United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant Quarantine
A PHOTOGRAPHIC METHOD FOR RECORDING SIZE OF SPRAY DROPS
By J. M. Davis
Division of Forest Insect Investigations
In analyzing the results of tests conducted with various types of
equipment for spraying insecticides by airplane, we found evidence
rather early that the degree of atomization was a governing factor in
the lateral distribution of the deposit. A search of the literature in-
dicated that the only method of determining a drop spectrum adequately
was to measure and count the drops recovered in a sample of the spray.
The requisite number of drops to be measured in an adequate sample
increases tremendously as the drop size increases. Since we were
dealing with sprays containing rather large drops, counting and meas-
uring them with a microscope was exceedingly impractical. Therefore,
we decided to avoid that method. Projected photographs appeared to
offer the best possibilities. Since some sprays evaporate rapidly, we
especially desired to develop a method in which the spray drops could
be photographed in the field. The method evolved is essentially as de-
Photographing the Drops
The samples are collected on microscope slides coated with an
oleophobic film. In a typical test from 21 to 61 stations are prearranged
at 10- or 20-foot intervals in a line perpendicular to the flight line, and
three slides are placed at each station. Usually the falling spray covers
only part of the line of stations. The spray is allowed to settle for 10
minutes after the plane has been flown over the sample line. The three
slides from each station are then picked up and put in a clip to hold them
parallel to each other. Each clip is placed in front of the lens of the
camera, as shown in figure 1, with the middle slide in focus and the
slides photographed. Only the middle third of each slide is recorded
by this camera, which was developed especially for this work. The lens
used most has a focal length of 4 3/4 inches but those of other focal
lengths can be employed, depending on the magnification required. The
bellows is rigid, being a plywood box with shields inside to subdue re-
flections from the sides. The film is held in the back by a 4- by 5-inch
film-pack adapter. This combination always gives the same magnifica-
tion (about 4 times), since nothing is adjustable.
The camera is shown in figure 2. Beyond the lens is a circular target
centered on the optical axis. The size and placing of this target should
be such that its image just completely covers the negative at the lens
stop used. The one shown here is 5 5/8 inches in diameter and is 21
inches from the lens. This arrangement gives negatives of high contrast,
in which the background is transparent and the images of the drops are
black circles or crescents. The exposures are made with the optical
axis pointing directly at the sun. At f:64 1- or 2-second exposures are
used for most panchromatic films on days of average brightness, and
longer exposures under overcast conditions. At f:64 the depth of focus
is sufficient to include the three slides used. Drops on the near slides
will occasionally conceal a drop on a more distant slide, but the re-
sulting error is negligible. With a somewhat larger f number, this
camera has been used to photograph as many as eight slides simultane-
ously. Figure 3 illustrates the type of record obtained with this camera.
If the spray consists of relatively small drops, having a mass median
diameter 1 of less than 100 microns, a 2-inch focal-length lens is sub-
stituted for the 4 3/4-inch lens, and a 2 1/2-inch target at about 12 inches
is substituted for the 5 5/8-inch target at 21 inches. Only one slide is
used, because at this magnification it is difficult to get depth of focus
for more than one. One slide provides a sufficient sample, since the
smaller drops are more uniform and fewer are needed. With this larger
magnification also a smaller area is included in the photograph, but with
smaller drops there are many more drops per unit area. It is possible
that lenses of still shorter focal length could be used with very small
drops, but we have not had occasion to measure drops where the mass
median diameter was less than about 80 microns.
The apparatus described is usable when the smallest drops to be
measured are about 30 microns in diameter. The mass median diameter
is then usually greater than 100 microns. With the 2-inch focal-length
lens the range is somewhat less than half the range with the 4 3/4-inch
Measuring and Counting the Drops
The drops are measured and counted on a projection of the photographic
negative. A cold-light photographic enlarger was altered so as to project
upward from below onto a ground-glass table top. The time required to
I/ The mass median diameter is the drop diameter which satisfies
the condition that half the volume of spray is in drops larger and half in
drops smaller than it.
count and measure the drops on one negative is usually too great to allow
use of a conventional enlarger; hence the cold light was used. The over-
all magnification of camera and projector must be known. We determine
this by photographing the jaws of a vernier caliper set at exactly 1 inch
or other convenient length and then measuring the final projection of this
opening. We use a magnification of 14.25, but this value has no particular
merit over any other. The diameters of the drop images are measured,
and the images classified according to size. Each class has a diameter
range of 1 mm., i.e., 0.5 mm. smaller and larger than the unit specified.
These data are combined and totaled for all stations in the flight and
sometimes for several flights, and the average-drop spectrum computed.
Determination of Spread Factor
In order to determine the actual size of the drops, it is necessary to
know the spread factor, or the number of times larger the resting drops
are than the spherical drops from which they resulted. This spread
factor is determined by a method similar to one described by May (1).
This method involves two steps--(l) measurement by microscope of the
diameters and focal lengths of a few representative drops, and (2) deter-
mination of the index of refraction of the material. It has been found
advisable to tap the slide a few times with a pencil or fingernail before
making microscope measurements to help the drops establish their
equilibrium shape. The data are somewhat scattering; nevertheless, one
is able to determine the average ratio, which is assumed to be a straight
line with drops of the size ordinarily encountered. A vertical line upward
from the point on the nomograph (fig. 5) where this straight line intersects
the line of constant index refraction gives the value of the spread factor
on the scale at the top.
The product of the spread factor and the over-all magnification
represents the number of times larger the images are than the original
spherical drops. The micron size is found for each class by multiplying
the class size by 1,000 and dividing by the product mentioned.
Each class size is cubed and multiplied by the number of drops in the
class. These products are then expressed as percentages of the total and
cumulated. The size corresponding to the 50-percent cumulation is the
mass median diameter. The sizes corresponding to 25- and 75-percent
cumulations indicate the range of size represented by the middle 50 per-
cent of the volume deposited.
In determining these various points we found it convenient to plot the
cumulative volume percent against the micron size on log-probability
paper, as shown in figure 4. On this curve the mass median diameter is
seen to be 237 microns and the diameter range of the middle 50-percent
volume is 164 to 353 microns. In the class having the smallest diameter
the images of the drops may be confused with those of dust particles,
pollen grains, etc. Therefore, data for this class are usually discarded.
Although there may be a large number of these drops, they do not rep-
resent much volume, and the mass median diameter and the middle
50-percent volume are affected only very slightly when they are discarded.
Advantages of the Photographic Method
(1) It provides a permanent record which can be measured when
(2) It is fairly rapid. Two workers can tabulate several thousand
drops per hour.
(3) It is fairly clean, involving no smoke or coatings other than
oleophobic material, which is simple to use, and is readily obtainable.
(4) It avoids errors due to evaporation from drop surfaces.
(1) May, K. R.
1945. The cascade impactor; an instrument for sampling coarse
aerosols. Jour. Sci. Instr. 22(10): 187.
Figure 1.-- Three paralled slides mounted in the field of the camera lens.
Figure 2.--Camera for recording drop size of aerial sprays.
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Figure 3.--Sample photographic record of drops collected on three
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20 30 40 50 60 70 80 90 95
PERCENT VOLUME (CUMULATIVE)
$8 99 99.5 99.8 99.9
Figure 4.--Cumulative percent volume plotted against diameter of drops.
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FOCAL LENGTH, MICRONS
Figure 5.- -A chart for determining the spread factor when the index of refraction of the
material and the ratio of focal length to the diameter of resting drops are known
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UNIVERSITY OF FLORIDA
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