What is the Difference Between UV-A and UV-C?

Ultraviolet light is almost as diverse as the various colors of the visible spectrum. Yet, when we think about ultraviolet, we tend to forget this, and simply categorize it as a range of wavelengths associated with its usefulness in fluorescence, curing, disinfection but also its potential carcinogenic effects. It is very important, however, to distinguish between various types of ultraviolet energy, as each type has very different properties. In this article, we discuss the primary differences between UV-A and UV-C radiation in terms of their applications and uses.

Look for the Wavelength Value First

Ultraviolet energy should first and foremost be identified by its wavelength. The wavelength value is measured in nanometers (nm) and is what determines the type of ultraviolet energy. UV-A encompasses wavelengths between 315 and 400 nanometers, while UV-C includes wavelengths from 100 and 280 nanometers. UV-B wavelengths fall in between at 280 and 315 nanometers.

Both UV-A and UV-C are not visible to the naked eye, so it can be a bit counter-intuitive as you cannot visually see the difference between these two types of UV, the same way we can visually check if a light source is red or blue. Therefore, it is even more important that you know what wavelength light source you will need for your particular application, and at the very least, to be aware of the differences between UV-A and UV-C radiation.

UV-A: Fluorescence & Curing

Most UV-A lamp applications can be divided into either fluorescence or curing applications, and utilize a wavelength of 365 nanometers. Fluorescence is a phenomenon where materials such as paints, pigments or minerals convert UV-A energy to a visible wavelength. UV lamps that are used for such applications are called blacklights, as the lights themselves appear dark, but upon shining them on various objects, they emit various visible colors.

Below is an example of a rock showing green fluorescence under the realUV™ LED flashlight. UV-A fluorescence is very helpful in various applications such as forensics, medicine, molecular biology and geology, where the ability to identify the presence of certain fluorescent materials that otherwise cannot be distinguished under normal lighting conditions is a significant advantage.

Not all fluorescence applications are confined to scientific applications. Fluorescence can be used to create a wide variety of stunning visual effects, and can be used for fluorescence photography and blacklight art installations. Many entertainment venues, including that blacklight party you may or may not remember, will also use UV-A to create fluorescence effects.

The most common wavelengths for UV-A fluorescence are 365 nm and 395 nm. Generally, both 365 and 395 nm will create fluorescence effects, but 365 nm will provide a "cleaner" UV effect with less visible light output, while 395 nm will have a small visible violet / purple component. For more information, see our article comparing 365 nm and 395 nm.

Unlike fluorescence, UV-A can also initiate chemical and structural changes in various materials, and is used in curing applications. Generally, curing requires a significantly higher level of UV intensity, but is nonetheless accomplished using the same UV-A wavelengths. As with fluorescence, 365 nm is a commonly used wavelength for curing.

UV-A wavelengths are used for emulsion paint curing in screen printing, as well as epoxies for industrial applications as well as nail gels. In addition to intensity, the total exposure time is also a factor in UV-A curing applications.

UV-C: Germicidal and Disinfection Applications

Unlike UV-A, UV-C wavelengths occupy a much lower wavelength range of 100 nm to 280 nm. UV-C wavelengths have been the center of focus as an effective way to inactivate pathogens including viruses, bacteria, molds and fungi.

UV-C is an effective germicidal wavelength because DNA and RNA are susceptible to damage at and around 265 nanometers. When pathogens are exposed to UV-C wavelength radiation, double bonds that tie together thymine and adenine are destroyed in a process called dimerization, altering the structure of the pathogen's genome. Due to this alteration, when the pathogen attempts to replicate or reproduce, the genomic corruption prevents it from doing so successfully.

UV-C is unique in its ability to perform germicidal functions because of the wavelength susceptibility of thymine (uracil in RNA). Below is a chart which shows that thymine and uracil do not absorb UV radiation at wavelengths higher than 300 nanometers.

As the chart shows, UV-A does not have the ability to initiate dimerization in the way that UV-C radiation does. Therefore, all evidence to date indicates that UV-A is not an effective method of disinfection as it is not able to target the DNA structures of pathogens.

For more information, see our page dedicated to UV-C LED technology.

UV-A is Present in Daylight, UV-C is Not

A common misconception is that the natural daylight includes all kinds of UV energy. While solar radiation includes all wavelengths of UV energy, only UV-A and some UV-B pass through the earth's atmosphere. UV-C, on the other hand, is absorbed by the earth's ozone layer, and does not reach the ground.

Significant precautions must be taken with all forms of ultraviolet energy, as all wavelengths of UV, including UV-A, UV-B and UV-C are presumed carcinogens, according to the US HHS. Because it is invisible, UV radiation can be especially dangerous as it does not induce a natural reaction to squint or look away the way we do with visible light. However, we do know that UV-A radiation is quite prevalent in natural daylight, and as a result, there are far more studies and population level studies that provide us with some level of understanding of the potential risks and harm that UV-A can cause.

UV-C on the other hand, is not a type of radiation that a typical person is exposed to on a daily basis. Most studies have been performed with occupational health and safety in mind, for specific industries and occupations such as welders. Therefore, far fewer studies have been performed on the risks and potential harm caused by UV-C. From a physics perspective, UV-C has a much higher energy level due to its shorter wavelength, and we know that it damages DNA molecules directly. It would be prudent to assume that it has the potential to cause more human harm than weaker forms of UV, namely UV-A and UV-B. As such, even higher levels of caution should be taken to avoid exposure to UV-C.