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.