URA Flexibility: PMT Window Coating

 

Just Another Big, Bulky Sample

Sample – The coated window material of an intact photo-multiplier tube, viewed end-on
There are many “real world” coatings that do not reside on nice flat one inch sized substrates. Many modern quality control specular reflectance samples are the finished manufactured part.
The picture at left shows such a sample.
The placement of this type of large sample on the Universal Reflectance Accessory is shown in the bottom left picture. Since the sampled area is flat and covers the entire beam entry aperture, there is no need for additional light cover. There spectra of the window coating are shown at bottom right. The PMT contents behind the window are far enough removed from the window (1 cm) so as not to interfere.

Sample Preparation
1) Sample is placed on the URA stage with the coated side contacting the opening in the sample stage.
2) The internal Common Beam Depolarizer was used to prevent polarization artifacts at incidence angles higher than 20 degrees.
Placement of the sample on the URA accessory is seen below.

Sample on URA

The spectral results of  an angular study of the reflectance of the PMT widow coating is seen below.

URA Specular Reflectance Results

Conclusion: The design of the URA , with it’s gravity held sample placement and lack of either width or height obstructions, makes it the reflectance accessory of choice for large, bulky, irregular shaped samples. For more info go to http://www.perkinelmer.com

Specular Reflectance Measurements with Spheres

 

Introduction

One of the most common measurements made by the solar energy industry today is quantification of a material’s surface reflectance. These materials are as diverse as metal coatings, semiconductor coatings, anti-reflective coatings on window material, as well as the window material itself. These measurements are most commonly made between 300 nm and 1500 nm. This is where the solar cell is responsive to energy from the sun. Reflection comes in two varieties, specular and diffuse.
Specular reflection (part A in Figure 1) is generated by a smooth surface. The light ray’s angle of incidence is equal to the angle of reflection; therefore, specular materials frequently produce images on their surface (mirror). Specular reflectance is measured by a number of different types of accessories (VW, VN, IV, and Universal Reflectance Accessory).


Diffuse reflection (part B in Figure 1) is generated by a rough surface. Here the light ray’s incidence angle gives rise to a multiplicity of reflection angles; therefore, images are not produced. Diffuse reflectance is how people see the world. This is because the vast majority of objects in the world are diffuse reflectors. Diffuse reflection is measured by an integrating sphere, which comes in two sizes, small (60 mm diameter) and large (150 mm diameter).
One of the unique requirements of the solar industry is for specular sample to be measured on an integrating sphere. Why? Because the solar industry needs to measure total reflection (specular + diffuse) even if the sample is predominately a specular reflector. Integrating spheres are the only devices which excel at total reflection measurements.

The 60 mm Integrating Sphere
The 60 mm integrating sphere is a low cost reflectance accessory that measures both “diffuse only” and total reflectance. It consists of a hollow 60 mm sphere of highly reflective Spectralon polymer with two holes for the sample and reference beam to enter and two ports for placement of sample and reference material. Background collection (autozero) is performed by placing two Spectralon plates at the sample and reference ports. Since Spectralon has a diffuse reflectance of 99.0+ %R, the reflectance of spheres can be assumed to be close to absolute %R. Because of the size of the sphere and the lack of “baffling” of the detectors in the sphere from first bounce sample reflectance; this sphere type is subject to several types of spectral artifacts such as incorrect %R values and steps at instrumental filter and detector change points.
In the solar industry however, total reflection measurements must be obtained for highly specular samples. Under these circumstances the 60 mm integrating sphere can provide acceptable spectra for diffuse, specular,  and combination diffuse/specular samples. We will show here that although the 60 mm sphere can be prone to artifacts, with the use of the proper background correction techniques, excellent and accurate specular reflectance data can be obtained on this type of sphere.

Figure 2

What happens when you measure a totally specular sample with little or no diffuse component in a sphere? In order to answer this question we need to understand what happens to the light inside the sphere. Figure 2 depicts how both diffuse and specular light reflect off the sample and into the sphere. The diffusely reflected light will evenly illuminate the entire area inside the sphere through 380 degrees (Figure 2, left). The specular reflected light, however, will strike only an area along the midline of the sphere in the vicinity of the transmittance port (Figure 2, right).

