Measuring > 8A Samples

 

Measuring > 8A using the PerkinElmer Lambda 1050 UV/Vis/NIR Spectrometer

Introduction 

The PerkinElmer Lambda 1050 is a state of the art research UV/Vis/NIR spectrometer utilizing 3D (three detector) technology. Because the Lambda 1050 incorporates holographic gratings in a true Littrow double monochromator design, the stray radiation level is specified at < 0.00007 %T in the UV/Vis range. Because the amount of stray radiation limits the dynamic range of a spectrometer, the ultra-low stray light level of the Lambda 1050 allows the instrument dynamic range to be specified to 8A. High absorbance measurements to 8A require reference beam attenuation, a process where neutral density screens are inserted in the reference beam path to help balance the sample and reference beam energies when high absorbance samples are being measured. Reference beam attenuation will improve the signal-to-noise levels when high absorbance samples are measured. The capability to measure to 8A is incorporated in the Lambda 1050 by the inclusion of automated sample/reference beam attenuator screens. The neutral density screens included allow attenuation levels of 1%T (2A) and 0.1 %T (3A) to be programmed in the method. When required, the screens will be automatically inserted in the sample or reference beam paths, or both. The 1%T attenuator works well with sample up to 5-6A, and the 0.1%T attenuator works well when sample absorbance reaches 6-8A range.  Though the Lambda 1050 is specified to 8A, valid data can be acquired in the 8-10A range as long as certain procedures are followed. The instrument’s parameters need to be optimized for wide bandpasses, slow scan speeds (high integration times), enable dark current (0%T) correction, and the use of supplemental reference beam attenuation. Discussed in this application note are the proper procedures for acquiring absorbance data >8A. A bandpass filter with out of band blocking specified at grater that 8A will be used as an example.  A PerkinElmer Lambda 1050 (S/N 1050L1109146) was used for measurements.

Experimental
MicroCoatings™ 337 nm bandpass filter was used an as example. This filter is pictured in Figure 1.
This filter is specified with an out of band blocking greater than 8A. The filter is a sandwich type, with one side mirrored, the other side a glass surface. For measurements, the glass side surface was inserted to face the beam. A standard solid sample holder (PE # B0080822) was used to hold the filter properly in the beam.

Figure 1. MicroCoatings 337 nm Bandpass Filter

A reference beam attenuator kit (PE Part Number L1160560), was used to provide supplemental reference beam attenuation (Figure 2). This kit consists of 5 circular magnetic ringed screens having different attenuation levels (32, 14, 6, 4, and 1%T) which can be adhered to the reference beam magnetic window. The attenuators in this kit provide a uniformly flat response as a function of wavelength. If needed, multiple reference beam attenuators can be stacked to produce a specific level of attenuation in the reference beam.

   Figure 2. Supplemental Reference Attenuation Screen Kit (PE # L1160560)

To construct a method to properly scan above 8A, the use of wide slits (bandpasses), slow scan speeds-high integration times, enabling dark current correction (0%T correction), and use of reference beam attenuation are all required. Because the Lambda 1050 incorporates motorized screen attenuators in sample and reference beams, a unique software feature can be used where the attenuation level is selected on the reference side, and the sample side attenuator set to “Automatic”. When the method is executed, this enables a three step measurement of the internal attenuators 1) Sample side, 2) Reference side, and 3) Attenuators in both sample and reference. This type of correction solves one of the problems of only using reference beam attenuation alone. By just adding screens to the reference beam path, the signal-to-noise at lower absorbance levels will be significantly degraded to obtain the benefit of better S/N at higher absorbance. By correcting with attenuators in both sample and reference, and then correcting for the contributions of the attenuators, the signal-to-noise will be enhanced not only at high absorbance but at low absorbance as well.

Figure 3. Scan parameter selections influencing high absorbance scanning include data interval (1), scan speed (2), response time (3), slit (4), and attenuator settings (5),

Under the Corrections menu in UVWinlab V6, is a check box that allows the 0%T dark current correction to be done. Correction at 0%T allows better accuracy when the sample absorbance exceeds 8A. In this example the MicroCoatings filter was scanned from 750 to 350 nm, using a 5 nm data interval, a 5 second response time, a 5 nm slit, and a reference attenuation setting of 0.1% and the front attenuation setting of Automatic. With the goal of verifying out of band blocking greater that 8A, it is shown in Figure 4 that that was accomplished without the use of supplemental reference beam attenuation. Because no part of the scan in the 670 to 400 nm range dipped below 8A, the conclusion that this filter meets is stated requirements can be proven.

