UV-Vis Frequently Asked Questions - Solid Sample Transmission
My sample’s not a liquid, now what do I do?
By far the most common application in UV/Vis spectrophotometry is the measurement of liquids in cuvettes. However, high performance UV/Vis and UV/Vis/NIR instruments are more commonly used for material’s characterization, which is an area dominated by solid samples. The diagram above shows the various transmission and reflection modes for a clear solid sample. A transparent or semi-transparent sample can exhibit reflection, both specular and diffuse, and transmission, both specular and diffuse. The diffuse mode arises from particle scattering of the sample. Diffuse reflection is also called back-scatter, while diffuse transmission is called forward scatter and both are typically generated by rough surfaces. Specular reflection and transmittance is the product of non-scattering smooth surfaces. If a sample is opaque it can only produce diffuse reflectance or specular reflectance depending on its surface characteristics. The accessory of choice for solid samples is the integrating sphere. Integrating spheres are able to perform a variety of measurements on transparent, translucent, and opaque solid or liquid samples.
A spectrophotometer functions by first performing a background correction (autozero) to calibrate the 100% T (0 absorbance) value. It is crucial that placement of the beam on the instrument’s detector be the same for both the background and sample measurement. If the beam changes either placement or shape during these two measurements, artifacts (inaccurate photometric values) can occur. Spheres can minimize this problem because the detector in a sphere is “looking” at the entirety of light integrated over the surface of the sphere. A sphere eliminates many types of artifacts for a variety if non-ideal samples. Sphere scatter transmission measurements are made by placing the sample on the scatter transmission port on the front of the sphere.
What are the relationships between different types of transmitted light?
Transmission measurements involve shining light on a sample and measuring the light transmitted through it. The transmitted light is generally classified as shown in the top figure. A spectrophotometer offers three types of measurements:
- Linear transmission measurements of light that passes straight through the sample
- Diffuse transmission measurements of light that scattered inside the sample when passing through it
- Total transmission measurements of all the light transmitted through the sample.
The characteristics of the measurements and the points to be aware of differ according to the type of transmitted light measured. We prepared a clear glass sample (glass filter) and opaque and translucent sample (opal glass). The bottom left figure shows a photograph of a linear transmitted light sample (blue) and a diffusely transmitted light sample (white). As one can see the glass filter is transparent, whereas the opal glass appears cloudy.
How do clear and diffuse sample spectra compare on a linear transmission accessory?
The figure here shows the measured results on the glass filter and opal glass. The transmission characteristics are clearly represented in the spectrum of a transparent sample, such as the glass filter. However, the transmittance is approximately 0 % for the opal glass which appears cloudy. The transmitted light for the opal glass is all scattered away at different angles due to the opaque nature of the sample.
If linear transmission measurements are performed on an opaque and translucent sample, some light is scattered in the sample as diffuse light and does not reach the detector. As the distance from the sample mounting position to the detector differs according to the type of spectrophotometer, the amount of diffuse light reaching the detector differs from instrument to instrument, even when the same sample is measured. Consequently, each instrument produces different measurement results. Total transmission measurements are also suitable for such samples.
The samples are displayed at bottom center.
Linear transmission measurements measure the part of the transmitted light that passes straight through the sample. The figure at the bottom shows a schematic view of the measurement. This method does not detect the diffuse transmitted light. This measurement method is generally used on thin transparent film or glass not exceeding approximately 3 mm thickness. It is also used to confirm almost 100 % transmittance in a sample with double-sided anti-reflection coatings applied to suppress reflection.
The film holder accessory allows easy sample positioning for linear transmitted light measurements. The photo at top right shows the appearance of the film holder. During actual measurements, the film holder is installed in the standard sample compartment. Baseline correction is then performed. Baseline correction generally involves measurements on air with no sample mounted. After baseline correction is complete, mount the sample at the sample side and perform sample measurements. The top left figure shows the sample mounting position, viewed from directly above the film holder.
How does sample thickness influence linear transmission measurements?
Linear transmission measurements are relatively simple to perform but there are several points you need to be aware of. One is the sample thickness. If the sample thickness exceeds about 3 mm, the focal point will change considerably between baseline correction and sample measurement. This results in changes in the beam size at the detector light-receiving surface, making it impossible to obtain accurate transmittance values. This change in beam size results from the difference in refractive index between air and the sample. If the transparent sample is sufficiently thin, the change in beam size will be small and not cause any problems during measurements. As samples become thicker, this effect becomes more difficult to ignore. The figure here shows a schematic view of the change in beam size. Total transmission measurements using an integrating sphere, as described below, are suitable for thicker samples. It is impossible to obtain accurate transmittance values for samples, such as lenses, which are thin but the focal point changes significantly between baseline correction and measurement. Total transmission measurements are also suitable for these samples.
Note that in a typical scatter transmission measurement, only the transmitted and forward scattered light is collected by the sphere. The backward scattered (diffuse reflectance) light escapes collection. If both backward and forward scattered light need to be collected, then a center mount accessory can be employed. The center mount produces an absolute absorbance measurement and is very useful in measuring highly scattering or turbid samples on a 150 mm sphere.