Figure 3A

Photographs of this area, taken via a webcam inserted inside the sphere, are shown in Figures 3 A, B, and C. The photograph in Figure 3A shows the even illumination of the sphere from a 100% diffuse reflectance sample; however, if a specular sample is placed at the sample port a “hot spot” is formed on the wall of the sphere.

Figure 3B

 

Figure 3C

This “hot spot” is photographed in Figures 3B and 3C for a 10% R and a 90% R specular sample respectively. Note that there is little illumination of the sphere interior, only the concentrated “hot spot” shows up. The brightness of the “hot spot” is directly related to the reflectivity of the specular sample. Errors result from the fact that the background correction performed with the diffuse Spectralon plate illuminates the sphere differently (see Figure 3A) than the specular sample “hot spot” illumination (see Figures 3B and 3C). The resulting different sphere interior images that the detector measures between background and sample can cause errors and artifacts.

The Test Samples
Four sample types that span a wide range of specular reflectance intensities were investigated. A piece of polished aluminum represents high reflectance (around 90%R), a wafer of silica has medium reflectance (around 50%R), and a piece of dark glass as well as a clear glass slab with back side protection represent low reflectors (around 4%R). The samples were measured in the total reflectance mode on a 60 mm PMT/InGaAs integrating sphere. A fixed 2 nm slit was used in the UV/Vis range and the servo slit mode at medium gain was used for the NIR region. Data was collected every 1 nm with a data point collection time of 16 milliseconds.

The Problem
Figures 4A and 4B graphically display the problems associated with measuring specular samples on a small integrating sphere. The data are obtained with a autozero correction using the typical Spectralon plate.

Figure 4A

The first problem is that the two highest %R samples, the NIST mirror (green) and the polished aluminum (red), have values over 100% R. This is an obvious physical impossibility and is due to the intense “hot spot” in a small, non-baffled sphere. The second problem is the step at the UV/Vis-NIR detector change for all samples. Note that if we calculated the size of the step as a percentage of %R, the step is the same size (between 5%R to 6%R) for all samples regardless of their %R intensity. Parameter juggling (UV/Vis silt vs. NIR gain) will not decrease or eliminate these steps. They are a physical reality of the specular “hot spot” inside a small sphere.

Figure 4B

When we measure diffuse samples under the same instrument conditions, there are no problems and the data are excellent. As seen in Figure 5, the spectra of the white, gray, and black Spectralon plates lack the artifacts observed with specular samples.

Figure 5

 

 

The Possible Solution
Since we know that the artifacts due to specular samples are generated by the difference in internal sphere illumination between a diffuse autozero target (white Spectralon plate) and the “hot spot” illumination of a specular sample, would it be possible to improve the situation by using a specular autozero target. To investigate this we used a NIST front surface aluminum mirror for the autozero target and then measured the test samples under the same instrument conditions as the data above. The results are displayed in Figure 6. The artifacts that appeared when a white Spectralon plate was used for the autozero target appear to be gone when we use the specular mirror in its place; however, on closer inspection we see that we may have traded on set of artifacts for another. The %R values are all below 100 and the steps at the detector change are gone, but there appears to be a “bump” in the spectra at about 818 nm and are the %R values correct?

Figure 6

How could using a NIST mirror as an autozero target cause artifacts? At this point a little information on how a UV/Vis/NIR instrument autozeros is appropriate. When a UV/Vis/NIR instrument is turned on it is literally “as dumb as a stump”. A procedure must be performed to calibrate 100 % (or 0 absorbance) on the instrument. The autozero (background correction) sets the 100 % level for the instrument and is usually performed with a “blank” or sample that is 100% R. A white Spectralon plate fits this criteria well since Spectralon is over 99% R for its usable ranger of 250 nm to 2500 nm. So when Spectralon is used as an autozero target, 100% R is properly calibrated.
But what happens when an aluminum mirror is used for an autozero target? Figure 7 shows the spectrum for a front surfaced NIST aluminum mirror. As we can see the reflectivity is well below 100% R over the entire spectral range. This means that when this mirror is employed for the autozero procedure, a %R value of less that 100% R be set in the instrument’s calibration file to that value. This means that an error in photometric accuracy (%R) is commensurate with a mirror autozero correction. In addition, the spectral features of the mirror in the background correction will be introduce into spectra measured with that correction. As a result the sharp downward peak at 818 nm (or any other additional spectral features) appear as an artifact in all spectra measured with a mirror correction.
Fortunately there is a easy solution to both of these problems.