Figure 4. Scan of MicroCoatings 337 nm bandpass filter for verification of out of band blocking performance. The filter does exceed 8A in the range of 670 to 400 nm.

To help reduce the noise above 8A, additional reference beam attenuation was applied by adding the 1%T screen from the attenuator screen kit, affixing this to the reference beam magnetic window. Additional reference attenuation often will reduce the noise seen above 8A. The spectrum shown in Figure 5 is the same MicroCoatings filter scan with an additional 1%T screen added to the reference beam. Note that the spectra of the screens from the attenuator kit were scanned earlier to allow the absorbance from the selected screen to be easily added to the sample spectrum (reference attenuation will reduce the absorbance by the amount of the screen – to derive the actual absorbance the screen absorbance is added to the sample absorbance spectrum).

Figure 5. The same filter shown in Figure 4 is rescanned with an additional 1.8A (1%T) attenuator screen inserted in the reference beam. The additional reference attenuation has the benefit of improving the signal to noise at high absorbance. In this example, not only can it be shown that the filter exceeds the stated performance of >8A, but it actually exceeds 9A!

 

Figure 6. The mean absorbance of this filter calculated from 660 to 400 nm, was determined to be 9.4613 A.

 

Conclusion

The Lambda 1050 is a state-of-the-art double monochromator research UV/Vis/NIR spectrometer with ultra-low stray radiation levels In this system the use of holographic gratings used in a double monochromator Littrow design allows the dynamic range to be specified to 8A. In instances where the absorbance levels exceed 8A, valid data can still be obtained when the instrument parameters are properly configured.  In the examples presented here, verification of > 8A out of band blocking can be achieved using the standard automated attenuators. To achieve better signal-to-noise when the sample absorbance start to approach 10A, additional attenuators can be used in the reference beam path, allowing sample absorbance measurements to exceed 9A
.

Calculating % Transmission from Energy Spectra

 

Calculating Transmission Spectra from the E1 and E2 Energy Spectra

Using a PerkinElmer Lambda spectrometer, it is possible to calculate the transmission spectra (uncorrected) from the E1 (sample) and E2 (reference) energy spectra. The procedure to do this is described herein. A PerkinElmer Lambda 1050 running UVWinlab V6 is used for this procedure example.

The first step in the procedure requires the user to determine the correct PMT energy setting required to operate properly in the E1 and E2 modes.
After power up, and loading of UVWinlab, from UVWinlab Explorer, click on the Instrument menu, and then click on the Lambda instrument icon and then click on the Manual Control icon.
Using the Manual Control instrument settings enter a wavelength of 520 nm, a 2 nm slit, and then set the ordinate mode to E1 – then click on the Apply button. The maximum energy in the UV/Vis range is typically around 520 nm. The wavelength of maximum UV energy is typically around 250 nm, but this will be less than the signal at 520 nm.
Observe the E1 signal level on the live display. If the energy is between 95 and 100, the detector will be saturated. The energy needs to be dropped. The energy level is controlled by the PMT Gain setting. The default is 30. Enter a value of 15 to start and then click on Apply. The value should drop somewhere in the 10 to 30 range. Note that the bandpass selected will have an effect on the displayed E1 signal level, as well as the type of detector module being used. A PMT gain of 15 can be set for the standard detector module, and a slit of 2 nm.
After the PMT gain is determined, close the Manual Control mode, and click on a Scan method. Set an ordinate mode to E1, and the PMT Gain determined to be correct (i.e., 15). In the example shown here, a scan range from 700 to 400 nm was set, E1, and a PMT Gain of 15.



Click on the Sample Info menu in the method, and enter three sample names, called… E1_Raw E2_Raw E1_Sample
Click on Start to collect E1_Raw. When prompted for the next sample, click on Cancel, return to the Data Collection screen, and enter E2 for the ordinate mode. Click on Start to collect E2_Raw. An example of the overlaid E1 and E2 spectra is shown below. It is important not to change any other settings between the E1 and E2 scans.

The ratio of (E1/E2)*100 is the uncorrected transmission baseline. This spectrum can be calculated in UVWinlab V6 by clicking on the Processing section, and then adding an equation. For Equation 1, click to set the equation (E1_Raw/E2_Raw)*100.

Spectra directly calculated with equations will be placed into the Results section, under Custom. For each equation the spectra will be sequentially named Equation1, Equation2, etc. The calculated uncorrected transmission spectrum can be view there. An example is below.

