Sun:
The sun makes an excellent light source if weather conditions permit. Exposure can be in direct sun or in the shade with differing results. If in direct sun, a short stick mounted perpendicular to the Contact Printing Frame will indicate when the frame is aimed directly at the sun when no shadow is showing. Printing times may vary throughout the year as well as throughout the day. Most consistency can be kept by printing from 10:00 am through 2:00 pm. Logging the date and exposure in the Notebook can eventually provide information as to the need for any correction for the time of year. For more controlled light and printing at night or anytime, artificial lamps can be used.
UV Lamps:
There are many lamps available for use as UV sources. The most important aspect is to have uniform intensity in the exposure area. The best way to accomplish this is to have the illumination source the same size (or larger) as the exposure area. The long tube lamps work well for this purpose. Several straight tubes may be placed side by side to make whatever size area desirable . U-shaped tubular lamps may cost less overall, and should be mounted in two layers one offset above the other (see following diagram).
Since the Pt/Pd coating is mostly sensitive to ultra violet and blue light, tubes should be chosen that have their maximum output in this part of the spectrum. These lamps may be designated "BL" or super actinic. It is not immediately obvious which lamp will have the highest output. These lamps are ratted by wattage, and it is soon discovered that the higher the wattage the longer the tube. The highest output lamp can be found by dividing the wattage by the tube length, and then, comparing this number (the larger, the more powerful). The trick here is that what is needed is the power output per area. The tube diameter is usually small in comparison to its length, so using just the length will provide an accurate enough calculation. Catalogs readily list the wattage and length of tubes.
An excellent U-shaped lamp is the General Electric F40BL/U/3. Six can cover a 20 x 24 inch area at a distance of 12 inches (or even larger, 30" x 36" with good uniformity at a little further distance). When ordering the lamps make sure to get the proper ballast for the lamps. And, this is a good time to get the electrical sockets. This way everything should be compatible.
A wooden housing may be constructed for the lamps. If wood is used running a ground wire or grounded metal strip within proximity of each lamp will help assure proper starting. The inside of the lamp housing should be painted with a highly reflective flat white paint. This will get more light to the working area. Do not use polished or metal reflectors as this will only cause non uniformity problems in the exposure area. The working area can be painted flat black. The housing should be light tight. A black cloth with 1" to 2" wide Velcro around the opening will make a good light seal and keep it easy to open and place the printing frame into the exposure area. The reason the exposure area should be light tight is so that the UV light will not interfere with other steps (such as clearing or coating) that may be occurring simultaneously in the same room. Having the exposure area in a separate room would be helpful especially when dodging on burning with the cloth open, but would require more movement from room to room.
For dodging and burning the cloth will have to be open. During that time make sure no other UV sensitive activities are occurring within proximity. Also protect eyes and skin whenever exposed to the light (see additional equipment below).
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Additional Equipment:
Glasses that block all Ultra Violet light - These must be used when dodging, burning, or looking at lamps or the reflected UV light from lamps. Make sure the the light is blocked from the sides of the glasses as well such as may be done with glacier glasses or goggles.Gloves and long sleeves - These will be used when dodging or burning, it is easy to get a burn from working under UV lamps. Sun-block may be used, but wash it off hands and fingers to avoid getting any on the print.
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Sun vs UV Lamps:
The following table lists and compares some of the differences between using the sun and using UV lamps.
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Difference Sun UV Lamps collimated light yes - parallel and sharp no - diffused spectrum change by atmospheric conditions by lamp type intensity can vary by hour of day and time of year fairly consistent availability partial always reliability by chance high sharpness high depends on contact temperature may get very hot cooler than sun exposure times typically several minutes usually longer than sun equipment cost minimal (stand and shadow stick) several hundred dollars exposure control by covering by timer switch
