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
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.
Operation and Results
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:
correct UVI = (Sunwatch II reading)* 1.162 - 0.195
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.
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)
The Sunwatch II can be ordered at the following places:
Personal Protection Technologies, Postbus 1049, NL-9400 BA Assen, fax (0031) 5920 176 31
Conrad Electronic GmbH, Klaus-Conrad Str. 1, D-92240 Hirschau, fax (0049) 96 22 30 434