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Constructing Your Own F-Box

# Introduction and Background

The F-Box is one of the most useful demonstration items in teaching
basic optics principles. With a variety of exercises, this system can be
used to teach everything from how images are formed to light gathering
power and image scale for CCD cameras.

The F-Box demonstration system consists of three basic components:

-   Scene Generator (The F-Box, itself)

-   Lens Assembly, with 3 lenses of different focal lengths

-   Projection Screens

You will also need a ruler of at least one meter in length. The F-Box
system should be used in a darkened room.

# Components and Parts

Building an F-Box demonstration system is quite easy, and can be
accomplished with basic hand tools found in any wood shop. There are
ways that the design presented here could be simplified if some tools or
parts are unavailable, but the present design has been found to be easy
to construct, durable, and flexible in the classroom.

To build one demonstration system, you will need:

-   1 Sheet of ¾ inch plywood (Only a fraction of the sheet actually
    gets used)

-   3 Pieces of ¼ inch thick wood or plastic, 4 x 6 inches each

-   3 Lenses, preferably 10 cm (4 inches) in diameter, with focal
    lengths of approximately 20, 50, and 100 cm, although smaller
    lenses, down to 2 inch diameter, can be used

-   1 Piece of ground glass, 11 inches square, 3/16 to ¼ inch thick

-   1 Ceramic lamp socket for a standard-base bulb

-   1 - 100 W frosted bulb

-   Electric line cord, approximately 10 feet in length, with a plug

-   6 Strips of flexible plastic, approximately 1/8 inch thick by 12 ½
    inches long, or six rings of plastic cut from tubing approximately 4
    inches in diameter (such as PVC pipe). If using smaller lenses, use
    tubing that is approximately the same size as the lenses chosen

-   Black card stock (poster board)

-   Aluminum foil

-   Graph paper

-   22 - #8 x 1 ¼ flat head wood screws

-   Paint or stain (Optional)

-   Velcro tabs, approximately ½ inch square

# Construction

*Scene Generator:* The first item to build is the scene generator. This
consists of a cubical wooden box approximately 12 inches on a side, with
one face replaced with ground glass. Figure 1 shows the five wood pieces
that should be cut from the ¾ inch plywood. Three of these pieces must
be grooved to hold the ground glass. This is most easily accomplished
with a table or radial arm saw, with the blade set to cut only ¼ inch
into the wood, or with a router and a square bit. A circular saw with
the blade suitably adjusted could also be used. Carefully label each of
the pieces, as their dimensions are close but they are not
interchangeable. Attach the boards together following the layout shown
in Figure 2. All joints should be glued together at the edges, and held
with three (3) of the #8 wood screws in countersunk holes. After the
boards have been assembled, drill a hole in one bottom corner of the
back board, as shown in the diagram, for the electrical cord to exit the
box. When the box has been assembled, line the interior with aluminum
foil, using some contact cement or other glue. The foil serves to
reflect most of the light from the bulb out the front, and also helps
keep the wood cool. Next, run the electric cord through the hole in the
back of the box, keeping the plug on the *outside* of the box. Bare the
ends of the two wires in the cord, and attach those ends to the screw
contacts on the socket. Screw the socket into the center of the inside
of the back board with wood screws. Insert the light bulb into the
socket. Slip the ground glass into the slot in the front of the box.
Attach tabs of velcro to the front of the box, on the edges of the wood
surrounding the ground glass. It is probably best to use the loop side
of the velcro here. If the velcro is not self-stick, use some white glue
or contact cement to attach the tabs.

The second item needed for the scene generator is a mask. Cut a piece of
black poster board to a square just large enough to cover the front of
the F-Box. In the center of the square, cut out the letter **F**, about
6 inches high, with each cut out strip about ½ inch wide. Attach the
other half of the velcro tabs to the corners of the square, so that the
tabs line up when the square is placed on the front of the box.

Congratulations! You've just finished the scene generator. You could, of
course, cut many different shapes into poster board squares, but the
letter **F** is a great place to start -- it's easy to tell if it's
upside down or backwards, and it's easy to recognize. You can also paint
or finish your F-Box, which is best done after the wood components are
assembled and before the foil, socket, glass, or poster board square are
attached.

