to be, a powerful and relatively inexpensive research tool. The new Red Tide spectrometer is $999, while the new USB-4000 starts at $2,199. The required SpectraSuite software is available for an additional $199. See a partial list of specifications in Table 1.
The spectrometers are very compact (about the size of a deck of playing cards) and light weight. These specs utilize USB technology, making then ideal for field use (in fact, many researchers have their spectrometers in underwater housings for work with corals in their natural habitats). All OO spectrometers are the crossed Czerny-Turner design.
|USB2000||USB4000||Red Tide USB650|
|Optical Bench||Asymmetrical crossed Czerny-Turner|
|Detector||Sony ILX511||Toshiba TCD1304AP||Linear silicone CCD|
|Grating||Choice of 14 holographic gratings||Preconfigured for 350-1000 nm|
|Entrance Slit (microns)||Choice of 5,10,25,50,100,200w X1000h, or none||Preconfigured – 25µ slit|
|Input Fiber Connector||SMA 905||SMA 905||SMA 905|
|Focal Length||42 mm (input), 68 mm (output)|
|Integration Time||3 µsec – 65 sec||10 µsec – 65 sec||3ms to 65 sec|
|Effective Range||Depends on slit and grating, ranges 200 -1000nm||350 – 1000 nm|
|Optical Resolution (FWHM)||Ranges from 0.3-10.0 nm, depending upon grating||~2 nm|
|Signal to Noise||250 to 1||300 to 1||250 to 1|
|Spectrometer Price||No longer available||$2,199||$999|
|Operating System||N/A||Windows 98/Me/2000/Xp, Mac OSX & Linux w/USB port|
|* Use of serial port requires 5v power supply at additional cost|
As mentioned, these spectrometers utilize diffraction gratings to split incoming light into its spectral components. This diffracted light falls upon the CCD array and specialized software analyzes and reports spectral characteristics. No single diffraction grating is efficient for broadband analyses (200nm -1100 nm, except in the case of the HC-1 grating available in the CG ‘composite grating’ series spectrometers, starting at $3,999), so Ocean Optics offers a choice of 14 gratings for the USB4000 (the inexpensive USB650 is preconfigured for analyzing wavelengths between 350 and 850 nm). Analyses of UV (down to ~220nm) and visible radiation will require a specific grating, while work with visible wavelengths and near-infrared will call for a different grating. At a minimum, one should decide what part of the spectrum is of interest for analysis. Even with the proper grating, the reported spectral quality is not 100% correct. A particular grating is efficient over a given spectral range (Ocean Optics uses 30% efficiency as the cutoff). This means these spectrometers generally under-report some wavelengths. The #3 diffraction grating is probably the best option for most. It is most efficient at 500nm and under-reports other wavelengths, particularly blue and red wavelengths. There are various remedies, but perhaps the best is free spectral correction software from Dr. Charles Mazel at www.nightsea.com. Charlie hopes to have this posted on his website ‘soon’ and will include it along with his new products for viewing fluorescence in aquaria. Ideally, one would use a reflectance standard along with the software to correct the spectrometer’s measurements, but it is not absolutely necessary. The data is easily exported to a spreadsheet program for further analyses (though this will require some time and effort invested in writing a few lines of code).
Another important consideration is of expected light intensity. Six variously sized apertures (called ‘slits’) are available for the USB4000 (5, 10, 25, 50, 100, 200 µ and ‘none’- the fiber optic size acts as the light regulating mechanism). Low light applications (such as fluorescence measurements) require larger slits (say, 200µ), while higher light intensity requires a smaller slit. The Applications Scientist will walk you through these choices. The question of suitability of a particular setup over a broad range of light intensities is obvious – can a spectrometer designed for low-light work in measuring spectra of ‘bright’ metal halide lamps? The short answer is ‘yes’ – but discuss this with the Ocean Optics Applications Scientist (a few ‘tricks of the trade’ are listed in the Helpful Hints section, below). Optical Resolution is a function of slit width and holographic grating. An Optical Resolution of just a fraction of a nanometer is possible. However, the Optical Resolution will be generally between 2 and 10nm.
