Several years ago I started my quest into non-toxic photo-polymer photogravure and in the process did some research on contact printing with UV light sources. (I don't have my notes to hand but will make a concerted effort to find them over the next few days and post actual test data). I will however try to summarize my findings (as memory allows) as follows:

a) different bulb types emit varying amount of UV; (big duh...!)

b) an integrator is a useful tool for measuring UV output especially as bulbs heat up (duh...!)

keep reading we're getting to the good part.... :{D


c) the between the source and the receptor glass filters out UV light transmission; some bulbs such as street light have a secondary glass mantle for exactly this purpose.

>>> How does this matter to Alternate Process practioners ? <<<

Consider the glass that stands between your receptor (print/neg/pos) and the light source in your contact-printing frame. It is filtering out (BLOCKING) some of the UV light coming in; this may explain why people are getting different results with the same light source.

TO VERIFY THIS: take a reading with your light integrator below the glass and compare the reading above the glass. The reading below will always be lower, the question is by how much. If the ratio is large then simply replace the glass with a high UV efficiency glass.

THE SOLUTION: [PHOTOCOPIER GLASS -> WHY? read on] Photocopiers work on the principle that UV light is used to adjust the static charge on the printing drum. The white areas of the paper reflect a lot of UV that destroys the static charge and thus the toner is not attracted to that area resulting in a clear or white area. Conversely where black lines do not reflect UV light the static charge remains intact and picks up toner resulting in a black area. To ensure that UV light passes through the glass efficiently photocopier glass is specifically manufactured to allow high rates of UV transmission.

When I built my contact-printing frame I contacted several parts suppliers for photocopiers until I found one with surplus glass for non-current models that I bought at a reduced price (it's somewaht expensive). I think the size was odd something like 15x19.5 and I didn't attempt to cut the glass. Instead I built the frame around it. My exposure times (I use a 1000 watt Olite) were about 40% of the old 1920's frame that I was borrowing.

As a side comment it is worth noting that for photogravure and other printing methods that involve burning a matrix a single source light such as the Olite must be used instead of a multiple source such as rows of fluorescent tubes. The reason is that the matrix must be projected into the rector media as a cylinder (single source) instead of a cone (multiple source). A cone result with multiple sources because the source on one side partially cancels out the projection from the other side and vice versa. This is not desirable because when the amount of development reduces the depth of the matrix and with a cone the cross-section decreases with depth whereas the cross-section is constant with a cylinder.

Anyway thatís probably enough for you to chew on for the moment.

Hope this helps. -Terrance