Building wearable UV Index sensor devices for protection from harmful sun exposure - Embedded.com

Building wearable UV Index sensor devices for protection from harmful sun exposure

Editor’s note: In this Product How-To, Silicon Labs’ Kevin Kilbane provides an overview of ultraviolet radiation from sunlight and its effect on human health, and explains how to use Silicon Labs’ Si1132 and Si114x UV index sensors to build a wearable UV sensor device to monitor for harmful levels of UV radiation.

Skin cancer has become an increasing issue in human health. Skin cancer has become the most common form of cancer in the United States, with more than 3.5 million cases diagnosed each year. Over the past three decades, more people have experienced skin cancer than all other types of cancer combined. As a result, people are becoming more concerned about gauging their exposure to the ultraviolet (UV) radiation that is, for the most part, responsible for the formation of skin cancer tumors.

UV radiation is a natural part of the environment and even has a beneficial effect in smaller doses. If we tried to eliminate all UV exposure, we would see an increase in skeletal diseases caused by a deficiency of vitamin D that is synthesised by the body with the help of UV. How much of a health benefit we see from UV depends on personal circumstances, since there are strong interactions between UV exposure and skin pigmentation. The key is to maintain UV exposure at an optimum healthy level but not so high that it becomes dangerous.

When developing UV sensing applications, it is helpful to distinguish between the different types of UV. The 1932 Second International Congress on Light defined three distinct types of UV that exist in the 100 nm to 400 nm wavelength range: UVA, UVB and UVC. Only two of these types – UVA and UVB – are important to consumer applications for ambient UV measurements.

The short-wavelength UVC photons from the sun do not penetrate the atmosphere and, for the most part, can be disregarded for use in personal healthcare and wearable computing products. UVC is primarily of interest for industrial applications, for example, to sterilize and disinfect equipment because of UVC radiation’s harmful effects on bacteria and other infectious organisms.

UVA and UVB radiation passes through the Earth’s atmosphere, although the shorter-wavelength UVB rays, which lie in the 290 nm to 320 nm range, are absorbed more strongly than longer-wavelength UVA rays, which lie in the 320 nm to 400 nm range. In addition to being more prevalent in the atmosphere, UVA penetrates human skin more readily than the more energetic UVB rays, as shown in Figure 1 .

Figure 1: Types of UV Radiation and Effects on Human Skin

UVA does have some health benefits as it activates melanin pigment already present in the upper skin cells, creating a tan that appears quickly but also fades quickly. But by penetrating into deeper skin layers, UVA also affects connective tissue and blood vessels. According to the World Health Organization (WHO), the skin gradually loses its elasticity as result of excessive UVA exposure and starts to wrinkle. Recent studies suggest that UVA may also enhance the development of skin cancers, although the mechanisms of this UVA damage are not well understood.

Scientists have known for a long time that UVB rays are more harmful to health than UVA. UVB exposure has been shown to cause damage to DNA, leading to potentially irreversible genetic damage. Mammalian cells have self-repair mechanisms that deal with low levels of DNA damage caused by phenomena such as UV radiation. However, once the damage reaches a certain point, the repair mechanisms cannot keep up, and under normal circumstances the cell triggers its own death, a process known by biologists as apoptosis. For example, this situation occurs when someone receives a bad sunburn. If a skin cell does not correctly perform apoptosis, the potential arises for it to form the core of a cancerous tumour.

Numerous factors lead to significant changes in UV exposure. Higher altitudes reduce atmospheric absorption of UV rays and therefore lead to higher UV exposure. Time of day and seasonality as well as the presence of clouds and dust affect the amount of solar UV radiation that a person encounters while outside. The level of UV radiation varies by approximately four times around the globe, and the situation is complicated by the way in which ozone – which strongly absorbs UVB – is concentrated in the atmosphere. At higher latitudes, less ozone is often present in the atmosphere, which increases the risk of DNA damage from UVB.

The incidence of melanoma tends to be higher for fair-skinned people living in higher latitudes. For example, skin-cancer mortality is six times higher in Nordic countries than in the Mediterranean countries, according to WHO figures. This situation is partly due to fair-skinned people receiving high UV exposure while on holiday in sunnier latitudes.

The WHO developed the UV index as a way to raise public awareness of the risks of excessive UV exposure and to help weather forecasters and consumers gauge how strong the sun is on a given day so that they can take preventive measures. The UV index provides a numeric value that is related linearly to the intensity of sunlight, as shown in Figure 2 .

The UV index forecast is based on what the UV index is expected to be at noon; the actual UV index changes throughout the day with the angle of the sun and with varying cloud cover. In addition, because human skin responds different to UVA and UVB, the UV index is calculated according to the International Lighting Commission (CIE) Erythemal Action Spectrum. The CIE provides a standardized method of weighting the UV index based on the normal human skin response and is important in gauging how much damage the sun can do.

Figure 2: WHO UV Index scale (1-2 very low risk  to 11+ high risk of UV exposure)

Preventive measures such as the UV index are beneficial to public healthy by providing an early warning of when people have reached unhealthy levels of UV exposure. Wearable computing devices and smart phones that actively measure UV can provide a convenient way of using the UV index to determine how long one can stay outside in the sun without adequate protection (i.e., use of sun screen, shades, hats and other protective clothing).

