Sun Sensor for Sensible Exposure



Every summer we are warned of the dangers of sunburn and the risk of skin cancer due to excessive exposure to UV radiation. The UV index is a daily feature of most weather forecasts. The ability to accurately measure personal exposure to UV radiation is fast becoming a ‘must have’ gadget, whether as a stand-alone device, smartphone app, or an added feature to a smart watch, fitness bracelet or heart rate monitor.

Manufacturers are continually seeking to differentiate and add value to their digital consumer products. Fortunately, component miniaturization through integration combined with low-power operation facilitates this trend. With the addition of simple ambient and UV light sensors, wearable digital devices can easily measure the UV index, monitor our cumulative exposure to sunlight and provide warnings when pre-set levels are exceeded.

This article will outline a number of approaches to building a UV exposure monitoring function, focusing on photodiodes for UV and IR sensing. A more detailed look at the integrated approach offered by Silicon Labs follows. Ideal for miniature, wearable devices, the Si1132 and Si114x single-chip solutions are complemented by evaluation boards and software.

Sunshine in moderation

The digital UV index is an international standard measurement of the strength of UV radiation from the sun. Developed in Canada, then adopted and standardized by the World Health Organization (WHO) and World Meteorological Organization in 1994, it is now used globally. The UV index is linearly related to the intensity of sunlight, and is weighted according to the Erythemal Action Spectrum, (adjusting for the UV wavelengths to which human skin is most sensitive) developed by the CIE (International Commission on Illumination).

Effectively, the higher the index value, the greater risk of sunburn. An index value of 10 was originally established to correspond approximately to midday summer sun with a clear sky in the northern hemisphere. However, in the tropics, or at high altitude, or in areas known to suffer from a depleted ozone layer, index values can be higher. Predictions are made by a computer model that accounts for the effects of sun elevation, stratospheric ozone, cloud, air pollution and ground altitude. With this linear scale, we can generally accept that an hour’s exposure at an index value of 4 is approximately equivalent to half an hour at an index level of 8.

Over exposure to UV radiation not only results in sunburn, and, ultimately, the development of skin cancer, but can also be the cause of eye damage such as cataracts. Importantly, skin damage from sun exposure is cumulative during our lifetime. Skin cancer is reported as the most common form of cancer in the US and its incidence is rising fast elsewhere.

While UVA radiation (315 to 400 nm) is largely responsible for skin aging and melanoma, UVB radiation (280 to 315 nm) causes sunburn. However, in small doses, UV radiation is beneficial to our health. Vitamin D, for example, is only synthesized by the body with the help of UV radiation. UVA is beneficial in activating the melanin pigment in the outer skin layer, giving us a tan as well as some protection. Thus, fair skinned people with less melanin pigment are at a greater risk of sunburn and melanoma.

The UV index changes throughout the day, and, while sunscreen provides effective protection when applied as directed, the SPF value is regarded as an imperfect measure of the level of protection delivered. For these reasons, the wearable UV index monitor is in demand to more accurately determine the current UV index and one’s accumulated UV exposure.


Figure 1: Wearable UV exposure monitoring comes in many forms. Bling jewelry – June from Netatmo and fun bracelet - Sunfriend, will both communicate with your smartphone. Meanwhile for the strictly techy, smartwatches, like this one from Apple, are likely to include the feature.

An expanding range of wearable UV monitors is becoming available, ranging in cost, style and functionality. The electronic design varies too. Conventional models use UV-sensitive photodiodes, microcontroller, A to D converter and signal processing firmware. Some models might use an inverted LED - blue (415 nm) is a favorite - instead of a photodiode. The downside is that it only monitors UVA.

Pre-programmed algorithms automatically calculate the UV exposure, taking into account the data from the UV sensor. Some units may not monitor UV continually, but just once when the device is activated. Typically, the user can set the device according to skin tone and sensitivity. Some can be programmed according to the SPF of sunscreen used. Alerts may be provided by the display, flashing LEDs or vibration.