![](media/image2.wmf){width="5.992361111111111in"
height="8.14861111111111in"}

**Figure 1. Wood Pieces for the F-Box Scene Generator. All F-Box parts
can be cut from a single sheet of plywood. Careful cutting would allow
construction of several such assemblies from a full plywood sheet.**

![](media/image3.wmf){width="5.207638888888889in"
height="9.122916666666667in"}

Figure 2A. F-Box Scene Generator Assembly Diagram. Glue and screw the
sides, bottom, and back. Mount the lamp socket on the inside back
surface.

#### ![](media/image4.wmf){width="4.4625in" height="5.493055555555555in"}

#### 

#### Figure 2B. F-Box Scene Generator Assembly Diagram -- Insert Glass. Note how the notches in the sides and bottom accommodate the glass plate, and secure it in position. It can be slid out of the assembly to allow changes of the light bulb, seen at the inside back of the box.

## Lens Holder

The second component of the F-Box demonstration is the lens holder. The
purpose of this piece is to hold three lenses in a fixed orientation.
Figure 3 shows the parts that are needed for this item. The three holes
should be cut so they are just larger than the lenses that will be used.
It's possible to build an F-Box demonstration with lenses of 2 inch or
larger diameter. It's best to keep the overall lens holder dimensions
the same, and drill or cut suitable holes in the same locations as those
indicated in the diagram for 4 inch lenses. Smaller lenses are easier to
find and less expensive; however, they also make it more difficult to
demonstrate different aperture sizes in the light gathering power and
magnification exercises.

![](media/image5.wmf){width="6.495138888888889in"
height="2.767361111111111in"}

#### Figure 3. Lens Holder Assembly. Cut two small blocks of wood for feet, and notch the main board to allow room for the feet to be attached with glue and screws. Keep the locations of the lens holes the same even if drilling/cutting them smaller for smaller lenses. 

First, cut the three wood pieces -- the main board and the two feet.
Drill or cut the three holes to hold the lenses -- make sure you measure
the diameter of the lenses carefully. It may be necessary to use a
rotary sander on a drill or to hand sand the holes so the lenses fit
smoothly. Attach the feet using wood screws. If you are going to finish
the lens holder with paint or stain, it is best to do so at this point.
Next, take the strips of plastic or the rings cut from PVC tubing, and
cut them so that they will fit snugly inside the holes cut in the lens
holder. These pieces will serve as retaining rings to hold the lenses.
Figure 4 shows how the lenses are held in by the retaining rings. Each
ring is split -- a small section has been cut away -- so that it fits
inside the hole where the lens is placed. It may be necessary to sand
the plastic rings to make sure they have a snug fit against the lenses
inside the holes. Make sure the lenses are centered and square to the
board. Next, place velcro loop tabs on the four corners of the front of
the lens holder -- the side with the convex side of the lenses.

![](media/image6.wmf){width="5.313888888888889in"
height="2.4555555555555557in"}

Figure 4. Installation of Retaining Rings. Cut a small segment from each
plastic piece, allowing you to snugly fit the ring inside the hole in
the lens holder block. Make sure the lens is correctly centered and
aligned to be straight inside the hole.

Cut three pieces of black poster board the same size as the lens holding
boards -- you'll need to cut notches so that they fit over the feet. In
each piece of poster board, cut a set of circles out, with each circle
centered over one of the lenses. On one piece, cut out 1 inch diameter
apertures; cut 2 inch apertures in the second piece, and 3 inch
apertures in the third. Attach velcro hook tabs to the corners of each
of these pieces. These aperture stops can be applied to the lens holder
to limit the effective size of each lens.

## Projection Screens

Cut three blocks of wood 4 inches square. Cut a notch in the center of
each board ¼ inch wide and 3/8 inch deep. Press in and glue the ¼-inch
thick, 4x6 inch boards into the slots, with the six inch dimension
vertical (see Figure 5). At this point the pieces could be painted or
stained. On one side of the 4x6 inch board glue a piece of white paper.
On the other side, glue, side by side, two pieces of graph paper. One
should have squares approximately ¼ inch on a side; the other should
have squares no more than 1/10 inch on a side. These pieces could be
hand drawn, drawn using a computer graphics program, or cut from
pre-printed graph paper. The grids so generated are used in exercises to
measure the sizes of images created by the lenses.

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height="3.5631944444444446in"}

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#### Figure 5. Assembly of Projection Screen

*This writeup was prepared by Douglas N. Arion and Kevin M. Crosby of
Carthage College, Kenosha, WI, with funding from the National Science
Foundation* Center for Adaptive Optics.