To scan a sample in the energy mode, insert the sample into the sample side cell holder. Set the ordinate mode to E1. Leave all other Data Collection settings intact. Click on Start. The example below is a holmium oxide filter scanned in the E1 mode overlaid with the E1_Raw curve.

To calculate the E1 sample curve, in Processing, and another equation (Equation 2) to subtract E1_Sample from E1_Raw. See example. Again, the calculated spectrum will be placed in to Custom tab under Results, called Equation2.Sample. The curve calculated Equation 2 is shown below.

To calculate the transmission spectrum, the spectrum from Equation 2 needs to be ratioed to E2_Raw. In processing, add another equation (Equation 3) and set the final calculation as… [1-[Equation2/E2_Raw]]*100

This will calculate an uncorrected transmission spectrum and place the result into the Custom tab. Shown below is the E1 and E2 calculated transmission of holmium oxide, overlaid with the corrected spectra of holmium oxide scanned in the normal %T mode.

Creating %RC Correction Files in UVWinlab

Introduction

Reflectance measurements can either be relative or absolute. Relative simply means that the data collected from the spectrometer is relative to the standard that was used to background the accessory. Absolute reflectance means that the acquired data is independent of the standard or optical components being used to background the accessory to set a 100% R baseline. Absolute reflectance can be obtained using the proper absolute reflectance accessory (i.e., such as the Universal Reflectance Accessory) or by mathematically correcting the relative data to absolute using a known, calibrated standard.

An important consideration is that relative data typically cannot be compared across spectrometers because different standards are used in different laboratories. Even the “same” type of standard (i.e., a front surface aluminum mirror) can vary significantly from manufacturer, and typically will change response with age and use. Relative spectral data collected using a mirror reference will be different than spectral data acquired using a BK-7 reference, or a Spectralon white plate reference Absolute reflectance data by comparison is comparable across spectrometers and laboratories.

This technical note will describe how to create reference correction files that can be used in UVWinlab V6. This will allow relative reflectance data to be converted to absolute reflectance data. Note that this procedure requires a calibrated standard of some sort, i.e., a NIST traceable mirror or a Labsphere calibrated Spectralon standard are common. The more accurate the standard is specified the more accurate the conversion to absolute reflectance data. This procedure is valid for integrating sphere accessories, and relative specular reflectance accessories.

 

Creating %RC Correction Files

A calibrated standard is required. Typically, this will be a NIST traceable mirror, or a calibrated Spectralon white plate from Labsphere. The %RC option in UVWinlab V6 is available under the Corrections menu, when the ordinate mode of the method is set to %R.

When %RC is selected as the Correction Type, options will be shown for Light Spectral Reference, and Dark Spectral Reference. The Light Spectral Reference is the calibration data file that was supplied with the standard. This is a required entry. The Dark Spectral Reference is optional, and usually applies to integrating sphere measurements. An entry here depends if the measurements are being performed near 0%R, such as with AR coatings. It is recommended to use a Dark Spectral Reference for measurements routinely below 1% R. A standard is not required for the Dark Spectral Reference.

 

Creating the Light Spectral Reference File

1. Examine the calibration sheet supplied with the standard. If not electronic, you will need to get this data into Excel. Examine the data interval - if the data interval is not uniform, you will need to interpolate this data in Excel to a uniform data interval, typically every 5 nm or 10 nm.

2. The calibration data needs to be descending from high to low wavelength. If not, reverse the order of the data in Excel. The data can entered in Excel as R or %R.

3. The next step is to scan a “dummy” file in %R on the Lambda XXX from the starting wavelength of the standard (commonly 2500 nm) to the ending wavelength (commonly 250 nm), at the data interval for the calibration data (typically 5 or 10 nm). Background in %R using the 100% and 0% baseline corrections (under the Corrections menu, make sure the 0%T option is checked).

Note - The quality of the data is not important, as it will be overwritten.

If using a sphere accessory, acquire scan with the standard white Spectralon plates in place on the sample and reference reflectance ports. If this is a relative specular reflectance accessory, acquire the scan with the supplied first surface mirror.

4. When completed, right-click on the filename and select “Save as ASC”, and save to a location on your PC.

5. Next browse to this file and open this ASCII spectral file in Excel, by right-clicking on the file and selecting “Open with…” and select Excel.

