When working with screen rulings, particularly those above 200 LPI or FM screening, you may need to know the size of dots in microns. This is to make sure that the plates, plate imaging system, press, and ink pigments are all capable of delivering the minimum printing dots through the process. For example, if the dot size is 10 microns but the ink pigment size is 25 microns there may not be enough dot surface area for the pigment to stick to and hence that tone will be dropped out on press. Or, if the screen calls for a 10 micron dot but the plate can only hold a 20 micron dot then, again, that tone will be lost or have to be compensated for by employing hybrid screening techniques.
Dot diameter in microns can be calculated using the following formula.
D = Dot area in percent (e.g. 1% dot equals .01)
F = Screen frequency in lines per millimeter (LPM)
To convert the screen frequency from lines per inch use the following formula (2,540 dpi device on left and 2,400 dpi device on right):
While this formula is not absolutely exact, it gives a close enough approximation for most practical purposes.
A few things to keep in mind about halftone dot size. The formula applies to the size of dot that is generated by the halftone screening algorithms in the RIP that will be sent to the imaging device. It does not calculate the size of the dot that appears in the final presswork which may have been affected by dot gain or loss. Also, a RIP will only image full individual pixels to form a halftone dot. So, in the case of a 2,540 dpi device, each pixel is 10 microns in size (10.6 microns for a 2,400 dpi device). Therefore, if, for example, the formula says that the final diameter of the dot is 15 microns what will happen is that the RIP will alternate between 1 pixel dots (10 micron) and 2 pixel dots (20 micron) which results in an effective 15 micron dot average for that tone value. You can see that happening by watching the dot formation in the lighter tones at the start of the video located HERE.
Below is a quick reference comparison chart showing the dot diameters for tone values of 1% and 2% for various screen rulings from 10 LPI to 400 LPI on a 2,540 dpi platesetter.Highlighted areas are where the required dot is smaller than a single pixel at the device's resolution and therefore will not be imaged.