Seven terms you'll meet in the first ten minutes of use. Short definitions, calibrated not to slow you down. If you want the exhaustive version — twenty-five terms — it's in the full glossary.
#Test chart
A grid of grey squares ordered from white to black, which you print, expose like a normal negative, and then photograph with Calibration Flow to measure the process's tonal response. The test chart is the central measurement object of the process. It's documented in detail in Understanding a test chart.
#Patch
A single square of the grid. A Calibration Flow test chart contains 25 patches in a 5×5 grid, from pure white (patch 0) to full black (patch 24), in steps of about 4% intensity. Each patch gets an independent measurement.
#Density (optical)
The opacity measurement of a patch. A very light patch has a low density; a very dark patch has a high density. Calibration Flow does not measure an optical density in the hardware-densitometer sense — it measures the perceptual luminance L* CIELAB of the pixel, the scale that matches human perception of grey.
#Curve (corrective)
The result of the calibration. A mathematical function that tells your image: "before printing on transparency, change your pixels like this to compensate for the process's non-linearity." The curve exports to
.acv#Black point (Dmax)
The highest density your process can reach on your paper. On an Arches 300 g cyanotype, that's typically 1.4. On a Hahnemühle Platinum Rag platinotype, up to 1.8. The corrective curve never exceeds this limit — it's physics.
#White point (Dmin)
The lowest density — the paper white after washing. Always non-zero (paper is never perfectly white spectroscopically). Typical density: 0.05 to 0.15 depending on the paper.
#Polarity (negative / positive — positive interpositive)
A negative chart has white on the right and black on the left (inverted image). A positive chart is in normal orientation. Vision Picturale rule: all VP processes call for a negative chart, except résinotype, which calls for a positive interpositive. Rationale: in résinotype, the pigments settle on the black zones of the negative (the unexposed zones). For the other processes (cyanotype, Aquaprint, bromoil, charbon, gumoil) and for printer calibration: negative chart. The Positive vs negative chart page gives the full table.
If you're missing a term, open the full glossary — it holds twenty-five.