The light intensity (or irradiance) of the Ember printer varies from day to day for a number of reasons. Ultimately, this variation can lead to failure in the printing process. Too much light, and the printed part will stick to the window, and jam the printer. Too little and no polymer forms.
The easiest way to account for this change is to measure the light intensity of the printer, and then adjust the exposure time, so that the dose (total amount of light received) is the same from day to day. That is
Dose of light (mJ/cm2) = Light intensity (mW/cm2) x exposure time (s)
This Instructable will outline how to measure the light intensity with a G&R Labs 221 radiometer with a 420 nm probe. An ILT1400 radiometer and an SEL005 probe and an A#28762 filter would also work. However, the build arm of the printer needs to be removed beause of the size of the probe
First the printer tray needs to be removed, drained, and carefully cleaned with a solvent (isopropanol preferably). Any resin left on the window will be transferred to the radiometer probe and gunk it up.