6. You will see about 80 lines of header info, and beneath that, starting with #DATA line the two column data, as wavelength and ordinate. Important entries to note are 1. Ordinate mode 2. Data interval and 3. Number of data points in the scan… The number of points must match between the calibration data and the dummy file. This is calculated by… ((starting wavelength – ending wavelength/data interval)+1).

7. Open the calibration data file in a separate copy of Excel, and copy the two columns calibration data (starting at the first wavelength) by selecting and then Copy.

8. Paste this data into the “Dummy” file Excel spreadsheet over the top of the prior data. Note if R data is being pasted, change the line 77 label to R from %R.

9. Save the modified Excel spreadsheet as text (click on the file type selector - important – don’t save an XLS).

10. Using Windows Explorer, browse to the saved file and make sure the extension is .ASC. If the extension is .txt, highlight and change to .ASC.

11. Copy this file to C:\Program Files\PerkinElmer\UVWinLab\V6.0\Data\Corrections Data.

12. The file is now ready to be used in the Corrections menu for Light Spectral Reference. You will need to browse to your file. Important – when browsing to your created reference file, you should see the spectrum graph be displayed for this file. If it does, it is a valid correction file. If an error is given, it is not a valid correction file, and likely a mistake was made in its creation. In this case, re-do the procedure to regenerate the file.

 

Creating the Dark Spectral Reference File

If using relative specular reflectance accessories a Dark Spectral Reference file will not be needed, as the specular accessory without a mirror in place will read 0%R.The Dark Reference File is typically only recommended with integrating spheres, and especially if the measurements are in the low %R range. For integrating spheres, the Dark Spectral Reference option corrects for the small offset from true 0% R due to the light traveling the length of the sphere (the detectors will see a small amount of air scattered light). To generate a Dark Spectral Reference file for the 150 mm integrating sphere accessory, perform the following steps…

1. Using the same range and data interval of the Light Spectral Reference “dummy” scan, background in %R using the 100% and 0% baseline corrections using the standard white plates.

2. When the background is completed, remove the Spectralon white plate from the sample reflectance port and replace the cover. The cover will act as a light trap.

3. Collect a scan. The observed ordinate readings will typically be low, somewhere in the 0.1 to 0.5% range. High ordinate readings (i.e., 5%) may indicate that the light beam is not freely passing through the sphere, and may be clipping on the edge of the port. Re-check beam alignment using the Align button on the top of the toolbar. If necessary adjust the sphere optics top center the sample beam on the %T port and the %R port so not clipping can be observed.

4. When completed, right-click on the filename and select “Save as ASC”, and save to C:\Program Files\PerkinElmer\UVWinLab\V6.0\Data\Corrections Data. The file is now ready to be used in the Corrections menu for Dark Spectral Reference.

 

Using %RC

After the reference files have been created, set the ordinate mode to %R in the method, and in the Corrections menu, select a Correction type of %RC. Assign you newly created Light Spectral Reference file, and optionally the Dark reference file.

The standard for the Light Spectral Reference needs to be used for the background correction (autozero). If using a relative specular reflectance accessory, position the certified mirror on the accessory to obtain a baseline. If using the 150 mm integrating sphere, place the NIST traceable mirror or calibrated Spectralon plate on the sample reflectance port and acquire the background.

To measure a sample, replace the standard with your sample. Data that is collected from the spectrometer is corrected point-by-point in real time. Note that it is normal not to observe 100% R readings after a baseline has been acquired, because the correction data is being applied to the live display for the standard. Important - If the ordinate data is 100 times higher than it should be, then the line 77 label (R or %R) needs to be edited to match the data format in the Light Spectral Reference file. Note that some earlier versions of UVWinlab stored an “nm” as the ordinate label when files were saved as ASC. If an “nm” label is observed change to %R. This has been corrected in V6.

 

Summary

With properly prepared %RC Reference files, and the appropriate standards, absolute reflectance data can be acquired from the PerkinElmer UV/Vis and UV/Vis/NIR spectrometers.

Visible-NIR Detector/Grating Change Steps

Steps at the detector change point in a UV/Vis/NIR instrument equipped with a specular reflectance accessory are due to four primary causes:
    (1) Lack of total depolarization of the native light beam in the instrument.
    (2) Change of either the size or shape of the beam image on the detector from the background correction to the sample.
    (3) Difference in the noise profile between the UV/Vis and NIR regions.
    (4) Lack of proper optical alignment in the beam path of the instrument.

