The present invention relates to the field of photography and, more specifically, to electronic strobe lighting units adapted to be built into or detachably mounted on cameras, and to reflectors and reflector assemblies forming part of such strobe units.
Compact, highly-efficient, electronic strobe lighting units are rapidly becoming the preferred source of artificial illumination for making flash and fill flash exposures with automatic cameras designed for the mass amateur market. The availability of high-speed color films (ASA 400-600) for amateur use has substantially reduced the light output requirements for these lighting units and, coupled with efficiency improvements in the light emitting gas discharge tubes and associated reflectors, their size has been scaled down to the point where it is practical to build them into compact cameras without inhibiting portability. Also, quenchable strobe units have been integrated into automatic exposure control systems which operate in both flash and proportional fill flash modes thereby substantially increasing the utilization of these lighting units for photographing both indoor and outdoor scenes.
Quantitatively, strobe units are rated in terms of peak light output power (effective range), recycle time, and the number of flashes per set of batteries. Qualitative figures of merit, however, define the light output characteristics in terms of wavelength spectrum, color temperature and optical performance in terms of field coverage (as it relates to the field of view of the camera lens) and the distribution of light intensity over the field of coverage.
These last-mentioned optical parameters are defined by the design of the reflector, the location of the light emitting gas discharge tube in relation thereto, the design of the light transmissive face plate (usually a Fresnel lens) and another factor that is not quite so obvious, the manufacturing tolerances to which the individual components are made and thereafter assembled.
For representative examples of contemporary optical design activity in this field, reference may be had to commonly-assigned copending applications, U.S. Ser. No. 183,105 filed on Sept. 2, 1980; U.S. Ser. No. 172,778 filed on July 28, 1980; and U.S. Ser. No. 175,060 filed on Aug. 4, 1980.
The first application discloses a reflector having a parabolic cross-section shape in the form of a seventh order polynomial which, when used with an artificial source of illumination of predetermined geometry, projects a light beam having substantially uniform intensity within a given solid angle.
The latter applications relate to a strobe unit having a reflector cross-section shape in the form of a high order polynomial and a strobe tube which is disposed with its center offset with respect to the reflector's plane of symmetry to provide a vertically asymmetric distribution of illumination intensity over the field of coverage so that higher intensities occur at the central upper part of the picture area where the most important subject matter of a scene is most often found. By selectively concentrating the light output in this manner, the effective range of the strobe unit is extended.
The nature of the technical problems encountered in the manufacture and assembly of such a high quality strobe unit to insure that the optical benefits of such a sophisticated design will be realized when the unit is incorporated into a camera is disclosed in commonly-assigned copending applications U.S. Ser. No. 175,250 filed on Aug. 4, 1980 and U.S. Ser. No. 187,311 filed on Sept. 15, 1980.
The strobe unit described therein comprises a housing; a main reflector section having a high order polynomial cross section; a pair of side reflector plates enclosing the open lateral ends of the main reflector section; a flash tube; and a Fresnel lens face plate that snaps onto the housing in front of the reflector and tube assembly.
The latter application is directed to a method for making a metal stamping die for forming the concave-shaped main reflector section in a manner to compensate for the spring back characteristics of the sheet metal used to form the reflector. First, a test die is made and the resultant reflector section is measured to determine a calibration function which is then used to determine a final die shape that will produce the desired polynomial curve following die forming and spring back.
The former application relates to a high quality strobe unit comprising a multi-component reflector (i.e., a generally parabolic main section and two flat side pieces) that is adapted to be assembled on a mandrel and then inserted into a receiving cavity in the housing wherein tabs on the reflector assembly fit into receiving notches to accurately locate it therein. After the reflector is located in the housing, the strobe tube is inserted through elongated openings in the reflector side pieces and is urged against tapered locating surfaces thereof by elastic rings or grommets for accurately positioning the tube in predetermined offset relation to the apex of the parabola. Thereafter, a face plate, having an integrally formed lenticular section thereon, is snap fitted onto the housing.
While this type of assembly performs satisfactorily, it is relatively expensive to produce because the reflector components must be first separately formed and preassembled and the general assembly technique employed (e.g., having to elongate the elastic grommets and hook them over capturing detents on the housing) do not readily lend themselves to automatic assembly methods.
U.S. Pat. No. 4,223,372 discloses several different embodiments of a small compact strobe unit. In some embodiments, the reflector is a one piece stamping made of aluminum which is inserted into a receiving cavity or frame in the strobe housing. Other embodiments employ a molded plastic housing, or plastic insert therefor, having an integrally molded plastic member that has the curved surface of a reflector. This reflector surface is then coated with a thin layer of aluminum, using a vapor deposition process, to give it the required reflective property.
Both types of construction have certain characteristics which inhibit cost effective automatic assembly. If the reflector is formed separately, before initiation of the assembly process, labor and material costs accummulate for post-forming steps such as removing the reflector from the forming die or mold; inspecting it for defects; loading it into a tote tray or other container to protect the reflector from damage during storage and transport to the assembly area, and then once again handling the reflector to insert it into its housing. Reflectors that are coated with a thin aluminum film must be batch processed for the vapor deposition step thereby precluding the use of more cost-effective continuous in-line assembly methods.
Also, when the strobe tube is flashed, a significant amount of heat is generated which must be effectively dissipated without causing distortion or other damage to the strobe unit components. Because the vapor deposited aluminum film is so thin, it does not have sufficient thermal mass to serve as an adequate heat sink. Therefore, in order to prevent the film from charring or discoloring, especially near the parabola apex, it is usually necessary to provide a relatively thick metal heat sink on the back side of the molded plastic reflector. This just adds another component and assembly step which increase manufacturing costs.
Therefore, it is an object of the present invention to provide a high-quality strobe unit and reflector assembly forming part thereof which are simple in construction and easy to assemble.
It is another object to provide such a strobe unit and reflector assembly which can be produced at relatively low manufacturing cost while maintaining high performance standards.
Another object is provide such a strobe unit and reflector assembly which are compatible with automatic assembly techniques.
Yet another object is to provide a high quality strobe unit and reflector assembly forming part thereof wherein the reflector is formed during and is an integral part of the assembly process.
Another object of the invention is to provide a strobe unit and reflector assembly wherein the reflector serves as a heat distribution member for facilitating the dissipation of heat generated when the strobe tube is fired.
It is yet another object of the invention to provide an effective method for reducing the manufacturing cost of a high quality strobe unit and reflector assembly forming part thereof.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.