TruFade from James Heal can operate in either "controlled irradiance mode" or "blue wool mode".
The term "blue wool mode" is also synonymous with "nominal power mode".
Blue Wool mode can be used when Controlled Irradiance is not required or not available.
The electrical power to the xenon lamp is set to a nominal value and the endpoint of the test is determined by the fading of the Blue Wool references. As the xenon lamp ages with use, the radiant energy declines and therefore the testing time will be extended (it will take more time).
With controlled irradiance, there is closed loop control. The radiant energy from the xenon lamp is measured and the electrical power to the lamp varied accordingly. This permits the dosage of radiant energy to be calculated as the test progresses and the test time remains more consistent. Some standards state exposure to a specified dosage of radiant energy without the use of Blue Wool references.
Nowadays, the use of controlled irradiance mode is becoming more prevalent. Many standards now require controlled irradiance but continue to use the Blue Wool references to determine the end-point of the test.
TruFade uses the SolarSens radiometer, which also incorporates a black standard thermometer, to measure the radiant energy with a 300-400 nm broadband sensor.