A standard inkjet printer renders grey flats through variations in droplet density — the darker the area needs to be, the more drops the printer lays down per unit of surface. On a normal paper page, the result is imperceptible: the eye sees a continuous grey. On an inkjet transparency, sometimes it's a problem — the drops can cluster into visible bands, or create stray patterns that contaminate the scan measurement.
The A4 high-resolution test chart with dithering fixes this problem by building the dithering into the source file rather than leaving it to the print driver. You download a chart that already contains a calibrated Floyd-Steinberg or halftone pattern — each grey level is represented by a pattern of black and white dots, at very high resolution. The printer just has to transcribe that pattern, which every printer does well (since they print in pure black or nothing).
#What you see
You download this variant from the types of test chart menu. The PNG is denser than the plain versions — 300 dpi instead of the 150 dpi typical of the standard versions. On screen, at 100% zoom, you see the chart look like a normal image with 25 grey patches. At 400% zoom, the fine structure of the dithering appears — each "grey patch" is in fact a dense pattern of black and white pixels.
You print this file on an inkjet transparency without color management and with the print settings typical for transparencies (maximum resolution or "photo"). The printed transparency shows the same structure: areas that look grey at normal distance, but made of a pattern of dots on close inspection.
When you print the test chart, you expose this transparency onto your sensitized paper like any negative. The chemistry receives light through the "holes" of the pattern (the white pixels of the transparency) and blocks it on the inked areas (the black pixels). The result on the final paper looks like a print made from a continuous flat — the resolution is too fine for the eye to distinguish the individual dots.
At the scan, Calibration Flow measures the average density in each patch of the printed chart. The fine dithering has been smoothed out by the chemistry, and the app gets a stable, reproducible measurement.
#Why it's valuable
For consumer printers that render flats poorly. Many consumer Epson, Canon or HP printers have limitations on solid grey flats — horizontal banding, density variations from one area to another, stray patterns. The chart with built-in dithering sidesteps these limitations: the printer prints only black or nothing, which is mechanically more stable.
For carbon and Aquaprint. These two pigment-based processes prefer patterns over flats — see Dither for the reasons. The chart with dithering gives you a suitable negative directly, with no need to rework the file after download.
For reproducibility across printers. If you change printers mid-run, the standard grey flat can come out differently from one machine to another — hence calibration charts that aren't comparable. With built-in dithering, both printers reproduce the same dot pattern, and the measurement becomes comparable.
#When you don't need it
For a high-end professional printer. If you have an Epson SureColor SC-P700/P900 or equivalent, flats come out flawless. The plain version is more than enough, and even gives you a finer reading (since dithering always adds a bit of noise).
For cyanotype, platinotype and other processes where flats aren't a problem. The plain chart gives a better rendering — smoother tonal transitions, more precise measurement. Dithering adds nothing and can introduce a light grain.
For everyday calibrations. If your test charts come out well without dithering, don't overcomplicate it. The dithered version is a solution to a specific problem, not a general improvement.
#Key points
| Element | Value |
|---|---|
| Format | A4 landscape high-resolution (typically 300 dpi) |
| Dithering used | Floyd-Steinberg or AM halftone built into the file |
| Polarity available | Positive and negative (two variants in the menu) |
| Relevant cases | Difficult printer, carbon, Aquaprint |
| Non-relevant cases | Pro printer, cyanotype, platinotype, beginner |
| Reading at zoom | Dot pattern visible at 400% |
| Appearance at a distance | Indistinguishable from a grey flat |
#The test
Download the plain variant and the dithered variant of the same polarity. Print both on the same inkjet transparency, with exactly the same settings. Compare the two transparencies at 10 cm from your eye under a desk lamp: the plain version may show banding or irregularities in the flats; the dithered version should show a uniform, fine pattern. If the plain version comes out flawless, keep it for your future calibrations — you don't need the dithering.