After all this talk about doing it using various hardware: If you are not interested in numerical densities, why not calibrate by printing? This does not involve any maths, and it has the additional advantage of accounting for paper characteristics, too (which becomes a disadvantage if you switch to a different paper.)(posted 8848 days ago)BTW: Pete: Remember reading in my description that I recommend measuring all negatives at the same spot of the baseboard? This eliminates uneven lighting and any angular sensitivity. As to stray light: Assuming that the stray light has a constant intensity (which is reasonable, as it usually comes - via walls and ceiling - from the enlarger head which is on during all measurements) its importance grows as the measured intensity decreases, i.e. at higher densities. So the uncertainty in the higher densities is greater. I think it might even be reasonable to infer that this is not an uncertainty but rather a systematic error, as falsely high intensities, whence falsely low densities are determined. So the method tends to underestimate the high densities, and consequently the gradient, but probably not too much if you take some care to minimise stray light. The critical low densities, however, are not affected to the same degree, because there, the signal to be measured (intensity transmitted by the negative) is high, stray light being constant.