What exactly is "65 microW/cm(2)" of light? I'm not familiar with this measurement.?

From this: "Light was administered via a portable light source comprising two light-emitting diodes per eye, with the irradiance of each diode set at 65 microW/cm(2)" http://www.ncbi.nlm.nih.gov/pubmed/14962066. What exactly is this measurement, how much light are these LEDs producing, and how can it be converted to lux or lumens?