The Principle
- To engrave a photograph into glass, a negative
artwork is required. The final halftone artwork for
PrecisionMasks should look something like the
images below..... a negative halftone image,
surrounded by clear space and with a border and
R as in the artwork instructions.
Preparing the Photograph
- We recommend engraving with a halftone resolution
of 100 lines per inch (lpi). This is because the smallest
dot that will reliably engrave is approximately 70micron
(3 thou or mil). It is the difference between this
smallest dot and the full sized dot of the halftone
that determines the available tonal range. So, whilst
it is perfectly possible to engrave halftones of 120
or even 140 lpi, this dot size differential - and thus
their tonal range - is much reduced.
- Scan (in full colour, even if it is a black and white
photo) or import the required photograph in to your
image manipulation program (e.g. Adobe Photoshop).
- If you wish to repair or cut out the image (as above)
it is generally easier to do this whilst it is still in colour.
- Convert to greyscale and resample the image to its
final size with a resolution of around 150 pixels/inch.
- Levels - adjust to make sure that you are using the
full available range.
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Unsharp Mask - Over sharpen a photograph for
engraving use. We regularly unsharp mask to
between 100 and 200% and 2 to 5 pixels, with the
threshold adjustment levels depending upon the
image - fewer levels for scenery, more for facial
tones.
- Invert the image (i.e. make it a negative).
- Curves - this adustment allows a fine control of the
sizes of the halftone dots, particularly at the ends of
the spectrum. At 100lpi, anything over 7 or 8% will
engrave and anything above 95% will blast away. If
you are doing a cut out image, ensure that shadow
dots at the image edge (in this case, the reins) are
reasonably large. They are adjusted to 20%+ here,
making the engraved image more visible. Whereas
they can be left at 7% in the uncut version because
they are surrounded by other detail with which to
contrast. You may wish to save and then temporarily
convert the image to halftone at this stage, re-invert
it and print it at 200%, simply to check the results.
-
Save as a greyscale (i.e. do not convert to a halftone
- let your bureau do that at the output stage). If you
wish to add text, the black border and the R and
so on, you may find it easier to do this in a vector
drawing program (such as Freehand or Illustrator),
in which case save your greyscale image in a suitable
format for later import (e.g. Tiff, for Freehand).
Output, Exposure & Blasting
- (3 thou or mil). It is the difference between this
smallest dot and the full sized dot of the halftone
that determines the available tonal range. So, whilst
A film positive output from a process camera or
image-setter is essential. Ask your bureau for "film
positive, right reading, emulsion up, 100lpi elliptical
dot halftone at 45°".
-
Preferably expose only one sheet at a time directly
over the lamp using 30% of the centre exposure
setting as described in our instruction sheets, "How
to Calibrate your Exposure Unit".
-
When blasting, do not attempt to cut any depth - this is a surface finish only. Depth will both wear away the smaller dots and generate additional reflections from the sides of the cut, spoiling the image. Using a fine grit, preferably one that is already well worn, a lower air pressure and, with the nozzle further away than normal (almost wafting the grit onto the mask), use a series of brisk, parallel passes, both across and up and down the image. This will help avoid banding.
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