Hello Pierre,
It was great to see you and other forum members at the show.
About overexposure.
Here is one way to look at it.
With a 50-100x microscope we examine a 40-50% halftone. We look specifically at the side wall of the halftone to see if there is a dark halo, or a light halo around the dot, or whatever dot shape you use, and we view the dot from the print side of the screen. A light halo inside the halftone dot shows overexposure. The dot has been undercut with too much light and the emulsion opening on the squeegee side is smaller than the print side. The sidewall is angling inward to create the smaller halftone opening on the squeegee side. With a dark circle the opening on the squeegee side is wider due to the developing water washing away soft emulsion and we are seeing the shadow and emulsion when viewing from the print side. The more the halftone dot shape has no dark or light halos around them the more vertical the halftone sidewall. A halftone like this will not dot gain as easily (underexposure) or require more pressure to clear <20% dots, (overexposure). Helps keep sim process sharp and tonal values accurate, although some dot gain is to be expected, thats why halftone outputs to film should be linearized to compensate for dot gain of 10-20%
This all assumes you have a well functioning lamp. If your lamp is old you can both overexpose and underexpose a screen, huh? OK I did have band practice last night but I've sobered up by now, so this can happen. What is going on is the exposure time in light units may be long due to a weak MH lamp or if you use weak chaotic fluorescent lamps. The time needed to get the emulsion film exposed to avoid slime on the inside may be so long that undercutting is occuring, and an older lamp's light may have lost UV power and is not cross linking the emulsion completely. Typically discharge or HSA inks will cause premature breakdown on what was supposed to be adequate or more than adequate time. The strength of the light in the 360-420 nanometer wavelengths is key. If the bulb has aged it may still shoot a screen well for plastisol, but for discharge it can breakdown in hundreds of prints once the lamp has lost strength. So yes you can over expose and under expose on the same screen but it just won't be visible until it breaks down.
However there are times where overexposing can help simple spot art for discharge. Bumping exposure 10-20% isn't going to hurt solid spot art and will give it a little more durability when shot with a fresh MH, or LED.
Al
