The instrumentation used to measure UV index for the program is directed toward the in a global normal (perpendicular to level ground) inclination. Therefore the UVI meter is designed to operate in the same fashion. Solar irradiance measured this way includes direct (reduced by cosine law depending on solar angle) plus diffuse (diffused by the atmosphere). This vertical (global) reading represents an average intensity value your body experience while being active on a tennis or volleyball court, or lying flat on a blanket on the grass. If however, you are reclined in a lounge chair facing the sun perpendicular to the sun angle, you experience a higher UV intensity, as represented by pointing the UVI meter directly at the sun. This value typically exceed the actual UV index number. Technical Parameter Index Range:0-15.0 Response:280~400 nm Resolution:0.1 Index Conv. Rat:3.0 Readings/Sec Display:3.5 Digit LCD Digit Size:0.4 inch high Oper. Temp:32 Fahrenheit TO 90 Fahrenheit Oper. Humid. :5% TO 80% RH Accuracy:±10% NIST Colour:yellow Material:plastic Package Contents: 1 * Digital UVI Meter Only the above package content, other products are not included. Note: Light and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.