These specs are compatible with USB 1.1 and USB 2.0; Use of a serial port is possible with the available 5v power supply.
Particular applications may require an upgrade to your spectrometer which is usually at additional charge. For instance, work with shortwave ultraviolet radiation will require special UV-transmitting optics (optional UV2 quartz window for work with 200 – 340nm) and fibers resistant to UV damage (‘solarization’). For low-light work (such as fluorescence) consider the L2 lens upgrade.
It is not absolutely necessary to use fiber optic patch cords if the goal is simply to measure lamp spectra – one can merely remove the protective cover from the input aperture and point the spectrometer at the light source. However, fiber optic cords do offer advantages in that they can be tightly attached to the aperture (via SMA 905 terminated fiber) thus protecting the internal works of the spectrometer (the thought of a drop of saltwater or debris entering the spec housing is frightening). The fiber optic cords also offer other advantages. While it is apparent that cords are a must for connecting optional accessories, it should be noted that the size (diameter) of the cord is also a critical consideration. Very simply, the larger the diameter of the fiber optic cord, the more light transmitted. This is useful to know if high light intensity ‘saturates’ the CCD array and causes the reported intensity to be above the maximum allowed – Use of a smaller diameter cord could attenuate (‘weaken’) the signal, thus allowing measurement. Fibers are available in the following diameters (in µ): 8 (VIS/NIR only, range of 450 – 1000nm), 50, 100, 200, 300, (for use with UV <250nm) 400, 600 and 1000.
Fibers are usually 2m in length, and custom lengths are available. Fibers terminated with SMA 905 connectors can be spliced with bushings (available from OO).
Software and Applications
SpectraSuite: Ocean Optics has upgraded its older OOIBase32 software to the new SpectraSuite software. SpectraSuite is Java-based software that operates with Windows 98/Me/2000/XP, Mac OSX and Linux w/USB port. Light measurements are usually reported as ‘counts’ (a generic term) but it is possible to measure absolute irradiance. See comments on OOIIrrad software below. Graphical data can be copied, and numerical data exported to spreadsheets such as Excel. Integration time is programmable, as are functions such as averaging and boxcar smoothing. Two spectral charts can be open at one time thus allowing simultaneous use of two spectrometers (especially useful when observed fluorescence excitation and emission wavelengths). Languages include English, German, Japanese, Russian, Chinese, French and Spanish. Lists for $199.
OOISensors: Software specifically for use with fluorescent pH and dissolved oxygen probes. Available for $199.
OOIIrrad:Software for measure of relative and absolute irradiance. OOIIrrad also requires the purchase of a LS-1-Cal Calibrated NIST-traceable Light Source for an additional $749. OOIIrrad software sells for $299.
OOIIrrad-C: In addition to the functions of OOIIrad software (above), this package also analyzes reflective and emissive color. $399.
Note- Ocean Optics is changing their software to for compatibility with various operating systems (Windows, Mac, and Linux). The prices listed are current as of early May 2006.
Ocean Optics offers a wide range of assorted accessories for assorted applications.
Specialized sensors utilizing fluorescence are available for monitoring of pH (TP300 pH probe, available for $750) and dissolved oxygen (‘FOXY’ probes, starting at $499). OOISensor software is also required (see above). These probes are of modest diameters and can test pH or dissolved oxygen levels with the interstitial spaces of ‘bushy’ corals. We’ll likely see important information generated with these sensors in print soon. (Jake Adams experimented with corals and an oxygen sensor and made a very interesting presentation at the 2006 IMAC.)
A variable neutral density filter is available for light attenuation (but see ‘Helpful Hints’ below). The neutral density filter is $499. If one is needed, consider purchasing the UV or Vis Fiber Optic kits. They are $999 each (specify ‘ultraviolet’ or ‘visible’) and include one variable neutral density filter, four 74-series optical collimating lenses, 4 fiber optic patch cords of various diameters, CC-3 cosine corrector, bulkhead and splicer fittings, a SEM wrench and some odds and ends. If purchased individually, the assortment of items in the Fiber Optic Kits is ~ $1,500.