Only two of the items above (1 and 4) are relevant to a serviceman performing an instillation or alignment of a reflectance accessory.
Item 2 is usually caused by the sample and the step can be more or less severe depending on the individual sample's optical characteristics.
Item 3 is controlled by the energy related parameters set in the method.
Items 2 and 3 are very important because they can be the source of detector steps in instruments where the reflectance accessory and instrument are properly aligned.

Steps at the detector/grating change due to noise are fairly easy to identify. These types of steps may be problematic for the customer but they do not indicate an optical alignment problem that needs to be fixed. This type of step can be minimized by adjusting the energy related parameters of the method (such as slit size, common beam mask size, and integration time. A noise step can be easily identified by performing multiple scans on the same sample under identical instrument parameters. A noise step will not be reproducible. It's direction and magnitude will be random from measurement to measurement. Here are some examples.
Figure 1 below shows three separate measurement runs on a NIST mirror at 8 degrees with a Universal Reflectance Accessory (URA). The appearance of a step at the detector/grating change point is due to the elevated noise of the NIR region in relation to the less noisy UV/Vis region. Note the random nature of both the size and direction of the step due to the random nature of the noise. The high %R of the mirror keeps the step to a minimal level.

Figure 1

Figure 2 shows two separate measurement runs of the blackened quartz window from a microcell. It was measured on a URA at 8 degrees. The %R of this type of sample is an order of magnitude less then the NIST mirror and is an excellent example of a low reflectance sample. Because of the lowered reflectance of this sample, the noise in the NIR region is larger and as a result the step appears more pronounced. But there is no doubt that this step is due to noise and not alignment.

Figure 2

Now let's take a look at another type of sample. Figure 3 is a spectrum of a low reflectance optical coating measured at 8 degrees on a URA. On this scale everything looks good, but what happens when we expand the scale?

Figure 3

This is the same low reflectance coating as above, but with the scale increased. Now it becomes apparent that we have a step at the detector/grating change that is not from noise. The spectrum in Figure 4 has a clear off-set for the %R value between the NIR and UV/Visible. To try and judge the size of the step by an empirical visual means will not be satisfactory. Visual inspection is too dependent on arbitrary scaling to be useful. Let's try another way...

Figure 4

In Figure 5 we have displayed the spectrum full-scale. There is no question at this scale that we have a step that is greater then the noise envelope on either side of the step.
To size the step we obtain the ordinate value on the high wavelength side of the step and the corresponding ordinate value for the low wavelength side of the step. Subtraction of the lower value from the higher yields the delta value of the step. This delta value is meaningless in itself and must be compared to the ordinate value (%R) to be meaningful. If we divide the delta value by the ordinate value at the step wavelength and multiply by 100, we obtain a relative percentage of the step to the ordinate signal.
For the spectrum below the step is about 1.6% of the ordinate value.

Figure 5

In Figure 6 is shown a spectrum of a NIST mirror measured on a URA. The URA is out of alignment and as a result has a step at the detector/grating change. The same procedure was used to obtain the relative step size. The value calculated for the step was about 0.2%R. Note here how looks can be deceiving. Although the relative step magnitudes are about the same for the spectrum below and the one above, the step in the spectrum here appears smaller than the step from the spectrum above. This is due to scaling differences. Note also that the relative here is 0.2% which is much lower than the 1.6% from the spectrum above; thus, the step below is small when compared to the level of reflectance signal.

Figure 6

Expressing the step as a percentage of the total reflectance signal is an excellent way to compare the relative magnitude of the detector change step anomalies.

Stray Light: The Max Absorbance Limit

One of the most frequently asked questions about UV/Vis/NIR dispersive spectrophotometers is: “How high an absorbance value (or how small a %T value) can I accurately measure on my instrument?”

In the ‘good old days’, circa 1970s, there was a general ‘rule of thumb’ that stated, “all instrumental absorbance measurements should only be between 0 and 1 units”. The driving force behind this ‘rule’ was the fact that instruments of that era had ‘needle meters’ or strip chart recorders as output devices. Since absorbance is a logarithmic scale, the readable number of significant digits was best between 0 and 1, but declined rapidly for higher orders of magnitude. With the advent of digital displays one can read 3.1026 abs just as accurately as 0.1026 abs. The upper absorbance limit or instruments today is related to an instrumental specification called ‘stray light’. Today the stray light specification is the primary delineator of instrumental performance and cost.

