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Halftone Screens
The term Halftone Screen refers to the pattern of dots of varying sizes applied to an image of varying tones, or same sized dots applied to a tint of colour, when output to - film for the printing processes - or laser printed artwork etc..

Grey Scale

Halftoned Grey Scale

Elliptical, Round and Square dot shapes

Rather than use an actual photo as a sample, a step wedge similar to the top picture is best used as a reference containing obvious (stepped) tonal variations. The second picture is a halftoned representation of the step wedge. The darker the image, the larger the halftone dots become up to and beyond a point where the dots begin to (virtually) overlap.

The following definitions affect the result (quality) of the final output:
Screen Ruling - the number of dots per linear inch measured along the axis of each row.
Screen Angle - defining the angle of the axis.
Screen Dot Shape - generally Elliptical, Round or Square.
Screen Dot Range - the minimum to maximum range always measured as a percentage, 0% = white, 100% = solid.
Highlight Dot size - the smallest dot (% - original copy white).
Shadow Dot Size - the largest dot (% - original copy black)
Midtone Dot Size - a grey dot (%) relative to the original copies middle greys.
Pre Desktop
To help understand the process of creating halftone dots we could look at the traditional method using prepress cameras (pre desktop computer - these days you will be using scanners).

The last era of Camera Operating (a trade qualification) used:

1.High contrast sheet film,
2.A process camera (large floor standing),
3. A Contact Screen.

Portion of a Contact Screen

I have displayed an enlarged portion of a Contact Screen (above). These high precision pieces of film had a cris-cross pattern of varying density in the emulsion. The Contact Screen was vacuumed in perfect contact on unexposed film so that when a given exposure / development time was applied using a special camera, - via light reflecting from the copy, passing through the lens and then the Contact Screen - it (the controlled exposure) created varying dot sizes on the film in response to the amount, or brightness, of light transmitted from areas of the original, once developed ( picture below).

Copy, to Lens, to Contact Screen, to Film

i.e. Enough light from the lighter areas of the original penetrates even the denser areas of the contact screen creating large dots on the final film negative, whereas the small amount of light reflected from the darker areas of the original could only penetrate the contact screen where it was of least density and create only small dots on the film negative.

 Copy, to Contact Screen, to Negative, to Final Print

The third image, (above), displays light from the original passing through the Contact Screen creating the Negative film images, above representative dot sizes as they would print. The Film Negatives mentioned would be used to make printing plates which carried the positive or reverse of the film negatives image.

Traditional four CMYK screen angles

Screen(#) Ruling and Angles
Also defined simply with the Hash sign, (i.e. 133#) the Screen Ruling is determined by the number of halftone dots counted per linear inch, measured along the axis of a row of dots.

Therefore a job sheet requesting the use of 150# implies that the screen ruling required would equal 150 halftone dots per inch. The image above displays the traditional angles used in four colour process work (CMYK) where the axis of the rows equals the angle.

Single colour jobs use a screen angle of 45 degrees for halftoned images and colour tints (flat fills of a percentage of the solid colour). The 45 degree angle is the least noticeable and offensive to the human eye whereas an angle of 90 degrees is very obvious and draws attention to the dots rather than the image detail (the picture).

Animated Screen Angles

Multiple coloured images have always created immense problems for the prepress technicians and printers. Ideally each colour in a multi coloured halftone picture must be EXACTLY 30 DEGREES apart. But 90 degrees divided by 30 equals 3 so we have a problem with a four colour process (CMYK) picture because of the fourth colour. (an halftone screen has two axis, one 90 degrees to the other. Therefore only three halftone screen reference angles, 30 degree apart, can be used per 360 degrees as the situation duplicates for each 90 degree set)

The pattern created by 2 or 3 halftone screens layed at 30 degrees to each other is called a rose pattern, a seemingly simple and unobtrusive pattern that normally does not offend the eye. However as soon as we diverge from this ideal we start see a moiré pattern that progressively gets worse. One colour out by even 1 degree will start to clash with at least one of the other colours and destroy the rose pattern; (= moire pattern).

To overcome the '4 instead of 3' situation, the colour of least contrast, Yellow, is angled precisely 15 degrees between two of the other colours. The 'black image on clear base films' used to make the printing plates look terrible when overlayed yet we do not see the error once printed.

Traditionally the colour of most contrast is set at the visually ideal angle of 45 degrees, and the colour is of course Black in a CMYK job. However a more conscientious tradesperson will often set a very predominant picture colour (Magentas or Reds = the Magenta, Cyans or Yellow-Greens = the Cyan) at the 45 degree angle and place the Black at 75 or 105 degrees - if the black is a normal or light 'Key or ghost ' image.

Note: There are variations to the common screening techniques discussed here such as Stochastic Screening, HiFi Colour etc. mostly applicable to specialised areas of imaging and printing that I may get time to look at in the future.

Halftone Screens CONTINUE on PAGE 2

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