MARVIN A. SALZENSTEIN and WALTER C. McCRONE Amour Research Foundation of Illinois Institute of Technology, Chicago, Illinois
A
NEED has arisen for a simple microprojection apparatus answering the following requirements: project an image of four square feet, bright enough to be seen
by an audience of three hundred persons; be portable and easy to assemble and dismantle; and operate on 110 volts a. c. The system described below was developed to answer this need. The basic optical requirements for a microprojection system are shown above. The condenser (G) gathers the light from the light source (0. The mirror (J) reflects the light to condenser (I)which focuses the beam on the microscope stage (K). An enlarged image of the specimen under o b s e ~ a t i o nis formed by the microscope and reflected by mirror or prism (L) to the screen. A number of microprojectors are commercially available but have the disadvantages of high cost, weak light source, lack of portability, etc.' The system described can be built for about $150, not including the microscope. The authors have used a petrographic microscope with the projector to utilize polarized light; however, biological or other types of microscopes can be used. The principal problem in obtaining adequate projec1 A new projection microscope has recently become available, namely the Promi instrument sold through W. H. K e s d and Co., Chicago. It should be investigated by anyone interested in acquiring a rnicroprojector.
tion has always been the light source. A carbon arc on 110 volts d. c. is highly satisfactory, but unless magnetically controlled, requires constant attention. Further, few lecture halls even in chemistry and physics laboratories have 110 volts d. c., and the a.-c. arc, especially with low amperage current, is noisy, hard to start, and much less intense a source than the d.-c. arc. A number of other light sources were considered, among them the zirconium arc, high pressure mercury arc, tungsten arc, and the tungsten filament. All of those listed except the latter required heavy auxiliary equipment. Since home movie projectors have essentially the same problem and solve it by using tungsten filament lamps, a system of this type was chosen. A 750- or 1000-watt biplane filament projection lamp, the same type used in 16-mm. film projectors, was used. A housing was built using a prefabricated metal box (available through any radio supply house) and incorporating a fan for cooling the bulb, a lamp socket, switch, and reflector. A Bausch & Lomb Model B microprojector was disassembled to provide a support for the optical system, heat absorbing cell, and microscope. The assembly is shown on the opposite page. The blower ( A )handles about 50 c.f.m. of air to cool the light source ( C ) . Thechimney (R) directs the air stream over the bulb. The metal reflector (D) is mounted behind the bulb to direct as much light as possible forward. The hot air is ejected
s c r e n end C-.
( ~ ~ f~h~ t ) soreen hangs from dowel md. ~h~ soreen may be up d.. .I.c,~ in the ha,, d,. the sYpportsfolded down to provide a oarrvins case. ( ~ i p h t )he "suitoase" houses the projector ready for traveling.
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APRIL, 1951
through the openings a t (E) and (F). The light emerges through the opening ( E ) and passes through the condensing system (G) consisting of two double convex lenses of focal length about ten inches. A water cell (H) is interposed to remove most of the infrared light. The light isthen reflected by the microscope mirror ( J ) through the condenser lens (I),a simple convex lens a t least one inch in diameter and with a focal length of about two inches. (This large lens in place of the usual microscope substage condenser is oneof thegreatest single improvements embodied in this projector.) The image leaving the microscope eyepiece is reflected by the prism (L) to the screen. The blower and bulb are wired in parallel so that the bulb cannot be on when the blower is not operating. With this precaution the life also acts as a carrying case for the rolled up screen. of the 750-watt lamp is rated a t 25 hours; of the 1000- With the screen in the case, the uprights are folded down and clamped into place. When folded ready for watt lamp a t 10 hours. The carrying case has been designed to serve not carrying, the case is only 3 X 3 X 38 inches. All accessories are tucked into unused corners of the only to carry the projector but also to open on three sides to permit access for projection without the neces- projector case when the projector is packed for carrying. sity of removing the projector from the case. This has The body tube of the microscope, for example, must be the further advantage of shielding stray light from the removed, wrapped in cloth, and packed away in the screen and room. Two wooden dowel rods are provided bottom of the case, since there is insufficient headroom on which black curtains are hung to completely enclose to permit closing the case with the entire microscope in the apparatus except for the top. In some cases it is de- place. The complete unit ready to travel weighs about sirable to cover a part of the top also with additional 75 pounds, which aids in the stability and absence of black curtains. After use the case is closed to form a vibration when projecting. No great difficultyhas been encountered due to its weight on a number of long trips "suitcase" as shown in the figure. Projection is carried out from behind a translucent by train or by airplane. screen. This enables simultaneous and completely unhindered visibility of the screen both by the lecturer and ACKNOWLEDGMENT the audience. The screen (see figure), a 36-inch square Grateful acknowledgment is made to The Research of tracing cloth, is hung from a wooden 3/ginch dowel rod and has a '/r-inch straight aluminum rod in the Corporation for a grant-in-aid t o one of the authors hem a t the bottom. The screen is hung from supports (W. C. M.) from which part of the funds were obtained formed by two uprights hinged to the base. The base for this work.
