Contrast enhancement and small detail blending of photographic images

A method and apparatus for improving the quality of photographic images by projection through a linear radial transmission filter. The invention yields small detail blending or contrast enhancement of small details with a minimum loss of sharpness. A simple enlarger and a linear radial transmission filter are used to process a wide variety of images and are capable of simple adjustment to vary the resulting degree of blending or enhancement.

BACKGROUND OF THE INVENTION 
This invention relates generally to photographic printing and specifically 
to a method and apparatus for the improving of print quality by blending 
or enhancement of small detail. 
Photographic negatives frequently contain two problems which give rise to 
unsatisfactory appearance and difficulty in information retrieval from the 
final photographic print. One problem, graininess, is typified by the 
granular appearance of a print caused by enlargement to such a scale that 
the elements composing the gray background of the picture show up as 
distinct areas. This problem also appears in photographs of television 
pictures, caused by the horizontal scan lines of the television picture 
and in the halftone dots in newspaper photographs. 
The second problem, lost detail, is one caused by the limited dynamic range 
of normal photographs in regard to contrast. This is apparent when high 
contrast between large areas of a photograph masks small detail which 
actually exists on the negative. This detail is not retrievable by normal 
printing methods because such printing methods result in very low local 
contrast in the extremely light and extremely dark areas. While methods 
exist to overcome these problems, all suffer from either the compromising 
of picture quality or from the need for complex and expensive equipment. 
For instance, graininess may be removed by simple defocussing but excessive 
small detail is thereby lost. Also a conventional unsharp mask technique 
without a linear transmission filter may be used to reduce large scale 
contrast while retaining small detail, but this method gives up some 
exposure control accuracy and cannot blend small detail. Spatial filtering 
techniques may be used to accomplish both goals, to enhance or to blend 
details in prints, but they require highly accurate and expensive 
equipment and are complex to use. Spatial filters also limit the size 
negative which can be used and limit the overall image quality available. 
The present invention makes available a simple, low cost method for both 
blending small detail and enhancing the contrast of details otherwise lost 
because of high contrast areas on the photograph. 
SUMMARY OF THE INVENTION 
The present invention uses an average quality projection system along with 
a linear radial transmission filter to project an image of a negative for 
printing. A typical arrangement can use a standard enlarger set-up. The 
linear radial transmission filter is an optical quality filter which has 
maximum transmission in the center, and whose transmission decreases 
toward zero at the outer edge in a linear function. 
When a linear radial transmission filter is placed in front of the enlarger 
lens the focused image is not affected (except for a drop in overall 
exposure intensity), but the out-of-focus image will change considerably. 
The out-of-focus image of each point of the negative projected through the 
linear radial transmission filter becomes a small image of the filter 
whose diameter depends only on the degree of defocusing. The brightness 
across any diameter of the out-of-focus spot varies exactly as an 
isosceles triangle. 
Mathematically, the addition of the heights of equal isosceles triangles 
whose base sizes equal twice the spacing between their peaks results in a 
single plateau with a height equal to the peak of the triangles and with 
the end slope identical to the sides of one of the triangles. 
In the same manner, the out-of-focus image will exactly blend equal 
intensity spots whose centers are spaced at distances equal to the spot 
radii into a larger area of continuous brightness. While the spots 
resulting from graininess in the typical photographic print are not, in 
general, equally spaced or equally bright so that no one setting exists 
which can perfectly blend all the spots, the goal is to arrive at evenly 
shaded areas in which the individual picture elements are least 
discernible. This goal is achieved by projecting and then adjusting the 
image to that setting which yields a smooth and even shading between the 
individual picture elements. 
In one typical arrangement, with the linear radial transmission filter 
directly in front of an enlarger lens, the adjustment of spot size is 
accomplished by simply varying the distance at which the print paper is 
placed from the point of sharp focus. Thus, greater blending of graininess 
and small detail is accomplished by moving the paper further from the 
in-focus point until the setting which most closely approximates the 
situation illustrated in FIG. 2 is achieved. This setting is determined by 
visual observation of two adjacent picture elements and is discussed in 
detail below. The above described technique permits the production of 
photographic prints or transparencies in which grain structure is blended 
but where there is very little sacrifice of sharpness. 
The same essential technique, with the addition of one step, can yield a 
print in which large scale contrast is muted to better bring out the 
contrast of smaller details. This process, small detail enhancement, is 
accomplished using the technique noted above to produce a positive 
transparency as previously mentioned. This positive transparency is made 
so that small detail, up to the size for which enhancement is desired, is 
blended during its production. The transparency is then used as a mask 
over the original negative during a second projection operation without 
the linear radial transmission filter for the making of the final print. 
