A simple, easy-to-use optoelectronic tool projects scale marks that become incorporated into photographic images (including film and electronic images). The sizes of objects depicted in the images can readily be measured by reference to the scale marks. The role played by the scale marks projected by this tool is the same as that of the scale marks on a ruler placed in a scene for the purpose of establishing a length scale. However, this tool offers the advantage that it can put scale marks quickly and safely in any visible location, including a location in which placement of a ruler would be difficult, unsafe, or time-consuming.
The tool (see Figure 1) includes an aluminum housing, within which are mounted four laser diodes that operate at a wavelength of 670 nm. The laser diodes are spaced 1 in. (2.54 cm) apart along a baseline. The laser diodes are mounted with setscrews, which are used to adjust their beams to make them all parallel to each other and perpendicular to the baseline. During the adjustment process, the effect of the adjustments is observed by measuring the positions of the laser-beam spots on a target 80 ft (≈24 m) away. Once the adjustments have been completed, the laser beams define three 1-in. (2.54-cm) intervals and the location of each beam is defined to within 1/16 in. (≈1.6 mm) at any target distance out to about 80 ft (≈24 m).
The distance between the laser-beam spots as seen in an image is strictly defined only along an axis parallel to the baseline and perpendicular to the laser beam (also perpendicular to the line of sight of the camera, assuming that the camera-to-target distance is much greater than the distance between the tool and the camera lens). If a flat target surface illuminated by the laser beams is tilted with respect to the aforesaid axis, then the distance along the target surface between scale marks is proportional to the secant of the tilt angle. If one knows the tilt angle, one can correct for it. Even if one does not know the tilt angle precisely, it may not matter: For example, at a tilt of 10°, the secant is approximately 1.0154, so that the tilt error is only about 1.54 percent, which is negligibly small for a typical application in which only approximate measurements are needed.
Each diode laser generates a light beam having a power of 3 mW and consumes an input power of 150 mW. The laser diodes are powered by a lithium cell that can sustain operation for an interval of an hour or more. Because the optical performances of the laser diodes are equivalent to those of most laser-based auditorium pointers, the use of the tool should not pose a major concern for eye safety - provided, of course, that one observes the usual precaution of not looking directly into the laser beams.
The tool can readily be attached to almost any camera by use of the standard tripod nut on the underside of the camera. Once the tool is thus attached and properly aligned, it projects the laser scale marks wherever the camera is aimed.
The basic principle of operation of this tool is amenable to a number of potential variations of its design. For example, the number of laser beams could be different from four. For another example, one of the laser beams could be aimed at a known angle relative to the others so that the different distances between laser-beam spots in an image can be used to estimate the distance between the camera/tool combination and the target.
For yet another example, one could reduce the cost of the tool by using a single laser in conjunction with a non-optimum inexpensive simple beam-splitting device to generate all four beams. In this case (see Figure 2), the beam-splitting device would be a flat glass plate coated to be partially reflective on one surface and highly reflective on the other surface. Because the parallelism of the output laser beams would depend only on the parallelism of the glass surfaces and the distance between successive beams would depend only the thickness of the glass surfaces and would vary uncritically with the tilt of the plate, this design would offer the advantage of simplification of alignment. The one shortcoming of this design is that the four laser beams would not be of equal power.
This work was done by Charlie Stevenson, Jorge Rivera, and Robert Youngquist of Kennedy Space Center and Robert Cox and William Haskell of Dynacs, Inc. For further information, contact the Kennedy Space Center Technology Commercialization Office at (321) 867-8130.