Omar Khalid
The transparency of plastic is important in products where transparency is essential, such as plastic films, transparent packing, or automotive windshields. Haze, which is caused by the scattering of light, can create a blurry appearance in transparent materials and compromise their clarity. To measure the transparency of plastic, one can use a transparency meter, also known as a clarity meter, which measures the ratio of transmitted light to incident light. Another method is to use a spectrophotometer, which can measure colour, visual light transmission, and haze on a moving production line.
| Characteristics | Values |
|---|---|
| Measurement | Total transmittance, i.e., the ratio of transmitted light to incident light |
| Influencing Factors | Reflection and absorption |
| Industry Standard for Clarity Meter | 1. Reference beam, self-diagnosis, and enclosed optics 2. Built-in statistics with average, standard deviation, coefficient of variance, and min/max 3. Large storage capacity and data transfer to a PC |
| Haze Measurement | Wide-angle scattering |
| Clarity Measurement | Small-angle scattering |
| Haze Value | 0% indicates complete transparency; up to 30% is considered diffusing or translucent |
| Tools | Spectrophotometers, traversing beams, and quality control software |
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What You'll Learn
Use a transparency meter
A transparency meter, also known as a clarity meter, is a device used to measure the transparency of an object. Transparency is the degree of optical clarity with which an object can be seen when viewed through a plastic film or sheet, glass, or a similar substance. The quantitative assessment of transparency is critical in the production of sheeting, film, or glass.
The transparency of an object is measured by its total transmittance, which is the ratio of transmitted light to incident light. Incident light is the amount of light that falls on an object, while transmitted light is the amount of light that passes through it. There are two factors that influence total transmittance: reflection and absorption. For example, if 5% of incident light is reflected and 1% is absorbed, the total transmittance is 94%.
To measure the transparency of plastic, the specimen is placed in front of an illumination unit. The appearance of the specimen is directly related to its scattering behaviour. Light can be scattered at wide angles with low intensity, or at narrow angles with high intensity. Wide-angle scattering leads to a loss of contrast and a hazy appearance, while narrow-angle scattering results in a high concentration of light intensity.
A transparency meter measures the degree of light scattering to determine the transparency of the plastic. It can also provide valuable data that can be used to analyse and address inconsistencies and ensure the product meets industry standards.
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Measure total transmittance
Measuring the transparency of plastic is important in manufacturing, as it ensures the quality of the product. Transparency is measured by its total transmittance, which is the ratio of transmitted light to incident light. Reflection and absorption influence this ratio.
To measure total transmittance, a specimen is placed in front of an illumination unit, with a ring and centre sensor located on the sphere side within the light trap area. The light strikes the specimen, and is partially reflected and absorbed, with the remaining light transmitted in the integrating sphere (ITT). The sphere's interior is coated with a matte white material to allow diffusion. The white cover on the light trap is closed for the measurement of total transmittance, and a detector in the sphere measures the total transmittance.
The combination of illumination and detection must meet the requirements of the 1931 CIE Standard Colorimetric Observer with CIE Standard Illuminant for daylight. The intensity of the incident light is determined during 100% calibration, where no specimen is placed in front of the integrating sphere. The light is scattered in the sphere, and a small portion escapes through the entrance port. When a specimen is placed in front of the sphere, the light that would have escaped during calibration is partially reflected back into the sphere, increasing the intensity on the detector. This effect can lead to higher total transmittance readings, so a modified measurement principle is described in the standard ISO 13468, which outlines methods to prevent too high readings.
The ASTM International (formerly the American Society for Testing and Materials) is the main body that works within the industry to develop standards for various tests and instruments. They outline the industry standard for the clarity meter, which includes a reference beam, self-diagnosis, enclosed optics, and built-in statistics.
Other methods to measure transparency include FTIR infrared spectroscopy, which can be used to measure plastics according to their forms and measurement positions. Transmission measurements can be performed on a monolayer film or equivalent, by holding the sample in a suitable holder, such as thick paper. ATR spectroscopy, on the other hand, is an effective measurement method for thick film, plastic, or rubber samples that do not transmit infrared light. For ATR spectroscopy, the sample is pressed into close contact with a prism, and during total internal reflection of the infrared light in the prism, a small amount of the light enters the sample, enabling an infrared spectrum to be obtained.
