UV Measurement of Sunglasses using a Back-thinned CCD Spectrometer

Introduction
In assisting thousands of customers over the years, we’ve discovered that everyone has a slightly different idea of what constitutes ultraviolet light. For some, it’s the light just beyond human vision that some animals can see (300-400 nm). For others, it’s ionizing radiation that propagates only in a vacuum, below 120 nm.  What we can all agree on, however, is that UV light is not good for our eyes, and that a proper pair of sunglasses needs to block these harmful rays. 

Ultraviolet light can be broken into several wavelength ranges. UVA, UVB and UVC can all damage tissue and accelerate aging of the skin. Since most UVC rays are filtered out by the atmosphere, sun protection focuses primarily on UVB rays, which can damage eye structures such as the cornea, lens and retina.  Also, UVA can contribute to tissue damage indirectly by generating reactive species, but is less damaging and not always blocked by protective gear.

Wavelength range Known for
UVA 315-400 nm Black light
UVB 280-315 nm Producing vitamin D in skin
UVC 100-280 nm Germicidal
Extreme UV 10-120 nm Breaking organic bonds

Experimental Conditions
We used a QE65 Pro spectrometer to measure the absorbance of several pairs of sunglasses, ranging in price from $5 to $300.  For comparison, we also included several clear glasses ranging from $1 safety glasses to higher end prescription glasses. These all contained plastic lenses, presumed to be polycarbonate in composition due to their weight and spectral signatures. Measurements were taken using a QE65 Pro spectrometer configured for 200-998 nm with ~1.6 nm resolution (FWHM) using an HC-1 grating (efficient from ~200-1050 nm) and 10 µm slit.  The HC-1 grating has a broad spectral range, making it ideal for applications requiring UV, VIS and NIR range analysis.

Output from a DH-2000-BAL deuterium halogen lamp was routed in and out of a CUV-UV 10 cm pathlength cuvette holder with 74-UV collimating lenses via two P400-0.25-SR 400 µm solarization-resistant fibers. (Tip: To minimize absorption below 300 nm, it is advisable to use extreme solarization-resistant fibers, short fiber lengths and UV-transmitting quartz/fused silica optics.)

Results
Good S:N was obtained down to ~200 nm using 18 ms integration time, with averaging set to 100 with 3 boxcars and nonlinearity correction enabled. (A baseline correction was performed on the data to account for vertical offset of the spectra due to each of the glasses having different curvature of the lens and thickness.)

In comparing the absorbance spectra (Figures 1 and 2), it is interesting to note that the $5 sunglasses performed almost as well as the $300 prescription pair in the UVB region. The prescription sunglasses did absorb just slightly higher with the highest optical density (OD) of almost 3.3 at 240 nm, likely due to additional filter coatings.  Additionally, the clear reading glasses and clear safety glasses were almost as good for protection in the UVB range, but not as good in the upper UVA range (protection dropped off from 350-400 nm). All the glasses had a similar spectral shape in the UV range presumably due to each pair being made of polycarbonate, which absorbs almost all UV. In this particular case, the $300 prescription sunglasses actually absorbed slightly less in the upper UVA range (dropping significantly over 375 nm), but absorbed slightly more in the VIS and NIR range. This pair measured no more than 2.0 OD at 390 nm, while the other sunglasses maintained an optical density of 2.0 out to ~402 nm.

Interesting to note is that the glasses labeled “UV400” suggested they would cover 100 percent UV absorption up to 400 nm. The spectra shown in Figures 1 and 2 of these “cheap” promotional sunglasses were actually stated to have this protection and compared closely with the other less expensive pairs of sunglasses in the UV range. Sunglass standards vary by country and depend on whether they are meant for sun protection or cosmetic purposes, with minimum standards covering up to 380 nm and blocking 70% of UVB and 60% of UVA.

Figure 1. Absorbance of eyeglasses compared across the UV-Shortwave NIR range.

Figure 1. Absorbance of eyeglasses compared across the UV-Shortwave NIR range.

Figure 2. Close-up view of absorbance of various eyeglasses in the UV range.

Figure 2. Close-up view of absorbance of various eyeglasses in the UV range.

Conclusion
Not all sunglasses offer the same protection and it is not possible to infer the amount of protection just by looking at the color or tint of the lenses. Quality sunglasses are a worthwhile investment for your health, but the QE65 Pro demonstrated clearly that almost any pair is better than none at all.  With high dynamic range, excellent sensitivity and a TE-cooled detector providing low noise, the QE65 Pro has the performance to make UV measurements as easy as those in the visible.

On the web:

QE65 Pro Scientific Grade Spectrometer

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