The most accurate form of measuring plastic film thickness in common industrial use is by using one of a range of infrared spectroscopy methods on a sample of the film. These include measurement by transmission, attenuated total reflection, specular reflection and infrared microscope methods.
Standard infrared spectroscopy methods assume the availability of an appropriate sample of the film in question. For items in use, or which cannot otherwise be sampled normally, a much smaller sample scraped off with a silicon-carbide sampler undergoes measurement of the infrared spectrum using a diffuse reflectance accessory. Commercial gauges use a range of these tests to provide the appropriate measurements, but producers must ensure that they are using the right tests for their product's composition and end use.