Foldscope calibrations and measurements – or pixels and spider silk

Objective:          To photograph the LED (light emitting diode) pixels on a computer screens with my samples of Foldscopes and to calibrate the lenses. In addition, the Foldscopes will be use to examine some spider web silk fibers.

Methods and Materials:       

Two Foldscopes from the Foldscope kits were assembled using the L1 140X lens and the L2 500X lens.

The lenses were then removed and taped onto the screen of a laptop computer (HP G72-260US, 17 in. screen, 1368 x 768 pixels with 0.259 x 0.259 mm pitch) [1].

A web browser was opened with a new tab to produce a blank white page where the RGB (red green blue) pixels on the LED screen could be photographed.

A cell phone (Moto G, 1280 x 720 HD, 329 ppi, 4.5 inches (11.3 cm) diagonal (~ 55 mm x 100 mm) display, 5 MP, 2592 х 1944 pixels) [2]) was attached over the lenses for “phototaking,” per Foldscope Instruction Manual v1.1.

The Foldscopes were then reassembled. Some spider silk strands harvested from my home basement were placed on a glass slide for the scopes.


Figures 1 through 4 are unedited images from the Moto G camera. Their actual screen sizes are 10 mm long x 5.5 mm wide.

Figure 1 shows the LED pixels on a random area of the computer screen at a comfortable brightness level, using the L1 lens.


Figure 1   Moto G display of LED pixels using 140X lens

As the center screen RGB pixel (0.259 mm wide) was measured at about 13 mm on the cell phone screen, the image enlargement was about 50x (cf 140X for L1).

Using Google Picasa Photo Viewer, at magnification 1:1 (100%), the pixels measured ~80 mm. The corresponding image enlargement equaled 309x, i.e. a calibration scale of 0.3 mm per micron.

There was a 6.2x enlargement over the cellphone display. With a 3.3x digital zoom, 1 mm on screen was about 1 µm real size.

The entire image area covered a circle of about 1.3 mm in diameter on the LED screen.


Figure 2. Moto G display of LED pixels using 500X lens

Figure 2 shows a pixel set on anther random area of the LED screen using the L2 lens.

The RGB pixel (width 0.259 mm) was enlarged to about 30 mm on the cell phone screen. The corresponding image enlargement was about 116x (cf 500X for L2).

L2 was about 2.3 times better than L1 in enlargement, but it had a smaller field of view.

Using Picasa Photo Viewer, with 1:1 magnification, the RGB pixel measured about 195 mm. The corresponding image enlargement equaled 753x, i.e. a calibration scale of 0.75 mm per micron.

This was a 6.5x enlargement over the cellphone display. With 1.3x digital zoom, 1 mm on the computer screen was about 1 µm real size.

The RGB pixel image area covered a circle of 30 mm on the 55 mm x 100 mm screen of the camera, and about 1 pixel (or 259 µm) in diameter on the LED screen.

The image sensor of the Moto G had 1456 pixels (from manual) in width, the number of sensor pixels over the image diameter was estimated at about 790, i.e. about 3 sensor pixels per µm.


Figure 3. A random clump of spider silk as seen using L1



Figure 4. A random selection of spider silk threads as seen using L2

Figure 3 and 4 show a couple of random shots of the spider web silk fibers using the Foldscopes.

Using Windows Photo Viewer, a few random strands of spider silk could be enlarged to a pixelated level.

The thinner spider silks strands were estimated at 3 to 4 sensor pixels wide, or about 1 micron in thickness. (Unfortunately, I was unable to include the snippets here as part of the results due to some software incompatibility.)

Conclusions and Comments:         Using the L1 and L2 lenses with my Foldscopes and a 5 MP camera (Moto G or equivalent), it was possible to visualize small objects with 50 times and 116 times enlargements, respectively. With a computer photo viewer such as Picasa, another 6 times magnification was obtainable. I was not particularly interested in the morphometry of the spider silk – as it is about 1 µm thick, it was chosen to assess the lens and camera resolutions. If you have a 40 MP camera, you are likely able to get better than 1 µm resolution.


  1. hp G72 manual c02641840.pdf



2 Comments Add yours

  1. Niramay Gogate says:

    All the three photos were just awesome. If you still have those spider web threads, can you just stretch them under foldscope ? Because that would be quite useful for determining their elastic properties. I would like to see how do those threads respond when forc is applied to them

  2. lawrence chik says:

    Hi Niramay, as a retired electrical engineer, I would speculate that the elasticity of spider threads is probably well known. Using the Foldscope, my best suggestion is that you can harvest a quantity of silk, stretch them over a range of interest, and then fixate them on the slides for “phototaking.“ Doing it real time would likely involve some serious modifications and clever “Rube Goldberg” implements – should be fascinating!
    On second thought, if you mount the L2 lens over a cell phone camera, and if you have extremely steady hands to stretch the spider silk over the FOV of L2, then you can possibly show off the whole gambit using video capture.

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