With the default configuration of two layers, the optical density of the dark circles is 9, so the reduction in light levels of 1/10^4.5, or 0.000000001 of the input light.

With a single layer, the optical density of the dark circles is 4.5, or reduction in light levels of 1/10^4.5 or 0.000316 of the input light. This should be adequate for most applications. 

To obtain valid measurements, the chart must be photographed in a completely darkened environment. Any reflections on the surface of the chart will invalidate the results.

Review the Operational Guide for the Lightpad 950.