The same window film can show different results because spectrum meters do not all measure the same wavelengths. Some instruments test selected points, while others measure a wider range. A film’s transmission also changes across the spectrum, so a result at 365 nm, 380 nm, 940/950 nm or 1400 nm describes only what that particular meter measures.
A meter reading depends on what the meter measures
A handheld meter does not automatically represent every UV or infrared wavelength. Its light source, detector, wavelength range, calibration and calculation method determine the result shown. Two correctly functioning meters can therefore display different percentages for the same film.
365 nm, 380 nm and the full UV range are not the same
Linshang lists several transmission meters that use a 365 nm UV peak. EDTM states that its SS2450 focuses on UV-A energy across 315–380 nm. However, the ultraviolet region extends to 400 nm. A strong result at 365 nm or across a range ending at 380 nm does not by itself confirm the film’s performance throughout the longer-wavelength UVA region up to 400 nm.
Infrared readings also depend on wavelength
Infrared performance can be reported at selected points such as approximately 940/950 nm or 1400 nm, or across a broader infrared range. Because a film may reject one infrared wavelength more strongly than another, changing the test point can change the displayed IRR result.
How to make a fair comparison
Compare films with the same calibrated meter, the same wavelength or measurement range, the same sample orientation and the same glass condition. Record whether the result is film-only or film-on-glass. For a complete performance decision, review the test range together with VLT, TSER, solar reflectance, absorptance and transmittance.
Frequently asked questions
Does a different reading mean one meter is faulty?
Not necessarily. Both meters may be working correctly but testing different wavelengths, ranges or using different calculation methods.
Can a 365 nm meter prove UV protection up to 400 nm?
No. A reading at 365 nm confirms performance around that test wavelength; it does not establish the film’s transmission at every wavelength through 400 nm.
Why can the same film show different IRR percentages?
The meters may test different infrared points, such as around 940/950 nm or 1400 nm, while the film’s rejection changes across the infrared spectrum.
What should customers ask when shown a meter result?
Ask which wavelength or range is being measured, which meter is used, whether it is calibrated, and whether the result represents film-only or film installed on glass.
Reference websites
Product information can change. Check the brands’ current official information: