A few years ago, in my graduate program I realized that I used a version of photography in almost every arena of my life. I was shooting hoop videos on a micro 4/3 digital camera, using UV-Vis spectroscopy and mass spectrometry for my research and building a variable focal length pinhole camera. It occurred to me to reconsider what a camera was and how it functioned.
At the heart of each method, there is a detecting field, a lens, and some sort of exposure control. In a camera, the light-proof box, aperture and shutter control the duration and intensity with which film or the photo sensor is exposed. With the rain camera, you instead use a sheet of plywood or plastic as the shutter and an array small plastic cups replace the detectors.

Step 1: Materials & Tooling

To make an exposure, you will need:

A mass balance
A covered place with doors wide enough for your detector to do your measuring. (Garage's work great)
Something interesting that rain falls through but is deflected by (In this example a tree).
a few small bricks or pieces of lumber to prop up/level the detector sheet.
a ran camera detector sheet (In the picture above I've used 4)
A computer with photoshop and statistical software like sigma plot or Matlab is useful but not absolutely necessary.

To build a single detector sheet:

Wood glue
3 - 8' 1 x 2 wooden boards ( I used spruce)
1 - 4' x 8' sheet of luan (underlayment) [A sheet of thin acrylic or plastic would probably be better I used luan for cost]
Plastic cups (I used 5 oz Solo refill cups. Do not use paper cups!! They disintegrate!)
Urethane

Tools Used

Japanese Hand saw
Block plane
Assorted chisels
Wood clamps
A sand bag
Hole saw
A hand drill
A square.
Table saws

Step 2: Sensor Array Construction

Work out the grid placement you are going to use. Spend some time on this as it dictates where youplace your crossbars. With a 2 inch hole saw bit, I made an 11x11 hole grid. Make sure there is at least a 3/4" gap between each hole to allow for the width of the crossbar. 1" is better.
Create the frame with the 1x2 s this can be done a few different ways. I constructed mine essentially as a 4' x 4' square. Depending on your materials, and doorway width, different sizes can be used. Make sure to place your crossbars in between rows of holes.

The nailed butt-ended corners shown in the diagram are easier to construct, but they are not as structurally sound as the joints I used. Box joints are a simple but sturdy corner joint, where a stile meets the rail. Mortise and tenon joints work effectively for where the crossbar meets the rails. I'm a fan of cutting all of the bars slightly large and planing the joint smooth to achieve a more finished look.
Plane the top edge of the frame flat and level. This can take some time but will tighten up the over all structure by being smoother and having more direct surface area.
Apply a thin bead of wood glue along the top edge of the frame.
Cut the top sheet to match and place on top of the frame. Having an assistant here is useful since it will prevent smearing glue to one side or another.
Place clamps along the edges and the sandbag in the middle to keep pressure.
Wipe up glue drips with a wet paper towel.
Leave over night.
Cut grid of holes.
Seal the frame with deck sealant, urethane or oil. In all cases, be careful to dispose of with oily rags carefully. Oxidation of the oil in the cloth can lead to fires!

Step 3: Making an Exposure

Number all of your cups.
Mass each cup and record the mass in an excel spreadsheet along with it's number.
Place your detector in the desired location. This could be under a tree, a structure or out in the open. Even simple air current movements may make for interesting patterns. Prop it up with bricks and pieces of wood so that it is level.
Determine a method of placing the cups that you will remember. I went with cup 1 in the top left corner and cup 484 in the bottom right, ascending left to right and down as if I were typing on a page. Do what makes sense to you.
Cover with plywood and wait for it to rain.
Once the cups are all partially filled cover again and either commandeer assistance transporting into your measuring location or wait until the rain stops.
Mass each cup again and record in your excel spreadsheet. Then subtract the empty mass from the filled mass to find the quantity of rain.
Now based on your map and the ascension numbers you chose, you have grid map and exposure values for each point. In this case I used four 11x 11 arrays arranged in a square for a total of 484 points of measurement. This makes each cup akin to a single pixel in a 22 x 22 pixel image. I used a contour plot in Sigmaplot along with some processing in photoshop to achieve the example image on the first page. However, this is essentially the "Raw" camera data. How you choose to process it is up to you! Don't feel bound by that for your final images!

Here are directions to build a contour plot in excel!