Physics · the two-photon microscope in 3D
Inside the sample
Stand in front of a block of tissue 600 µm deep, scaled up to your height, with 420 glowing cells inside. Then look at it through a confocal microscope and a two-photon microscope, and see where each one goes dark.
The 3D companion to Seeing deep: the same cells and the same physics.
01 · Look around
screen, cardboard or headset
Three ways to look
No viewer? Use the buttons under the picture, or make one. The tissue surface is at the top, and depth runs down. The blue square marks where a confocal image disappears and the orange one where a two-photon image does. Switch microscope and the deep cells fade, the way the images on the microscope page break up.
3D without a viewer
On a laptop, Use your hands turns on the webcam. Pinch your thumb and finger and drag sideways to turn the block, or up and down to move the focus; pinch with both hands and pull apart to zoom; sweep an open hand sideways to switch microscope. Move your head and the block stays put behind the screen, as if through a window. The video is read in your browser and never leaves your computer.
In a cardboard viewer, turn your head to look up and down the block, and tap the screen (or press the viewer's button) to switch microscope. Close the view with the ✕ or your phone's back gesture.
Brightness here is on a log scale: full at the surface, half at 10% of the signal, nothing at 1%, where the images on the microscope page are gone. Same laser power at every depth; scattering lengths 60 µm visible and 200 µm near-infrared (Helmchen and Denk, 2005). Cells are simulated, not measured. The cardboard view has no lens correction yet, so the edges bend a little.
02 · Make a viewer
an afternoon, a few euros
A cardboard viewer from a cereal box
A viewer is a box that holds the phone in front of your eyes, with one lens per eye and a wall between them so each eye sees only its own half of the screen. The lenses let your eyes focus on a screen only 4 cm away.
You need
- Two lenses: convex, about 25 mm across with a focal length of 40 to 45 mm. Search for "cardboard VR lenses" (a few euros a pair), or take them from a cheap 5× magnifier or loupe, which is close enough.
- Stiff card: a cereal box or a shoebox lid.
- A ruler, a craft knife, tape or glue, and a rubber band.
Build it
- Lens board. Cut a piece of card 14 cm wide and 8 cm tall. Cut two holes a little smaller than the lenses, their centres 6.4 cm apart (the average distance between eyes) and 4 cm from the bottom edge. Cut a notch for your nose between them at the bottom. Tape the lenses over the holes.
- Box. Make a sleeve with four sides as deep as the lenses' focal length, about 4.5 cm, so that the lens board sits on one open end and the phone rests on the other.
- Divider. Glue a strip of card down the middle, from the lens board to the phone end, so the left eye can't see the right half of the screen.
- Phone. Hold the phone across the far end, screen in, with the rubber band. Black out any light coming in from the sides with tape.
- Focus. If the picture is blurry, slide the phone a few millimetres closer or further away until it is sharp, then mark the spot.
Google publishes a free print-and-cut template for its original Cardboard viewer (search for "Google Cardboard template"), and if you have a 3D printer, there are printable viewers for modern phones that take the same lenses.
Take breaks: a viewer with no strap and no lens correction tires the eyes faster than a headset. Not for children under 7, whose eyes are still developing.
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