Halftone Screen

Ink the brayer and roll it once, firmly, on paper. The ink is one density everywhere it touches. There is no half-ink. Write down what that means for printing a photograph.

Examine a newspaper or magazine photo under the magnifying glass. Find the dots. Note that they sit on a regular grid but differ in size.

On tracing paper, rule parallel ink lines 2 mm apart across a 100 mm square. Keep them straight and evenly spaced.

On a second sheet, rule lines 2 mm apart but alternate thick and thin. This is Levy's actual claim — varying line thickness, not uniform ruling.

Lay the second sheet over the first with the rulings at right angles. Hold to the light. You now have a grid of transparent apertures — and because one set alternates, the apertures come in different sizes, in groups. That is US 521,659.

Paint a strip on watercolour paper grading from near-white to near-black in five steps.

Hold the crossed screen a few millimetres above the wedge and photograph it with a phone camera. Keep the screen parallel to the paper.

Zoom in on the photograph. In the light zone the dots are small; in the dark zone they merge. Tone became dot size.

For each of the five wedge steps, measure the dot width on screen. Tabulate tone versus dot size — this is the transfer curve every printer still works with.

Count lines per centimetre on your screen and convert to lines per inch. Levy's commercial screens ran from about 65 to 175 lines per inch; the linked Science Museum example is 133.

Turn the top sheet to 45°. Levy's Figure 3 claims four sets of lines with the second pair crossing diagonally. Note how the pattern regularity changes.

Now rotate slowly through small angles and watch interference bands appear. Moiré is the failure mode that dictates screen angles in colour printing to this day.

Ink the brayer lightly, roll over the crossed screen onto paper, and lift. You have physically screened an area of flat tone into dots.

US 521,659, “Screen for Half-Tone Process”, granted 19 June 1894, filed 1 March 1894.

The patent names Max Levy alone. This is worth stating carefully. Max and his brother Louis Levy together produced the first commercial halftone screens in Philadelphia in 1890, and the credit for that work is genuinely joint. But this particular document carries one inventor. Sources that describe “the Levy brothers' patent” are compressing a company into a filing — the same confusion that surrounds the so-called Rice–Kellogg loudspeaker patent, which likewise names one man.

The Levys' screens were good enough that, as Britannica puts it, there has been no significant change in the method of making halftone screens since. Their manufacturing process — rule, etch with hydrofluoric acid, fill, cement at 90° — remained standard for decades.

Halftone predates Levy. Frederic Eugene Ives patented halftone processes in 1881 and introduced an improved screen in 1885, and credited himself with the first crossline screen made by cementing two single-line screens with Canada balsam. Levy's contribution is the screen's manufacture and geometry, not the idea of screening. Do not read this patent as the invention of the halftone.

What survives today: every printed photograph you have ever seen, and the CMYK screen angles chosen specifically to keep the moiré you produced in step 12 from appearing.