# How do you measure the irradiance of UVC LEDs? askjohn@luxreview.com was asked this question by Joe Van Vooren: ‘If a UVC LED specification states 6µW/m2 (microwatts per centimetre squared) at 100mA, does that mean radiation over one square centimetre at a distance of one centimetre from the LED?’

We asked Dr Kevin Kahn, a field application engineer at Crystal IS, for a definitive answer.

A dimension expressed in microwatts per centimetre squared refers to the irradiance (or intensity), and is the optical power per unit area collected upon a surface at a specified distance. For the specifications mentioned, this means that when the LED is operating at an input current of 100mA, the UVC power received at a particular distance from the LED is six microwatts on an area that is one centimetre squared.

The relationship between irradiance and source-surface distance can be expressed by the inverse square law: the peak intensity on a surface decreases exponentially with distance. For instance, let’s take a 10mW UVC LED in your example, with a 6µW/cm2 peak intensity at the surface. You could estimate the distance between the LED and the surface as being √(P/I) = √(10/0.006) = 40.8cm.

Of course, this is an approximation. The real value would require to take into account the viewing angle, radiation pattern, area of interest on the surface, and other external factors such as potential reflection from the environment. In most applications, design engineers are using these variables (and others like drive current) to maximise the irradiance to meet their application requirements.

Because of these variations, UVC LED manufacturers typically do not specify a standard irradiance value on their data sheets, but rather the characteristics of the LED itself. 