Figure 7

 

 

The Final Solution (%RC)
 The solution is a simple mathematical calculation that uses the known values for the autozero target mirror to eliminate both the photometric and wavelength dependent artifacts (see article “Procedure for Creating %RC Correction Files in UVWinlab V6”). Equation 1 below displays the math used for this correction. The R100 spectrum corresponds the the autozero target mirror values, while the R0 spectrum corresponds to the spectrum obtained with a light trap at the sample port or open sample port. The R0 spectrum corrects for the small amount of atmospheric scattered light in the sphere.

Equation 1

When this methodology is employed and the calibrated mirror is measured for the autozero and then run as a sample, you obtain the spectrum seen in Figure 7. Here we do not see the familiar flat 100 %R spectrum obtained before, but rather the actual values for the calibrated (NIST) mirror. With the combination of a mirror used for autozero and the %RC correction math, a spectrum of a specular sample can be obtained that is both photometrically accurate and free of any wavelength dependent artifacts.
In Figures 8, 9, and 10 we see the spectra for the polished aluminum, silica wafer, and back side protected clear glass respectively. The spectra represented in each graph are:
Green Spectrum = white Spectralon plate target for autozero
Blue Spectrum = mirror target for autozero
Red Spectrum = calibrated mirror target and %RC correction used

Figure 8

 

Figure 9

 

Figure 10
 

Polarization in Specular Reflectance Measurements

Radiation emitted by a spectrophotometer’s light source is by nature unpolarized; however, this light will inevitably become partially polarized by a spectrometer’s optical components. The primary causes of this polarization are due to:
(1) The reflection type diffraction grating used in the instrument’s monochromator. This is because of the very narrow ruling distance of the lines that make up the grating, radiation with an electric vector parallel to the ruling is reflected preferentially over the perpendicular electric component.
(2) Each of the spectrometer's mirrors has the potential to contribute additional polarization. The degree to which each mirror polarizes light is a function of the angle of the incident light. For aluminum mirrors at near normal angles (less than 10 degrees from the perpendicular to the mirror surface) the polarization contribution is zero; however, as the angle of incidence increases so does the polarization. Polarization is total at incident angles close to 56 degrees from normal (Brewster’s angle).
(3) Narrow monochromator slits introduce additional polarization. To achieve the minimum spectral bandwidth of 0.05 nm on the Lambda 900 spectrometer, a geometrical bandwidth of 18 micrometers is needed.
(4) The photomultiplier tube detector for the UV-VIS range exhibits slightly different sensitivity for different directions of polarization.
As seen in Figure 1 below, the measure of polarization is generally non-zero for any given spectrometer. Most spectrophotometers have their radiation partly polarized parallel to the slit. This effect is especially strong at the wavelengths for Wood's anomalies, which reside at around 500 nm and 2000 nm for the Lambda 900. This figure clearly shows the characteristic polarization profile for each of the respective gratings used in the UV-Visible and NIR spectral ranges. The instrumental polarization increases as a function of increasing wavelength in a similar fashion for both grating regions; thereby, reinforcing the fact that the diffraction grating is by far the most dominant element contributing to the inherent polarization of the spectrophotometer’s light.