Reflectance standards (diffuse and specular) are available and should be used for reflectance determinations.
Various excitation and calibration light sources are available, including deuterium, tungsten-halogen and light-emitting diodes (LEDs) of various ‘colors.’
Scratch and Dent Surplus Equipment
Ocean Optics offers trade show items for sale at bargain prices (I got a ‘like new’ Spectralon 99% diffuse reflection reference for $199 – it normally sells for $329). Check http://www.oceanoptics.com/PriceList/Scratch&Dent_FiberPriceList.pdf for available inventory.
Ocean Optics suggests using this formula for determining approximate optical resolution:
Optical Resolution = Dispersion X pixels in slit or fiber, where:
Dispersion = Spectral Range of the Grating/Number of Pixels in CCD array.
Resolution – Use Table 2. (Results are in pixels, typical value of slit width or fiber diameter).
|Slit Width / Fiber Diameter (µ)||Pixels|
If spectral analysis of lamp(s) is your only concern, then perhaps observations of percent spectral change would be of most use. Spreadsheets, such as Excel, allow for relatively quick and easy manipulation of exported data.
One of the most common challenges facing users of these spectrometers is one of excess light. Attenuation is possible by obvious means (such as moving the spectrometer or the fiber optic cord away from the light source). Reducing integration time is also useful as is use of a smaller fiber optic cord. Fiberglass window screen material, acting as a neutral density filter can also be replaced over the aperture or fiber cord terminal. Light intensity can also be reduced by applying a layer (or two) of transparent tape over the end of the patch cord or over the light entrance aperture.
Attach the spectrometer to a small piece of plywood with nylon cable ties. This will protect it from those inevitable bumps (See Figure 1).
I learned more than I had ever known about lamp spectra (especially in the UV range) in a few minutes’ use of my first OO spectrometer. Their relatively low cost instruments have made possible many experiments concerning lighting and its effects on zooxanthellae photosynthesis. These spectrometers’ small size, light weight and USB power have made field research easy. The Ocean Optics equipment is relatively inexpensive, but by no means cheap. Workmanship and overall quality of the hardware is first class.
There are a few areas in which products could be improved. The new software, SpectraSuite, is not as user friendly as the older OOI32Base software. The ‘Help’ section is on a CD (which is of course the rule today). Perhaps I’m just old-fashioned, but I much prefer a printed User’s Manual straight out of the box as opposed to printing a hardcopy myself.
Fully read and understand the directions for connecting the spectrometer to your computer. Resist the urge to plug the USB and spectrometer in! Read the directions! Plan to devote an hour to installing the software and configuring the spec.
Still, the equipment is not for everyone. Personally, I would recommend the purchase of a quantum or PAR meter before purchasing a spectrometer – a PAR meter will be of much more use. However, if one has a PAR meter, then spectral analyses are the next frontier. Hobbyists should honestly evaluate the return on investment as well as the amount of time one has to dedicate to instrument use and experiment set up. Sanjay Joshi did (and is doing) a wonderful service to the hobby by providing spectral data for hundreds of lamps (see archived articles in Advanced Aquarist, or visit Sanjay’s website: http://www.reeflightinginfo.arvixe.com).
On the other hand, Ocean Optics has earned the reputation of quality equipment at reasonable prices and any really serious hobbyist or small budget-minded laboratory would benefit from the spectrometer’s flexibilities. There are many unanswered questions about artificial light quality and its effects on zooxanthellae photosynthesis and coral host pigmentation. Especially with the introduction of the ‘Red Tide’ USB650 spectrometer and software package costing a total of only $1,200, Ocean Optics equipment will be an important key in our quest of unlocking mysteries and unknowns the hobby still faces.
See http://www.oceanoptics.com for an online catalog, contact information and more technical data.
I’m in the process of writing an article or two around the practical applications using Ocean Optics equipment, how to interpret the results and what they tell us.