With an integrating sphere, diffuse reflectance measurements can be made on opaque, translucent, and transparent samples (see picture above). Care must be taken with any sample that is not totally opaque. Sphere sample holders are designed to hold the sample against the reflectance port, so light from a non-opaque sample can pass through to be trapped and dispersed outside the sphere. It is very important not to place any material on the back of the sample, as this may cause an artifact due to a “double pass” through the sample which includes a reflectance contribution from the backing material. Spheres have the ability to measure total reflectance (specular plus diffuse components) or diffuse only reflectance. It is possible to subtract the diffuse from the total reflection to get an approximation of the specular component; however, a dedicated specular accessory will yield more accurate results. The primary utility of the diffuse only mode is to eliminate the specular gloss from a sample such as the quartz window on a powder cell.
What is a total transmission measurement?
Total transmission measurements measure all of the light passing through a sample, combining the linear transmitted light and diffuse transmitted light. The top left figure shows a schematic view of the measurement. A spectrophotometer fitted with an integrating sphere is used for the measurements. Therefore, such measurements are sometimes called "integrating sphere measurements." The bottom right figure shows an example of an integrating sphere attachment used for such measurements.
When a transparent sample is measured, total transmission measurements acquire the same data as linear transmission measurements because there is no scattering of diffuse light in the sample. If the sample is adequately thin, linear transmission measurements provide data with less noise. This is because much of the light entering the integrating sphere used for total transmission measurements does not reach the detector. Several types of integrating spheres are available for measurements. The sizes of the integrating spheres can differ dramatically. A 60-mm-diameter integrating sphere is normally used but a large 150-mm-diameter integrating sphere attachment is also available. Integrating spheres dedicated to transmission measurements are also available with different numbers of openings. Such transmission integrating spheres are suited to the measurement of samples when the focal point differs considerably between baseline correction and sample measurement.
For total transmission measurements, care must be taken when comparing measured data. It is sometimes impossible to accurately compare data measured using different integrating spheres. This situation often occurs with samples that scatter light and create a lot of diffuse transmitted light. For baseline correction, the light incident on the integrating sphere first hits a standard white plate and makes multiple reflections inside the integrating sphere before reaching the detector. During sample measurements, however, the light first hits the interior of the integrating sphere before it reaches the detector. Consequently, differences occur in the data due to differences in reflectance at the position of the first reflection. When comparing total transmittances, you are recommended to use data measured with the same integrating sphere.
To measure an actual sample, install the integrating sphere in the instrument and perform baseline correction. Baseline correction generally involves measurements on air with no sample mounted. After baseline correction is complete, mount the sample and perform sample measurements. The figure here shows the sample mounting position, viewed from directly above the integrating sphere.
How do clear and diffuse sample spectra compare on a total transmission accessory?
The graphs at the top show the measured results on the glass filter and opal glass. For a transparent sample such as a glass filter, there is virtually no difference between this spectrum and the linear transmission measurement spectrum. With a translucent sample such as opal glass, on the other hand, as total transmission measurements also detect the diffuse transmitted light, a transmittance approximately 40% higher than the linear transmittance of approximately 0% is produced.
The samples are displayed at bottom center.
Diffuse transmission measurements measure the part of the transmitted light that is scattered and does not pass straight through the sample. The top right figure shows a schematic view of the measurement. The linear transmitted light is not allowed to enter the detector during these measurements. Diffuse transmission measurements are commonly used to evaluate the scattering performance of translucent film. They are also used for haze (cloudiness) measurements. However, a special integrating sphere attachment is required to perform haze measurements according to several standards, which strictly define the beam size, aperture ratio, and aperture size of the integrating sphere.
As in the case of total transmission measurements, an integrating sphere attachment is used for diffuse transmission measurements. But it is used slightly differently. To measure an actual sample, install the integrating sphere in the instrument and perform baseline correction. At this time, mount standard white plates at the prescribed positions. Baseline correction generally involves measurements on air with no sample mounted. Next, mount the sample and perform measurements with the standard white plate on the opposite side of the integrating sphere. (In some cases, a light trap is mounted at the position where the standard white plate was removed.) This allows the linear transmitted light to exit the integrating sphere, such that the integrating sphere captures only the diffuse light. The bottom figure shows the sample mounting position, viewed from directly above the integrating sphere.
How do clear and diffuse sample spectra compare on a diffuse transmission accessory?
The graphs at the top shows the measured results on the glass filter and opal glass. As a transparent sample such as the glass filter produces no diffuse transmitted light, the diffuse transmittance is approximately 0 %. A translucent sample such as opal glass, on the other hand, produces almost only diffuse transmitted light such that the transmittance is approximately 40 %. This does not differ significantly from the total light transmittance value.
The samples are displayed at bottom center.
Can I measure linear transmission measurements on integrating spheres?
Relationships Between the Types of Transmission Measurements:
- Subtracting the diffuse transmitted light from the total transmitted light produces the linear transmitted light. However, in actual measurements, the linear transmitted light spreads out before it is detected.
- Consequently, the linear transmitted light results measured in the standard sample compartment may differ from the linear transmittance calculated as the difference between the total transmitted light and diffuse transmitted light measured using an integrating sphere.
How do I baseline correct when measuring a coating on a solid sample?
- For solid sample transmission measurements, baseline correction generally involves measurements on air with no sample mounted.
- However, to measure the transmission characteristics of only the film or coating on a substrate, before measuring the sample, perform baseline correction on a blank substrate with no film or coating applied.
- As it may be impossible to obtain accurate measured results at wavelengths where the blank substrate exhibits high absorbance, it is extremely important to confirm the transmission characteristics of the blank substrate.