Figure 1

One possible way to control the effects of polarization would be to take into account the degree of native polarization for a given instrument; however, this can never be applied in practice because of the number of dependencies involved. Another possibility would be to use deliberately polarized radiation; however, the most practical procedure is the removal of existing instrumental polarization, i.e., to depolarize the instrument beam. The best choice for this is an optical device, called a depolarizer, which is mounted in the instrument’s light beam and generates pseudo-depolarized radiation by scrambling; however, absolute depolarization is not easy to perform across the entire wavelength range of the instrument.
The depolarizer used in the Lambda 950/1050 is of the Hanle type, which consists of two wedges of differing optical material fastened together. The first wedge is made of double refracting natural quartz material, while the second wedge, manufactured from silica, is used to correct the direction of the beam. In order to achieve maximal depolarization the angle of rotation of the depolarizer must be individually adjusted for the spectrometer in which it is installed. This is accomplished by rotating the depolarizer around the instrument’s beam axis in a trial-and-error fashion until maximal depolarization is obtained.
The spectra in Figure 2 is from a NIST mirror measured at 30 degrees on a URA. The red spectrum was measured with a common beam depolarizer in place. The black spectrum was measured with the depolarizer removed. In the previous figure we saw the dramatic difference in the polarization state of the Lambda 950/1050 at the detector/grating change. This is a result of the fact that the change occurs at the low wavelength end of the NIR grating where the polarization is highly negative and the high wavelength end of the UV-Vis grating where the polarization is highly positive. These opposite polarization states of the two gratings are incorporated as artifacts into the spectrum of a polarizing sample when there is no depolarization of the sample beam.

Figure 2

The next two figures display the results of the effects from lack of proper depolarization as a function of reflection angle on URA measurements.
As can be seen in Figure 3, there are no apparent polarization effects at angles below 15 degrees. The first effects are noted in the spectrum at 20 degrees as a minimal step at the detector/grating change point. The step continues to increase in size with increasing angle at 30 degrees.

Figure 3

As can be seen in Figure 4, the step continues to increase in size as the angle approaches Brewster's Angle (56 degrees) for aluminum where the step would be maximal.

Figure 4

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.

Validating a Universal Reflectance Accessory

When a Universal Reflectance Accessory (URA) is installed it is important that the accessory be aligned properly so that there is not a step due to accessory or instrumental misalignment. It is important that the validation samples selected be free of artifacts and cover the frequently used ordinate ranges of the accessory. The NIST mirror satisfies this criteria for the high %R ranges. The first surface reflection of quartz (protected from the back surface reflection) works well for low reflectance.

A good sample to use for back side protected quartz is a self-attenuating microcuvette. The shiny black surface of the microcuvette is a thin quartz window place over optically black glass. This makes a useable easily available first surface quartz sample.

Below are spectra of the NIST mirror measured on a properly aligned URA over a series of angles that comprise the angular range (8 degrees to 68 degrees) of the accessory. An angular profile such as this will validate the alignment if there is not a step that exceeds 0.2 %R relative to the reflectance intensity.

Below are spectra of a black microcuvette measured on a properly aligned URA over a series of angles that comprise the angular range of the accessory. Note that there is a noise step due to the lower ordinate values but no step due to misalignment. An angular profile such as this will validate the alignment if there is not a step that exceeds 0.2 %R relative to the reflectance intensity.

If a black self-attenuating microcuvette is not available, then a simple 1 cm clear quartz cuvette can be used instead. The ordinate values are not as low as a black cuvette because of the added reflection from the back side of the quartz wall, but it can still give useful results.

Below are spectra of the 1 cm quartz cuvette measured on a properly aligned URA over a series of angles that comprise the angular range of the accessory. Note that there is a noise step due to the lower ordinate values but no step due to misalignment. An angular profile such as this will validate the alignment if there is not a step that exceeds 0.2% R relative to the reflectance intensity.

Haze Measurements

In materials characterization, haze is often as important as color in the quality control of manufactured products. The term haze refers to the visual clarity of a material. The historical evaluation for haze was simply visual, where the operator would hold a piece of the material up to a bright light and check for cloudiness The presence of haze can either be wanted or unwanted. For example, the clear plastic panels which cover the LED’s of an instrument display are intentionally hazy. In other materials, however, the level of haze is tightly controlled, such as, for example, in the polymer sheets which are layered in between automotive glass.
A standard UV/Vis spectrophotometer fitted with an integrating sphere accessory can accurately measure haze. A % haze calculation is quite easy to perform on a Lambda UV/Vis spectrophotometer fitted with one of the Labsphere™ integrating sphere accessories
The procedure is described in the ASTM 1003-92 “Standard Method for Haze and Luminous Transmittance of Transparent Plastics” The procedure involves acquiring four transmission scans of the sample from 780 to 380 nm, each with a different configuration of the integrating sphere, integrating the area under each curve, and plugging the values into the following equation…
Haze = [(T4/T2) - (T3/T1)] x 100%
where T1 through T4 are changing configurations of the sample placement on the sphere.

