already optimum, being over 100 lines/mm on axis and 60 lines/mm
in the corners, referred to the negative plane. A gross estimate
of the radial distortion A in the projected image can be given
as a function of the enlargement V from the linear function
A = U • D.01 mm. It is, then, appreciably smaller than the
acceptable plotting accuracy af + D.1 mm and can therefore be
treated as negligible even at 7x enlargement.
For exploiting the blue-sensitivity of the emulsion as much as
possible in order tD shorten exposure times, the short wave
lengths were taken into consideration in computing the Reprogon.
The lens is corrected for the spectral range of approximately
350 to 700 m^u.
The very small fall-off in luminance in the Reprogon meets the
requirement for uniform illumination. The light source is a 120
lilatt mercury vapor lamp with a high proportion of actinic light.
It is housed in a reflector in which it can be centered. A four-
lens, optically-corrected condensor provides uniform illumination
of the whole negative and, at all enlargements, concentrates the
light into the diaphragm. According to the enlargement, the
height of the lamp housing is adjusted automaticallyfor this
purpose by a curve-controlled system. The negative is protected
from the effects of heat by a heat filter. Further, a ventilator
can be switched on to cool the lamp and the condensor space.
In the case of thin negatives or small enlargements, the use of
an absorption filter is indicated so that the exposure times are
not too short on account of the brightness of the illumination.
Similarly, use of a graded-density filter is recommended for en
larging negatives made with lenses having a marked off-axis de
crease in luminance. The stage on which these filters are placed
is directly below the lamp housing, that is, not in the path of
the rays coming from the lens. It is therefore not necessary
that the filters be of expensive quality. The filter stage can
therefore also be used to carry masks for suppressing gross
contrasts.