In this operation, since the positive transparency and the negative are 
the same size, details which have not been blended in the positive 
transparency exactly counteract the same details in the negative. However, 
small details which have been blended in the positive transparency are 
only projected with generally lower intensity. A resulting print therefore 
has much lower contrast between all large areas, that is, the large dark 
areas are printed much less darkly, while the small details are shown with 
only slightly less contrast than originally. It requires only the use of 
higher contrast paper to result in increased contrast for the small 
details and approximately the original contrast in the large areas. 
Small detail enhancement therefore results. Appropriate use of varying 
contrast positive transparencies and varying contrast paper for the final 
print can yield a wide variation in enhancement of small detail or 
suppression of large high contrast areas depending upon results desired.

DETAILED DESCRIPTION OF THE INVENTION 
Structure of the Preferred Embodiment 
The preferred embodiment selected for illustration is similar to a 
conventional photographic enlarger and is shown in FIG. 1. The apparatus 
shown includes an optical projector 10 within which is a light source 12 
controlled by light control 13. The light source illuminates an image 
source 14 such as a conventional photographic negative. The image created 
by the light source 12 and the image source 14 is projected through lens 
16 onto projection surface 18 or onto any surface covering the projection 
surface such as the photographic paper 20. The optical projector 10 and 
projection surface 18 are maintained in rigid alinement regardless of 
their separation by upper holder 24 slidably attached to alinement rod 26 
by clamp 28 and by lower holder 22 similarly attached by clamp 30. The 
entire structure is maintained in a vertical orientation for convenient 
access by base 32 to which alinement rod 26 is conventionally attached. 
Lens 16 of optical projector 10 is maintained in linear alinement with 
light source 12, image source 14 and projection surface 18 by holder 34, 
which is attached to threaded extension 36 by drive mechanism 38. Threaded 
extension 36 is, in turn, rigidly attached to upper holder 24. Holder 34 
also rigidly attaches linear radial transmission filter 40 to optical 
projector 10 so that linear radial transmission filter 40 is held in close 
proximity to lens 16 and intercepts the projected image. The filter 40 may 
alternatively be placed in contact with lens 16. 
The linear radial transmission filter 40 is a device whose light 
transmission characteristics vary linearly with the distance of any 
particular point from the center of the filter. The light transmission of 
the filter is maximum at the exact center of the filter and no light is 
transmitted through the filter at its outer edge. The variation of 
transmission characteristic between the center and edges vary directly as 
the distance from the center. Thus the filter, generally constructed as a 
circle, has a transmission characteristic linearly decreasing with the 
distance from the center. The effect is that the measurement of light 
transmission of such a filter taken across any diameter begins with no 
transmission at one end, rises linearly to a peak at the center and then 
falls linearly to no transmission at the other end of the diameter. The 
graph of such a characteristic is best represented by a simple isosceles 
triangle. 
OPERATION OF THE PREFERRED EMBODIMENT 
The operation of the preferred embodiment shown in FIG. 1 is particularly 
dependent on the above-described characteristics of linear radial 
transmission filter 40. When the focus of optical projection 10 upon 
projection surface 18 is adjusted by sliding either upper arm 24 or lower 
arm 22 upon alinement rod 26 or adjusting drive mechanism 38 such that the 
projected image upon projection surface 18 is sharply in focus, linear 
radial transmission filter 40 has no effect upon the image. But when the 
apparatus is adjusted off the sharp focus point, for instance, by moving 
projection surface 18 closer to optical projector 10, each spot of light 
from which the image is composed is modified in such a way as to blend 
small detail or eliminate graininess. 
Because the image is transmitted through linear radial transmission filter 
40, each of these light spots or picture elements themselves take on the 
characteristic pattern of the filter. Thus the defocused light spots each 
appear as a spot varying linearly in intensity across any diameter and 
peaking in the center of the spot. The spot characteristic, like the 
filter characteristic can be represented by an isosceles triangle. 
The mathematical property of a series of isosceles triangles is that they 
can be overlapped and their heights added so that they form a perfect 
plateau with sides equal to the sides of the end triangles of the series. 
This result can be accomplished when the triangles are oriented with a 
peak-to-peak spacing of exactly one half their base length. This situation 
is illustrated in FIG. 2 where isosceles triangles 50, 51, 52, and 53 are 
added to form plateau with sides 54 and 56 and top 55. The same effect 
occurs when the light intensity of equally spaced picture elements is 
filtered through a linear radial transmission filter and the defocusing of 
the image is adjusted to expand each picture element to a size such that 
the edge of each picture element just reaches the centers of adjacent 
picture elements. The result at this defocus point is a large area of even 
intensity of light where previously there existed a multiplicity of 
individual picture elements. While the situation described above of 
equally sized elements exactly equally spaced does not exist in any actual 
physical case, the perfect case can be approached closely enough so that 
the average picture element size and spacing yields a very satisfactory 
blending of graininess or other small detail. 
