The Sunwatch II sensor from SAITEK is an inexpensive small, portable UVI sensor and exposure-time watch for general use. Powered by a small battery, the Sunwatch II can be clipped to a skirt-pocket or worn like a normal watch. Skin type can be programmed into its memory, and the sensor shows the remaining "safe" exposition time for the day. Besides this information, the display shows in small, difficult to read letters the actual UVI (with a 0.1 resolution).
The UV sensing element is placed under a curved plastic window; readings vary considerably with its orientation in respect to the sun, as the mountings of sensor may partially block the sun-light reaching the sensible element. To get coherent and reproducible results, the Sunwatch II was tilted and turned until the readings were maximum. These maximum values are used in the calibration check. When the sensor was hold straight, without moving, the readings were always stable; after a change, readings took about 2 to 4 seconds to stabilize definitively. All checks were done in the open, at the Lycée Classique de Diekirch (latitude ca. 50 degrees North, altitude ca 180 m), 6 to 15 June 1996, morning to late afternoon hours.
The UVI readings given by the Sunwatch II were mostly compared to the readings given by a Solar Light UVB_Biometer 501A; a small number (about 6) of comparisons were done with the hand-held Model 3D UVB meter from Solar Light. The readings of this latter were adjusted to those of the UVB_Biometer. The Solar Light UVB_Biometer is a high quality, temperature stabilized professionnel instrument, whose readings can be assumed correct. To convert the given MED/h to UVI, the formula UVI = 25 *(MED/h)/9 was used.
The following graph shows the "correct" UVB_Biometer readings versus those of the Sunwatch II. The Sunwatch readings are always too low, but can be expressed as a linear function of the correct readings. To get the correct UVI, the Sunwatch II readings should be multiplied by about 1.16. The correct relation-ship is:
One can not expect from a cheap general purpose instrument the same accuracy as from a professional sensor. Nevertheless, a mean difference of less than 20% between the readings of the Sunwatch II and the correct value seems tolerable. As only one single Sunwatch II has been checked, we can not be sure if the relation-ship found in this report will hold for all sensors.
It should be noted that all UVI measurements of the Sunwatch II reflect the maximum reading given for a proper orientation.
In summary:
To get a meaningful UVI result from the Sunwatch II, do the following:
- turn and tilt the Sunwatch II to get a maximum stable reading
- multiply this reading by 1.16 and subtract 0.2 from the result (this is not much different from adding 20% to the Sunwatch reading)