Irradiance is the measurement (milliwatts per centimeter squared or mW/cm²) of how much light energy a surface receives from a light source. The more powerful the light source, or closer in proximity the light is, the higher the irradiance.
Why is irradiance so important?
Other than the price tag, irradiance along with wavelength are the two most important factors when choosing an LED light therapy device. If irradiance is not taken into consideration, then you could end up with a low-power device that will have little to no effect on the body. Even if the light is the exact wavelength you are after—if the irradiance is low, it's not going to have that great of an effect on the body, thus the results and benefits will be much more limited.
This is why a red LED Christmas light is not as effective as a red light therapy device. They both emit red light, but the irradiance of the LED Christmas light is so low that it's not going to produce any benefits.
Some products claim to have a certain amount of irradiance, but neglect to mention that irradiance was measured at the surface of the light itself, rather than at the actual distance you would use a red light therapy device: around 6-12 inches away. When measured at these distances, Orion devices produce the highest irradiance levels compared to other devices.
Having adequate irradiance is the difference between needing 5 or 30 minutes of exposure. Orion simply offers the best value on the market. You get the most effective wavelengths at the highest irradiance, and for the most affordable price.