First measurement: Background Correction T1

 

The Second Measurement T2

    

The Third Measurement T3

    

The Fourth Measurement T4

The figure below displays example spectra for these four haze component measurements made on a clear polymer sheet sample.

The areas for the entire spectral range is calculated and the data is processed in an Excel spreadsheet.

% Haze = [(T4/T2) - (T3/T1)] x 100%
      = [(1940/11781) – (0.194/40000)
      =[(0.1647 – 0.00000485)]
      = 16.47 %

Calculation of % haze is also part of the PerkinElmer Architectural Glass Software package.

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”.

Design Considerations of Integrating Spheres

Integrating sphere reflectance accessories offer a simple method to extend the measurement capabilities of a UV-VIS or UV-VIS-NIR spectrophotometer. By adding a sphere reflectance accessory to a spectrophotometer, one is able to expand from traditional transmittance measurements of liquid samples and non-scattering solids to incorporate both reflectance measurements of opaque solids, powders, or pastes, as well as total transmittance scans of translucent films and scattering creams. Minimal preparation is required for most samples when a reflectance accessory is used, as samples can often be measured in an unprocessed form. Factors such as sphere efficiency, signal throughput, spectrum noise level, measurement accuracy, sphere port fraction, detector baffles, and sample beam size are key components of sphere design that are considered here.

 

 

Integrating Sphere Efficiency and Throughput

Smaller, 60 mm, integrating spheres are efficient collectors of light. The throughput of the system, that is, the amount of incident light which reaches the detector, is governed by the laws of probability. In a 60 mm integrating sphere, a photon will have to take a certain number of bounces before it reaches the detector. In a 150 mm sphere system, however, statistics dictate that a photon will have to take more bounces than were required in the 60 mm integrating sphere to reach the same detector. In the 150 mm sphere, since more “bounces” are required, the photon must undergo many more interactions with the sphere wall and, therefore, is more likely to be absorbed before it actually reaches the detector. Thus, a large 150 mm integrating sphere acts as an attenuator of signal and is inherently less efficient than a 60 mm diameter sphere. The larger the sphere, the greater the attenuation which results. As a rule of thumb, all other factors being equal, the relative attenuation is roughly equal to the square of the ratio between the two sphere diameters. This rule can only be used as a rough guide.

Spectrum Noise Level

A sphere’s efficiency may play an important role in the measurement of highly absorbing samples. When the high absorbance sample being tested limits the amount of light reaching the sphere’s detector, a high throughput, 60 mm sphere will generate better spectra with less noise than a 150 mm sphere mounted to the same spectrophotometer and using the same detector. Thus, spectral results of 60 mm spheres tend to have greater signal-to-noise levels than those of 150 mm spheres. In addition, the 60 mm sphere does not attenuate the instrument’s sample beam as significantly as a 150 mm sphere, thereby, only slightly affecting the overall linearity range of the instrument.

Port Fraction

Customers of spectrophotometer sphere reflectance accessories generally have access only to generic sphere designs which cannot be modified to fit individual needs. In this case, it is important to understand the effects which the sphere’s diameter and port fraction have on the performance of the integrating sphere accessory.

The port fraction is defined as the ratio of the total port area relative to the total internal surface area of the sphere. All beam entrance ports, sample ports, and detector ports which are filled with material of lower reflectance than the Spectralon sphere wall contribute to the calculated port fraction. The port fraction is significantly lower for 150 mm diameter spheres than it is for 60 mm spheres. For example, the port fraction of a representative 150 mm double beam integrating sphere accessory is 2.5 %, while a 60 mm sphere for the same instrument has a port fraction of 11.3%. The design of both accessories includes sample and reference beam transmittance and reflectance ports, as well as PMT and PbS (or InGaAs) detector ports. In order to adhere to many ASTM and CIE methods for measurements using integrating spheres, the port fraction of the sphere must be minimized. For instance, CIE recommends that the sphere’s port fraction be lower than 10% for color reflectance measurements, whereas ASTM D1003-95 requires the sphere to have a total port fraction less than 4% for haze measurements on transparent plastics. Thus, 150 mm diameter integrating spheres can be used for these methods. A 60 mm diameter integrating sphere with the standard transmittance, reflectance, and detector ports is often unable to meet these strict port fraction requirements. A low port fraction ensures good integration of the sample signal before it reaches the sphere’s detector. The influence of port fraction on sphere radiance is discussed further in the next section.

