Understanding UV light
Ultraviolet (UV) light accounts for 10% of the sun’s total output, but is completely invisible to the human eye. There are three ranges of UV wavelengths, classified as: UVA, UVB, and UVC. The lowest wavelengths of UV light (100-280nm) are heavily obscured by the atmosphere.
For imaging purposes, UVB (280 - 320nm) and UVA (320 - 400nm) are the common wavelengths that are imaged. Even if UVC light were able to reach a camera sensor, the silicone in the sensor reacts less and less as the wavelength of light decreases in the UV spectrum.
Like UVC, some UVB is partially blocked by the atmosphere. However, it is more difficult to detect with a camera than UVA. The most common wavelengths of UV light that can be imaged are in the UVA spectrum. 95% of the sun’s UV radiation that reaches the earth is UVA light, with UVB being the remainder.
The impact of glass on UV imaging
A significant challenge for UV imaging is the fact that glass absorbs UV light. Therefore, only specially designed optics made with very pure glass (silica) with few impurities can fully take advantage of the full UV exposure being imaged. Impurities such as iron and sodium are common within glass and are the cause of most UV absorption. In Figure 2, the top image is captured outdoors and the bottom image is captured indoors from behind a window. The glass in the bottom image of Figure 2 shows a reduction in exposure after having most of the light absorbed by the glass in front of the camera. So, when imaging UV-sensitive subjects, minimizing the amount of glass can help with exposure and capturing more detail.
Using camera filters to image with UV light - Visualizing sunscreen efficacy
A simple experiment was conducted to determine the efficacy of sunscreen. Using a Teledyne Lumenera Lt345R monochrome camera (with infrared blocking filter), combined with a MidOpt BP324 bandpass filter, the amount of UV absorption with sunscreen was observed. The infrared blocking filter used in the Lt345R monochrome camera is normally used in Teledyne Lumenera color cameras. In this case, it was used with a monochrome camera to account for the BP324’s transmission band centered on 710 nm (seen in Figure 3). This way the image sensor only received UV light between 260 and 390 nm, or more accurately, 280 nm to 390 nm. Since UVC is mostly being blocked by the atmosphere, the sensor would be barely able to register UV light of such low wavelength. Given silicone’s very low sensitivity to UV light and the combination of filters further reducing light transmission, high gains and longer exposure times were used to capture the images on a sunny day.
In Figure 4, sunscreen is applied on the subject’s right arm (left side of the image). The dark tone highlights the UV light being absorbed by the sunscreen on the arm. The monochrome camera visualizes this as a strong contrast since there is no “color” that can be attributed to the light reflecting off of the arm on the right side of the image, as UV light is outside of the visible spectrum and must be measured in terms of intensity.
With this set up, the intensity of UV light can be measured based on how light or dark the arm is as a result of the sunscreen. To show the difference in UV intensity, varying SPF value sunscreens were applied to the arm in Figure 5. The gradient of dark tones gets stronger as the SPF value increases. However, the tone of SPF 60 is not visually darker than SPF 30. Therefore, there might be less UV absorption due to either the amount of sunscreen or how evenly it was applied. This example shows that the SPF value is not the only factor when it comes to the effectiveness of sunscreen.
By using filters to accomplish UV imaging, more information and detail can be revealed. For other applications where the use of camera filters can help enhance vision systems to get every last bit of information, read our white paper, "Maximizing Camera Performance with Filters."
For more information on imaging with UV light and how it can benefit your application, please contact Teledyne Lumenera’s imaging experts. They can also help with selecting the cameras to best meet your requirements. Reach out at firstname.lastname@example.org.
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