232 
PHOTOGRAMMETRIC ENGINEERING 
plates are mechanically transported 
in the Y direction during the scan opera 
tion at a constant rate of approximately 
7 mm per second. 
2. A line-scan cathode ray tube is the light 
source for scanning and recording. The 
light-producing electron beam is incre 
mentally deflected in the x-direction by 
digital and analog electronic circuitry at 
a 15-kc rate. 
3. Optical elements (lens, mirrors, beam 
splitters)—The light beam emanating 
from the phosphor surface of the CRT 
is split, focused, and directed to identi 
cal photo-coordinate positions by a lens 
system. The light passing through the 
diapositives is directed by an optical 
collector to the sensing photomultiplier 
tubes. 
4. Photomultipliers—which produce a volt 
age output proportional to the imping 
ing light and, hence, to the transmissiv 
ity of the area of the diapositive being 
scanned. 
5. Linear amplifiers, with adjustable gain, 
which match the photomultiplier out 
puts to the A/D converter inputs. 
6. A/D converters—A voltage level of 0 v 
to 5 v is provided at the A/D converter 
input. The exact voltage level is propor 
tional to the “gray shade” of the incre 
mented area being scanned. The voltage 
signal is quantized by the converter in 8 
(or 16 or 32) equal increments, and the 
appropriate number of bits is assigned, 
based on the quantization level. 
7. Control logic—which allows the scanner 
to be operated in a continuous sequen 
tial mode and is also used to initiate and 
halt operations. Figure 11 shows a 
schematic of the Scanner System. 
OPERATION 
Since digital computers operate only with 
discrete data, the conversion of the pictorial 
content to digital data is a prerequisite for 
this form of photographic data processing. 
The Scan Mode of the modified WILD STK-1 
is used to sample the approximate transmissiv 
ity (T) of definite areas on the photograph 
and convert them to an 8-gray-shade (or 
“density” level) code. Each gray-shade value 
is coded into a binary number and combined 
with an error-check code before being trans 
ferred to magnetic tape. 
Because of the line-scan tube character 
istics and the limited size of internal memory, 
the data on tape is grouped in convenient- 
size blocks, called a scan record. This record 
consists of the 1728 gray-shade values of a 
4.32-inch scan (linear density of 16/mm). 
Since the scanner digitizes two photographs 
simultaneously, two gray values are packed 
into a single character. The three low-order 
binary bits carry the right photo gray codes; 
the three high-order bits carry the left photo 
gray codes. The IBM 7094 takes its input