I have a lot of information on the construction and method of operation of each of these three units. I'll post more about them when all the results are in. Having said that, I do want to say that the ESECO unit is by far the most sophisticated of the bunch. It actually has photoreceptors in the light chamber that monitor the LED intensity and correct the time based on the intensity. That takes care of any warm up and decay issues with the LEDS. It also addresses any LED intensity changes as the battery ages. As a practical note, the ESECO prints a much nicer step wedge on the film (compared to a Stouffer wedge). The numbers are clearer and it is much easier to align the test patches with the densitometer aperture and still see the wedge number.