Measurement Accuracy

Obviously, the integrating sphere’s design will affect its measurement accuracy. The size and location of ports, detectors, and baffles will influence how the light bounces around the sphere. As will be discussed in this section, large 150 mm diameter spheres have better light integration and their measurements are less likely to be affected by hot spots. The signal integration is not as good in smaller spheres, and the large port fraction typically found in 60 mm spheres can introduce significant errors in measurement due to flux loss. All of these factors must be considered when choosing an integrating sphere accessory which is appropriate to the user’s application.

When designing integrating spheres, it is important that the detector’s field of view does not include any portion of the sphere surface directly irradiated by the sample beam or the first reflection from the sample. This would introduce a false response into the measurement. Baffles are typically made from thick pieces of Spectralon or from metal which has been coated with the same material as the integrating sphere wall. Baffles are used to block the detector’s view of light which has not undergone at least two reflections from the sphere surface. Thus, the baffle is positioned to prevent the so-called ‘first-strike’ reflections from entering the field of view of the detector.

The size and position of baffles within the integrating sphere are very important factors which influence the system’s measurement accuracy. As described in ASTM E903, “large errors can arise if the angular distribution of the light reflected from the specimen is different from that reflected by the standard.”An example is found in transmittance measurements of translucent samples. The translucent sample, which scatters light, is measured with respect to the non-scattering open port (air), which is used for the background correction. Careful baffle design can substantially reduce errors due to the different light scattering distribution of samples and standards. However, baffle design must always be performed with respect to the overall radiance characteristics of the sphere. The balance between baffle design and sphere flux is an important consideration when choosing an integrating sphere design.

The distribution of light within an integrating sphere will drastically affect its measurement accuracy. While small spheres do have higher energy efficiency than their 150 mm diameter counterparts, large integrating spheres will yield measurements with greater accuracy since the light in large systems can be ‘integrated’ or distributed evenly about the sphere’s surface. The large internal surface area and the small overall port fraction of 150 mm spheres allows the light to reflect properly around the sphere, creating a homogeneous flux. However, in the design of small integrating sphere accessories, sphere flux homogeneity must often be compromised in an attempt to reduce the effects of hot spots.

Hot spots are areas within the sphere which appear brighter to the sphere’s detectors than other portions of the sphere. Measurement errors, sometimes termed regular reflectance screening errors, can result from hot spots, especially when measuring glossy or specular, mirror-like samples. The reflectance of mirrors may appear higher than the true value if the sphere’s detectors are not baffled from the spot on the sphere wall where the first-strike radiation hits. As described above, spheres are designed with baffles between the detectors and each sample port to minimize such occurrences. A sphere’s baffles are intended to eliminate hot spots or to shield the sphere’s detectors from viewing them directly. Hot spots are more prevalent in small 60 mm diameter spheres. In these spheres, it is often impossible to design baffles to the proper dimensions needed to adequately shield the sphere’s detector from view of a sample port or another bright spot.

Sample Beam Size

Due to their size, 150 mm integrating sphere accessories have proportionally larger sample beam spot sizes, typically 50% larger than those of small 60 mm spheres. A large spot size is an advantage for inhomogeneous samples, where large beam coverage ensures representative reflectance measurements over the entire surface of the test sample. However, the typical large beam size of the 150 mm integrating sphere accessory is not optimized for the measurement of samples smaller than one inch in diameter. For such small samples, the sample beam must be reduced so that it does not overfill the sample. Two methods can be used to reduce the size of the sample beam so it better matches small samples; 1) either a lens can be used to focus the beam down, or 2) the sample can be masked so that the correct portion of the beam strikes the sample. Both beam reduction methods result in sometimes considerable loss in beam energy, which can increase the noise in the scan and will require slower scan times to compensate for the energy loss.

