PRINTING PROCESSES 435 



Sources of Illumination. — Present-day enlarging is done largely with incandescent 

 lamps. At one time daylight was utilized, either by pointing the negative end of the 

 enlarger to the sky or by directing skylight into the projection equipment by means of 

 a mirror. Daylight is verj^ seldom used, today, however. 



The incandescent lamps may be of the ordinary house-lighting kind designed to 

 have a normal life of 1000 hr.; they may be of the photoflood type which are operated 

 at higher than normal temperatures so that the illumination is increased at the 

 expense of decreased life; or ordinarj^ electric lamps may be burned at a voltage 

 slightly above normal so that they furnish more than their rated amount of light but 

 less light than a photoflood lamp. 



Mercury and other vapor lamps are occasionally recommended for printing. A 

 A^apor tube is much more efficient than an incandescent lamp. A given amount of 

 power consumed will produce a greater quantity of light. The lamp will run cooler 

 than the bulb type of illuminant, and there will, therefore, be less danger of harming a 

 negative from excessive heat. 



These vapor lamps, however, have certain disadvantages. They require time to 

 heat up. They cannot be turned off and on like an incandescent lamp. When first 

 turned on a mercury lamp is very rich in ultraviolet light, but after 10 or 20 min. the 

 intensity of the ultraviolet decreases, and therefore the printing time varies as the 

 tube warms up. To get even illumination special reflectors are needed, and somewhat 

 heavy diffusion must often be used between light source and negative. The over-all 

 efficiency, therefore, may be no higher than with an ordinary incandescent bulb. 



Effect of Lamp Voltage upon Exposure. — Few photographers realize the close rela- 

 tion between light output of incandescent lamps and the voltage at which they are 

 operated. Not only the luminous output, but its spectral distribution as well, is 

 dependent upon the operating voltage. For example, a 1000-watt lamp will emit 

 only 50 per cent of its rated output if its voltage is reduced by 16 per cent. If expo- 

 sures are required that seem too long, the trouble may be due to reduced voltage on 

 the light source. Printing papers are not sensitive to yellow light, and yet the output 

 of an incandescent lamp becomes more and more yellow and less and less blue as the 

 voltage is decreased from normal. A paper that requires 10 sec. exposure when 

 exposed to a 115-volt incandescent lamp operating at normal voltage may require about 

 30 per cent more exp osure if the lamp is operated at 110 volts. 



Chlorobromide papers are somewhat more critical in this regard because their 

 sensitivity is largely in the violet region of the spectrum. Enlarging lamps are 

 available in voltages of 110, 115, and 120 volts and the photographer should use the 

 lamp whose rated voltage is nearest his normal line voltage. Lamps operated at 

 higher than rated voltage will permit shorter exposure times. The lamp life -will be 

 shortened, however. 



Lamp voltage should be accurately regulated by means of resistance or by means 

 of an autotransformer of the Variac tj^pe if exposures must be critically controlled. 

 This will enable the separate exposures, as in color photography, to be made at the 

 same voltage, and color balance in the final print will be more easily attained. The 

 autotransformer is a means of raising or lowering the lamp voltage compared to 

 the line voltage. A resistance will lower lamp voltage but cannot raise it above line 

 voltage. 



Exposure and Enlargement. — Although modern projection papers have certain 

 latitude as regards correct exposure, it is advisable, alwaj's, to come as close as possible 

 to the "best" exposure. Not only must the photographer determine the absolute 

 value of the exposure, but he must determine the contrast grade of the printing paper 

 he is to use. A negative of considerable contrast maj'^ require a paper of less than 

 normal contrast. Negatives made in miniature cameras, for example, are usually 



