No cloudy outlook for acrylic glass
Easy to determine haze value thanks to LabSolutions UV-Vis
Dr. Benjamin Thomas, Shimadzu Europa GmbH
Products made of acrylic glass have become an integral part of our everyday lives – from watch crystals to submarine pressure hulls. Hopefully, those transparent screen dividers from the pandemic are forever a thing of the past. Depending on whether a crystal-clear view or frosted glass is required: The cloudiness of acrylic glass is defined as its “haze value”, which is determined according to the ASTM D 1003 standard. The haze value can be determined using a UV-Vis spectrophotometer (or “spectrometer” for short). However, the measurement process is cumbersome and there isn’t enough time in everyday laboratory work to manually analyze the required data. That’s why it became necessary to automate the process while keeping the requirements of the standard in mind.
Acrylic glass is a plastic that has been used in a wide range of applications since its invention. And no wonder, not only is the material lightweight, it can also be thermoformed without any issues. In most cases, plastic glass replaces mineral glass, meaning it should be just as transparent. However, translucent acrylic glass is also sometimes used, for example as a diffuser in LED panels or for privacy lighting solutions (Figure 1). The degree of cloudiness of an acrylic glass pane is therefore an important specification for its future use and must be checked during quality control.
According to the ASTM D 1003 standard, the optical quality of a plastic pane is given as a haze value. This is defined as the percentage ratio of diffuse to total transmission of visible light through the sample. Put simply, the lower the haze value, the clearer the material.
The transmission values needed to calculate the haze value are either measured with a special haze meter, which is described in detail in the standard, or calculated from spectra taken with a UV-Vis spectrometer.
Haze meters vs. spectrometers
Both devices use an integrating sphere between the sample and the detector to collect the diffusely transmitted light. This is a hollow sphere with a highly reflective inner surface that collects the light, like a large funnel. The specification of this sphere is described in the standard and is met by Shimadzu integrating spheres with an internal diameter of 150 mm (ISR-1503 and ISR-1053F). Application note A525A explains the technical details.
The biggest difference between a haze meter and a spectrometer is the monochromator. It is located in a spectrometer between the light source and the sample and splits the white light from the light source into the different wavelengths – in the case of visible light, from 380 nm to 780 nm (Figure 2).
A haze meter does not have a monochromator, so it only ever measures a single value: the brightness of the light source behind the sample. On the other hand, a spectrometer measures the transmission of the sample for each wavelength. This is advantageous as other parameters can be calculated from the spectrum in addition to the haze value, such as the color or the transmission of ultraviolet or infrared light in particular. Depending on the specific application, these values are required by other standards, such as DIN EN 410 for the color fastness and thermal insulation of window glass.
But it’s precisely this versatility that is the spectrometer’s downfall: Determining the haze value requires extra work that a haze meter avoids. Calculation and evaluation still have to be done manually to a large extent. This meant that laboratories were previously faced with the decision of either purchasing a haze meter, which only serves one purpose, or putting up with the extra effort.
Calculating the haze value
Both instruments can measure the total transmission by imaging all light onto the detector and the diffuse reflection by imaging only the light scattered by the sample onto the detector. In this way, a data set is made up of the raw data from four measurements:
- Empty measurement with measurement setup for the entire transmission, T1 ~ 100%
- Total transmission through the sample, T2 ≤ 100%
- Empty measurement with measurement setup for diffuse transmission, T3 ~ 0%
- Light scattered on the sample, T4 ≤ T1
Since no light is reflected, absorbed or scattered by a sample during an empty measurement, the transmission measurement value of the empty measurement ideally corresponds to 100 % for the setup for the total transmission and ideally 0 % for the setup for the measurement of the diffuse transmission. If the measurement is transmitted through a sample, reflection and absorption will result in the total transmission equally less than 100 %. The measured value of the diffuse transmission depends heavily on the haze value and ranges between 0 % (0 % haze) and the measured value of the total transmission (100 % haze).
For spectrometer measurements, the entire spectrum must first be converted into a single transmission value for each of the steps.
The transmission value of each wavelength is weighted with the intensity of a simulated light source and the sensitivity of the human eye to brightness. The sum of these products across all wavelengths in the visible light range (380–780 nm) is then calculated to convert the entire spectrum (intensity versus wavelength) into a single intensity value. To achieve comparable values, the emission curves of the permissible light sources (tungsten lamp or daylight) and the sensitivity of the human eye for each wavelength are described and summarized in tabular form in more detail in ASTM D 1003.
The data can be converted directly in LabSolutions UV-Vis for each individual measurement as described and can be preset as automatic data processing. The corresponding tables are already stored in the software so that only the appropriate light source needs to be selected.
The corrected haze value is calculated from these four individual measurements by subtracting the haze value of the empty measurement (ideally 0) from the haze value of the sample:
This last calculation step is usually not possible from within the spectrometer software, instead requiring the raw data to be exported to evaluation software such as Microsoft Excel, MathWorks MATLAB or OriginLab Origin. This manual step is highly vulnerable to errors.
The automation process
To make this time-consuming process easier for users, the LabSolutions UV-Vis software had to be expanded and automated accordingly. After careful consideration of ASTM D 1003, an Excel template was first drawn up for the evaluation after manual export of the data and then expanded to include automatic measurement.
In the now customized software, an Excel template with prepared formulas or macros can be used to simplify the creation of the final report. For use in a regulated environment (21 CFR Part 11), Shimadzu offers the LabSolutions Manager multi-data reporting tool, a spreadsheet application with full traceability thanks to an audit trail and electronic signatures. Prepared haze templates can be requested for both cases.
Laboratory software in practice
Four differently clouded acrylic glass panes with the corresponding transmission spectra and haze values (Figure 3) are used to demonstrate the effect of the haze value. They were placed in front of a photo of the face of Genzo Shimadzu Sr., the founder of Shimadzu.
The top sample with a haze value of around 0 % has almost no diffuse transmission and the image is clearly recognizable. While the total transmission remains almost the same for all other samples, the measured value of the diffuse transmission increases with the haze value; the contours of Gen-san’s face become increasingly blurred. In the lowest sample with almost 100 % haze, the contours are barely recognizable; the curves for total and diffuse transmission are almost the same.
The measurement of the four required raw data sets and the analysis are now simplified by a dedicated measurement program. It guides the user through the various steps and ensures that the raw data is entered into the report template in the correct format and analyzed.
The graphical user interface of this program is shown in Figure 4. Clear instructions guide the user through the analysis step by step and illustrations, if required. The relevant metadata (Is it a sample or empty measurement? Should the total or diffuse transmission be measured?) are automatically generated in the background and forwarded to LabSolutions UV-Vis. A report is then generated using Excel or a multi-data report, depending on the regulatory requirements, where the formulas described here are already stored in the report template.
This eliminates the need to transfer the data to an additional program for evaluation. Automating the final and decisive calculation step saves time and helps prevent errors that can occur during transmission.
The haze value of acrylic glass is decisive for the technical application of the respective product. A clear lens with a low haze value allows an unclouded view, while a cloudy lens with a high haze value blurs the contours of any objects behind it. Manually determining the haze value requires some background knowledge. This has been facilitated by a program with automated measurements and clear instructions that now allows less experienced users to carry out analyses. The LabSolutions UV-Vis software allows automation via macros and the export of data to Excel. With the package of device, software and report templates presented here, this process is automated. Now, all the user has to do is enter the samples and press “Start” to receive a finished report.