Options for 150 mm Spheres Only

While all sizes of integrating sphere accessories are able to measure diverse samples such as powders, liquids in cuvettes, and translucent or opaque solids such as fabrics or syringes, the 150 mm diameter integrating sphere has extended sampling options that are unavailable on its smaller counterpart. Integrating spheres of 150 mm diameter or greater are able to accept center mount sample holders. These center mounts enable variable angle reflectance measurements of opaque samples, absorbance scans of thin films or translucent samples,or fixed angle liquid measurements to be performed. In addition, the standard 150 mm sphere accessories have removable reflectance port covers for the measurement of large or bulky samples.

Integrating Spheres: The Swiss Army Knife of Spectroscopy

150 mm Integrating Sphere

Integrating spheres, in combination with UV/Vis and UV/Vis/NIR spectrophotometers, are extremely versatile accessories for high precision reflectance and scattered transmittance measurements on virtually any solid or liquid. Application areas range from surface characterization of solids to the photometric analysis of turbid, colloidal, transparent and translucent samples. Typical uses encompass quality assurance testing and product development measurements on textiles, dyes, paper and glass. Integrating spheres are the most commonly used accessories on UV/Vis/NIR spectrophotometers.

Light Paths in a Typical Integrating Sphere

The PerkinElmer 150 mm integrating sphere optical design is shown above. The transmittance sample holder at the entrance of the sphere enables the measurement of light scattering solutions or solids more efficiently than in a standard focusing UV/Vis spectrometer with a conventional detector arrangement. In the latter configuration, sample transmitted light will be lost before it reaches the detector, resulting in significant photometric errors and uncontrolled variation between samples. The integrating sphere, however, collects all the light which has passed through the sample.

Total Reflectance and Diffuse Only Reflectance Modes

For reflectance measurements,samples are mounted in the rear diffuse reflectance sample mount. Either total reflectance or diffuse reflectance only can be measured by placing either a light trap or Spectralon plate at the specular reflectance angle. Measurement on irregularly-shaped solids such as solar cells, textiles, prisms and lenses is also possible using the center-mount option in which the sample is suspended in the middle of the sphere. The transmittance and reference ports are also shown. The reflectance port islocated under the light blue coveron the right-hand side of theaccessory. Even very large samples,for example sheets of glass, can bebrought up to the reflectance port for analysis. Additionally, for small samples or to sample small areas on larger samples, a small spot kit is available to focusthe beam at the entrance, reflectance or center-ports of the sphere.

Common Applications

The characterization of solar cells - UV/Vis spectroscopy is a convenient method of characterizing the transmission loss of silicon photocells. Since the samples are highly scattering, an integrating sphere must be used. Using the center-mount facility, the reflectance of the solar cell can be measured at various angles to mimic the passage of the sun across the sky. The thickness of any coating over the silicon cell can be calculated if necessary.
 

The analysis of security ink

Color analysis - For highly accurate color measurements, ASTM recommends the use of a 150 mm integrating sphere. This yields absolute color values enabling accurate color matching and inter-lab transfer of color values. Smaller integrating spheres, such as the 60 mm sphere, are also used for the measurement of relative color measurements, for example when the color of a product is verified over time against an internal standard. The spectra are traceable to international standards such as CIE, ASTM, ANSI, DIN and EN.

Distinction between specular and diffuse reflectance - Samples with different gloss characteristics or varying surface polishing can be analyzed in specular and diffuse reflectance mode. A standard gloss trap in the 150 mm integrating spheres allows the measurement of the diffuse component only.

Concentration determination of dyes in textiles - In diffuse reflectance measure- ments, the Kubelka-Munk function provides equivalence to absorbance in transmission spectroscopy. It is proportional to concentration and thus can be used for quantitative analysis of chemical compoundsin solids. little or no sample preparation is required. An integrating sphere system provides excellent qualitative as well as quantitative answers. After calibration the content of fat, protein and water can be obtained in a single measurement.

SPF value for sunscreens and cosmetics

Particle sizing