Power is an important issue, but not covered in this article. Briefly, some consumer devices are sealed with a coin cell battery, and while they are usefully waterproof, the battery cannot be replaced, giving a limited product lifetime. Others use rechargeable batteries but are not waterproof. Energy harvesting using a miniature solar cell is the obvious solution, and this has been implemented in some designs.

Integration is the key

Silicon Labs is taking a lead in the UV index and exposure sensing and monitoring business, with its Si1132. Designed as a single-chip UV index sensor, it will not only track UV exposure, but also provides heart rate monitoring and blood oximetry measurement for fitness applications, as well as proximity/gesture control for remote interfacing. The device includes UV index and ambient light sensors, with an industry-standard I²C interface to read the digital UV index values.

Complementary devices include the Si1145/46/47 UV and IR proximity/ambient light sensors, offering a choice of one, two or three integrated LED drivers with fifteen selectable drive levels for gesture detection. The ambient light sensor is useful for minimizing the use of LCD backlighting, both to ease eyestrain and reduce power.

In fact, Silicon Labs has implemented an ultra-low-power architecture with these UV index sensors, enabling thinner wearable designs with smaller batteries, and extending battery life with as little as 1.2 µA average current for once-per-second UV measurements. Standby power is quoted at less than 500 nA.

For designers of multifunction wearable devices that include UV exposure monitoring, the Si1146 and Si1147 include two and three IR LED drivers respectively, and are designed for advanced motion and gesture sensing. The Si1146 enables motion sensing and touchless control in the z and x axes, while the Si1147 enables 3D motion sensing.

A key advantage of the Silicon Labs solutions for wearable devices is the high level of integration into a small 2 x 2 mm package, reducing the design footprint and the bill of materials.

Designers keen to try out this device have a couple of options. The UVlrSlider2EK evaluation kit supports both the Si1132 and the Si114x families. The software contains a demonstration of the 11 point UV index and a five LED display shows a user’s accumulated sun exposure. The kit also demonstrates the gesture recognition capabilities and proximity sensing features of the devices.

A programmer’s toolkit is freely available, which includes an API to enable rapid development of software using a C compiler in a PC environment in conjunction with the evaluation board. A waveform viewer application is also included to display and debug the measurements taken from the evaluation board.

Conclusion

Monitoring UV exposure is set to become a standard feature on a wide range of wearable devices, including smart watches and glasses, and health and fitness monitors. Low cost and light weight, single-function UV Index monitors, in the form of bracelets, rings or lapel buttons, are likely to increase in popularity as awareness increases and concerns grow about the cancer risk of over exposure to the sun’s rays.

As a result, designers will be seeking low cost, low power, small size (even flexible) UV sensors and photodiodes, plus ancillary circuitry, to incorporate into the next generation of wearable devices. Highly integrated ICs, such as the components illustrated in this article, will be in demand and proliferate.

Additional Information

Organic option

A potential alternative to silicon photodetectors is the organic photodiode. Already being used to improve light sensitivity in cameras and for checking color homogeneity, stability and brightness distribution in displays, organic photodiodes are powerful, lightweight, cheap to produce and can be flexible. They can be integrated onto polymer foils, for example, for wearable applications.


Figure 2: Organic photodiodes are lightweight, cheap to produce and can be integrated onto flexible substrates. (Courtesy of Fraunhofer COMEDD)

Organic materials tend to be sensitive to a particular wavelength range, making each suitable for specific applications. Scientists at the Fraunhofer Research Institution for Organics, Materials and Electronic Devices (COMEDD)¹ in Dresden, Germany, have trialed a range of materials covering a broad wavelength spectrum. For use in the UV or near-infrared range, the team is further developing tiny micro-sensors that combine organic semiconductors with silicon technology. The ability to use 200 mm silicon wafer substrates or foils on carrier wafers will facilitate the transfer of this technology to the broader scope of industrial and even consumer applications.

References:
  1. Fraunhofer Institute COMEDD
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