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Control transmission haze
Haze is an optical phenomenon that occurs when light passes through a material and is affected by irregularities within it. These irregularities can include poorly dispersed particles, contaminants, and/or air spaces, which cause the light to scatter in different directions. Haze is typically measured using a transmission-type hazemeter, which measures the amount of light that passes through a material without being scattered. This is known as the material's total transmittance, which is the ratio of transmitted light to incident light.
To control transmission haze during the manufacture of plastics, it is essential to minimise the presence of irregularities that can cause light scattering. This can be achieved through various methods, such as ensuring the proper dispersion of particles, reducing contaminants, and minimising air spaces within the plastic material.
One common method to control transmission haze is to use a Taber abrader or a similar tool to create a smooth and uniform surface on the plastic. This helps to eliminate microscopic structures or textures that can cause scattering and reduce the overall visual appearance of the plastic.
Additionally, the thickness of the plastic material can impact light refraction and transmission haze. By maintaining a consistent thickness and avoiding internal refraction variations, manufacturers can reduce the haze and improve the clarity of the plastic.
Another factor to consider is the background colour of the plastic. The measurement of reflection haze, typically used for high-gloss paints and coatings, can be affected by the colour of the plastic material. Thus, by choosing an appropriate background colour that minimises scattering and reflection, manufacturers can help control transmission haze.
Overall, controlling transmission haze during the manufacture of plastics involves optimising the material's properties to minimise light scattering. This ensures that the plastic has the desired level of clarity, transmissivity, and visual appearance for its intended purpose.
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Monitor colour during production
Monitoring colour during the production of plastic is essential to ensure consistent quality and prevent costly errors. X-Rite, for example, offers inline solutions that provide real-time colour information, allowing operators to make necessary adjustments and avoid costly reworks. This process involves measuring colour, visual light transmission, and haze on the moving production line using a spectrophotometer.
The traversing beam, an essential component of the X-Rite system, enables measurements from left to middle to right and throughout the production run. This beam is custom-fitted to the film or plastic production machine, holding the inline spectrophotometer in place to take measurements. The instrument is automatically moved away during a sheet break and returned to its measurement position when needed.
The choice between transmission and reflectance measurement depends on the type of film being produced. Transparent films are measured in transmission to quantify colour, haze, and light transmission, while opaque films are measured in reflectance. The placement of the measurement can vary, but it is most commonly set before reel-up.
Additionally, advancements in colour measurement technology have led to the development of instruments that can measure both colour and transmission haze. For instance, the Konica Minolta Spectrophotometer CM-5, paired with QC analysis software, provides fast and easy quantification of correlated haze. By defining a reference correlated haze value, each batch of material can maintain consistent transparency. If the haze value deviates from the established standard, the formulation can be corrected and documented to prevent issues in future batches.
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Understand optical clarity
Optical clarity is a highly desirable feature in plastic. It is achieved when the refractive index is constant throughout the material in the viewing direction. The refractive index measures how much light is bent when it passes between mediums. A constant refractive index ensures that light passes through the plastic freely without significant distortion or scattering.
The optical clarity of plastic, such as acrylic, starts at the molecular level. Acrylic's molecules are arranged in a way that maximises light transmission. The crystal clear molecule structure, free of impurities or irregularities, means light that passes through is not scattered or distorted. This structure also helps to minimise light dispersion, preventing colour separation or distortion.
However, any surface irregularities or scratches on clear plastic acrylic sheets can disrupt light transmission. Surface reflections at the air/plastic interface create significant transmission losses. These surface reflections can come from specular reflection, the normal reflection from a smooth surface, or diffuse reflection, which is dependent on the surface flatness of the sample.
The transmission loss due to surface roughness or embedded particles is often termed 'haze'. Haze is a production concern and not a property of the material. While haze measurements depend on wide-angle scattering, clarity is determined by small-angle scattering. Thus, haze measurements cannot provide information about the clarity of a specimen and vice versa.
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Frequently asked questions
A transparency meter, also known as a clarity meter, is an instrument used to measure the transparency of an object. Transparency is the optical distinctness with which an object can be seen when viewed through a plastic film/sheet, glass, etc.
Transparency is measured by an object's total transmittance, which is the ratio of transmitted light to incident light. There are two influencing factors: reflection and absorption. For example, if the incident light is 100% and absorption and reflection are -1% and -5% respectively, then the total transmittance is 94%.
A non-contact spectrophotometer can be used to measure the transparency of plastic film or panes during production. It continuously measures colour, visual light transmission, and haze on the moving production line.