Definition - Stray light is any light outside the spectral region isolated by the monochromator that reaches the detector. It is mainly produced by scatter from the optics and walls of the monochromator and is present in varying amounts in all spectrophotometers. Stray light is a constant for any given instrument design and is usually expressed in the specification values for that model as a %T value.

Example: Stray Light = 0.00007 %T for the UV/Vis region of the Lambda 950.

Stray light most frequently leads to deviations from the Beer-Lambert law and subsequent inaccuracy in photometric values. Double grating systems have lower stray light than a single grating systems, but are as one would expect more expensive.

The graph below is for a high performance instrument having very low stray light. The stray light level of an instrument fixes the maximum "actual" absorbance (blue line) measurable for the instrument (here 6A), Stray light also defines the maximum linear range (here 0A to about 5A). Remember: 0.0001 %T = 6 A

The % error (difference) between the absorbance with zero stray effect and the absorbance including stray light gives a sensitive measure on the non-liniarity of measurements with a given instrument. This % error can be used as a tool to help define the linear range of an instrument. Usually 5% error is the cut-off.

  Conclusion: Knowing the stray light of a spectrophotometer enables one to calculate its maximum linear absorbance range and maximum upper absorbance limit. For more info go to http://www.perkinelmer.com

Step-Scan Technology of the Lambda Spectrometers

The new PerkinElmer Lambda 650/850/950/1050 UV/Vis and UV/Vis/NIR spectrometers incorporate the latest technological advancement in scanning, termed “Step-Scan Integration”. This technology is enabled by a unique CSSC (Chopper Segment Signal Correction) high-speed chopper design having 4 segments. The ultimate goal of advancing scan technology is the elimination of photometric errors. When scanning a monochromator in a conventional time shared optical system, the wavelength changes during the measurement cycle and can lead to distortions in the spectral data. At each wavelength, a conventional double beam spectrometer records a Sample signal, a Reference signal, and a Dark signal. This is most commonly accomplished by use of a three segment chopper.

The problem with a conventional instrument design, is that the chopper spins continuously, collecting readings from the three chopper segments, while the monochromator is actually moving. The result of this is that the Sample (S), Reference (R) and Dark (D) readings are actually taken at three different wavelengths. This produces a derivative error as scan speed is increased, causing shifts in absorption peaks towards the blue, and a suppression of the true absorption. This effect is commonly known as “tracking error”. Below is a holmium oxide doped glass sample scanned with a conventional wavelength drive spectrophotometer illustrating the “tracking error” effect, where peaks positions will shift to the blue with increasing scan speed, and peak values decrease.

 

Step-Scan Technology

Step Scan Integration is a digital scan drive system that synchronizes the chopper and grating movement. This yields two benefits not available on conventional drive systems…

1) Elimination of “tracking error”, regardless of scan speed selected.

2) The exact integration time at each digital wavelength step can be controlled by the researcher.

The Lambda 650/850/950/1050 spectrometers use an advanced chopper design, called CSSC which is an ultra-high speed chopper (operating at a 20 millisecond measurement phase) utilizing four segments, a Sample, Reference, and two Dark portions. In the PerkinElmer CSSC chopper design, the chopper spins first to collect the Sample, Reference, and Dark signals, and then the gratings move on the next non-measurement spin of the chopper. This design is a significant improvement over the earlier “Dark Cycle Stepping” technology, in that the readings for all photometric components are recorded while the gratings are truly stationary.

The earlier “Dark Cycle Stepping” design would actually move the grating while integrating the Dark portion of the signal. Unfortunately, this design had the effect of compromising the accuracy at higher absorbance levels (low %T).

The unique PerkinElmer patented CSSC chopper design having two dark chopper segments provides the following additional benefits…

1) Real-time dark current corrections are taken 2X more frequently than a conventional chopper, yielding better accuracy for optically dense samples, with less signal fluctuation.

2) The ultra high-speed chopper allows the instrument to be operated with the cover open. Most conventional systems (slower chopper, less frequent dark current correction) utilize a cut-off switch on the sample compartment door to prevent detector damage. The Lambda 650/850/950 spectrometers are immune to damage from room light.

3) Digitally synchronizing the scan drive to the chopper allows the Sample/Dark and the Reference/Dark signals to be collected while the monochromators are stationary, completely eliminating tracking error.

Shown below are an overlaid holmium oxide scans acquired with a Lambda 950 with slow to fast scan speeds. The peak positions and absorbance values remain constant when moving from slow to high scan speeds. Digital Step Scan Technology eliminates “tracking error”.