I am the author of the article here on the use of a spot meter to measure negative (transmission) densities. The use of the spot meter under the conditions specified gives repeatable though not necessarily accurate data. I have NEVER been able to get repeatable data from reflective (print) readings. I attribute this to my inability to furnish a reliable and repeatable light source. There are just too many uncontrollable variables for this type of use IMO anyway. I’m sure with the application of funding and considerable thought, it can be done, but by then wouldn’t it be just as economical and much easier to purchase a densitometer?