To accomplish the blending of graininess in the preferred embodiment, an 
image source 14, usually a photographic negative, is projected onto 
projection surface 18 through linear radial transmission filter 40 by 
optical projector 10. By sliding either upper holder 24 or lower holder 22 
along alinement rod 26 and adjustment of drive mechanism 38 the desired 
size of image is secured and the image is sharply focused. The image is 
then defocused to a point where the desired degree of blending of small 
detail or graininess is attained by further adjustment of drive mechanism 
38, upper holder 24, or lower holder 22. This desired degree of blending 
is attained by observing two adjacent typical picture elements and 
defocusing to that point where those elements of the picture, the "grains" 
of a typical photographic projection, are seen to blend into the adjacent 
elements. The defocusing is continued until the blending is complete as 
indicated by no substantial difference in shading between the points where 
the previous spots existed. At this point the situation illustrated in 
FIG. 2 is approximated. Should the defocusing progress too far, a picture 
spot will begin to form midway between the previous focused spot, and the 
adjustment must be backed off. When the desired setting is reached 
photographic paper 20 is placed upon projection surface 18 and 
conventional exposure is accomplished by optical projector 10 through 
linear radial transmission filter 40. This is followed by conventional 
processing to yield a print or transparency with graininess and small 
detail blended. 
The second objective of the invention, enhancement of small detail, can be 
accomplished by a continuation of the above described operation. For this 
result a positive transparency mask is produced by the method described 
above. This transparency is made either the exact size of the print 
ultimately required or of the original negative. The choice of either size 
is largely influenced by the factors such as size of the negative, 
capability of the optical projector for projecting an image the same size 
as the negative and the number of prints to be ultimately produced. For a 
high quantity of prints, for instance, it is easier to make the mask the 
size of the negative because it requires the alinement of negative and 
mask only once. 
When the positive transparency mask is produced as discussed above, the 
small details for which enhancement is desired in the final print are 
completely blended during the production of the positive transparency. It 
is this action which creates the opposite result in the final print. Once 
the positive transparency is processed it is used during a second 
projection operation to mute contrast of the final print by preventing the 
projection of large areas of light which later become dark, high-contrast 
areas. 
This problem and the method can best be understood by the attempt to print 
a large black square with several small gray dots close together within 
it. Typically this is difficult to accomplish because the high exposure of 
the black square and the limited print paper exposure range "washes out" 
the presence of the gray dots. To overcome this, the positive transparency 
mask is produced after blending together the several dots as described by 
the method above. This yields a positive transparency mask with 
essentially one object, a large dark square slightly lighter in the area 
of the previous dots. If the mask was produced in the appropriate size it 
is taped to the previous negative so that the dark square on the mask 
exactly overlaps the light square of the negative. The darkness of the 
square is determined by the contrast of the film used to create the 
transparency and will itself determine the degree of enhancement in the 
final print. When this combination of mask and negative is now projected 
for a printing operation, this time with the linear radial filter removed 
from the projection system and the system in sharp focus, the squares on 
the negative and the mask essentially counteract each other and project 
only dimly. The original dark dots on the negative, however, are not 
counteracted by the mask since no dots exist on the mask. These dots 
therefore project less light than the rest of the square and show up as 
spots with greater contrast on the final print. The degree of enhancement 
of such features as the spots may be controlled by the use of varying 
contrast paper in the final print as well as the contrast of the film used 
for the mask. The mask may also be laid directly upon the final print 
paper, where a sharp image is produced, and, if alined with the projected 
image, will accomplish the same result as when attached to the negative. 
To aid in the alinement of the mask with either the negative or the 
projected image, it is desirable to use several holes on the negative 
placed outside the area of interest on the final print. These holes may 
also be used during the production of a mask of the same size as the 
negative by using the measurements between them on the negative to check 
the image size before exposure. 
Another embodiment of the invention is pictured schematically in FIG. 3. In 
this embodiment the linear radial transmission filter may be used in 
conjunction with an immediate-viewing projection system to blend undesired 
picture elements. In a television projection system, for example, the 
horizontal scan lines become more and more objectionable as the image is 
enlarged. As shown in FIG. 3, linear radial transmission filter 64 is 
attached to projector 60 in close proximity to lens 62 by holder 68. The 
image is then projected upon projection screen 66 and the projection 
system focus adjusted by adjustment 70 until the picture elements are 
blended for satisfactory viewing. 
It is to be understood that the forms of the invention herein shown are 
merely preferred embodiments. Various changes may be made in the shape, 
size or arrangements of parts; equivalent means may be substituted for 
those illustrated and described and certain features may be used 
independently from other features without departing from the spirit and 
scope of the invention. For example, the linear radial transmission filter 
may be placed on the side of the lens nearest the light source rather than 
as shown on the side nearest the projection surface. In addition, the 
linear radial transmission filter may be located internal to the lens at 
the lens stop. Moreover, in FIG. 1 base 32 may be substituted for 
projection surface 18 by the simple removal of projection surface 18. And 
relating to FIG. 3, the projector may be one used for motion pictures as 
well as one used for projecting television images.