Since wearable devices and smart phones can store data over a long period, consumers can use these devices to determine their cumulative UV exposure, which may occur while sunbathing, exercising or working outdoors. UV exposure measurements can provide vital information for people with an elevated risk of sunburn, especially when caused by UV light accumulated over days of exposure.In addition to addressing the serious health concerns over UV exposure,the ability to determine the degree of UV exposure received during theday has value in terms of comfort and convenience. Vacationers sometimesforget to apply sunblock before venturing outdoors or may be unaware ofthe strength of the sun’s rays when they first arrive, leavingthemselves vulnerable to sunburn.

A wearable device that alertsusers to the strength of the ambient UV radiation can help protect usersfrom the discomfort and danger of an unexpected sunburn. The UV sensorcan also provide reminders to reapply sunscreen based on UV exposureover time. For these reasons, wearable computing products such asfitness trackers and smart watches equipped with UV index sensors arebeginning to appear in the consumer electronics market.

Althoughindustrial UV sensors are in widespread use, these sensors focusprimarily on artificially generated wavelengths, normally in the UVCrange, to ensure that industrial workers are not affected by UVCradiation used to sterilize tools and equipment. UV index sensors usedin wearables (Figure 3 ) and even smart phones are designed tofocus on the UVA and UVB ranges and how each these wavelengths affectour skin according to the CIE Erythemal Action Spectrum.

Figure 3: Wearable and smartphone products can help protect consumers from excessive UV exposure by integrating UV Index sensors

Traditionally,UV sensors for consumer applications have been implemented as discretesolutions, typically consisting of a photodiode tuned to be sensitive toa range of UV frequencies. These photodiodes emit a current that isdigitized by an analogue-to-digital converter (ADC) before it can beprocessed by a microcontroller (MCU). The sensitivity of photodiodes canfluctuate greatly, requiring calibration for them to be used reliablyin a consumer application. These discrete UV solutions tend to rely oncompound semiconductors, making them difficult to integrate withCMOS-based signal-conditioning and processing circuitry.

Size isalso an issue in wearable designs as the discrete packaging consumesspace that is difficult to justify in wearables. Because the wrist areais usually exposed to sunlight when someone is exercising outside,sports watch and heath/fitness wristbands are well suited to theapplication of incident UV measurement. However, these wearable devicesare already heavily space-constrained due to the need to incorporateadvanced digital signal processing and antenna functions within a verysmall “wrist-top” form factor.

Integration onto a siliconsubstrate provides not only space savings but improvements in UVmeasurement itself. If a sensor IC contains not only the UV sensor butalso signal-conditioning circuitry such as op-amps and an ADC, it ispossible to perform device calibration and programming at the factory toensure consistent UV readings from product to product. This ability isfurther enhanced if an MCU can be integrated into the solution toperform calibration, take readings and provide the data in a usableform. Silicon Labs has implemented this highly integrated approach inthe design of its Si1132 and Si114x digital UV sensors, which combine aUV index sensor and digital processing circuitry into a single-chip ICthat fits into a tiny 2 mm x 2 mm package. In fact, the resulting UVindex sensor IC product is so small that 655 of these sensors can fitacross a typical 8.5-inch-wide page.

Because most wearables arebattery-operated, they are highly power constrained, requiring veryenergy-efficient components. With this consideration in mind, SiliconLabs designed the Si1132 and Si114x UV index sensors (Figure 4) toensure very low current consumption, using as little as 1.2 µA for UVmeasurements performed once per second.

Standby current is lessthan 500nA. The Si1132/4x sensors are factory-calibrated to addresspart-to-part variation for more accurate measurements. The sensorsinclude an industry-standard I2C interface to communicate digital UVindex values to the host processor. To ensure that the sensor’s UVreadings match medically important frequency ranges, the UV index sensorperforms measurements that closely match the weighted UVA and UVBwavelengths of the CIE spectrum.

Figure 4: Example of single-chip UV Index sensor IC architecture

Asubsystem that already includes acquisition and processing electronicscan readily incorporate other forms of light sensing, all of which canbe accessed over a single I2C bus. This approach is used by the SiliconLabs UV index sensor family to incorporate not only ambient lightsensing – used to detect visible light levels – but also infraredsensing for proximity detection.

In addition, the Si114x sensorsintegrate up to three LED drivers that can be used to develop gestureinterfaces, which are becoming increasingly important for wearablesystems. The LED drivers can also be used to enable heart-rate andpulse-oximetry measurements when the LEDs and associated sensor arepressed to the skin. Development of advanced motion and gesture sensingis aided by a programmer’s toolkit and API.

The ambient lightsensor enables easy integration of other wellness functions in additionto UV measurement. For example, an ambient light sensor in a wearabledevice that is designed to be worn night and day can be used to indicateto the host MCU when it is dark, indicating that the wearer is likelyto be asleep. Readings from the wearable system’s accelerometers canindicate whether sleep patterns are disturbed during the night. Theambient light sensor also helps improve the wearable product’s userinterface and battery life by adjusting display brightness levels basedon the level of incident visible light.

By integrating UV indexsensing and processing capabilities into a tiny, energy-efficient,single-chip solution and including other features for user interfacefunctions and wellness monitoring, Silicon Labs has taken a major steptoward making UV sensing an integral feature for the fast-growingconsumer wearable-device market.

Kevin Kilbane servesas a senior product manager for Silicon Labs’ optical sensor products.Mr. Kilbane joined Silicon Labs in 2010. He holds a bachelor’s degree inElectrical Engineering from Cornell University.

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