“Small changes in cover glass thickness or refractive index produce a disproportionately large optical path error, reducing image contrast and resolution.”
— Inoué, S, Video Microscopy, 2nd ed., Plenum Press (1997)
This gadget is like a tiny playground for cover slips, featuring two razor-thin steel blades acting as barriers, with a shim creating a precise offset. When you drop a cover slip into the gap, Geometry happens and makes the cover slip to tilt depending on its thickness. Even Hair thin differences in thickness become easy-to-read
Cover-slip thickness is critically important in microscopy because high-resolution optical systems are designed for specific optical path lengths between the specimen and the objective lens. Even small deviations from that ideal thickness can cause spherical aberration, loss of contrast, and degraded resolution.
Yes, professional manufacturers nowadays do a decent job of reporting cover-slip thickness correctly (Types 0 to 2). However, not all producers follow these standards, so having a tool like this can be really useful especially when working with objectives designed for specimens very close to the cover slip (corrected for 0.17 mm thickness). If you happen to use a cover slip that’s too thick, you could accidentally scratch your very expensive objective in an irreversible way.
“In fluorescence, phase-contrast, or quantitative polarization microscopy, consistent optical path length ensures reproducible intensity and retardation measurements”
— Inoué, S., Collected Works, Article 4: Cover-Slip Thickness Gauge (1951)
Shinya Inoué built his cover-slip thickness gauge (1951): to quickly select slips within ±0.005 mm of 0.17 mm. (Check the paper atached in files).
In 1950, while teaching cell biology at Princeton University, Shinya Inoué realized that commercial “#1.5” cover slips, however, often varied by several micrometers. Measuring each one with a micrometer was slow and risked contamination, so Inoué devised a simple mechanical gauge in 1951.
His device used two razor blades set at a fixed angle: the cover slip itself acted as a tiny lever, tilting according to its thickness. A scale converted that tilt into a direct reading in millimeters fast, clean, and accurate to a few microns.
Remarkably, the same gauge built more than 70 years ago was still in use in Inoué’s laboratory half a century later a small but elegant example of his ingenuity in combining optical precision with practical design.
This is a tribute to the great Shinya Inoue, but also a 3D STL file designed to be used for microscopy enthusiast and professionals.
The modern 3D-printed version of Inoué’s coverSlip thickness gauge preserves his elegant simplicity while embracing digital fabrication. Inspired by traditional Japanese joinery (kigumi) the craft of creating strong, precise wooden joints without screws or nails this design uses interlocking parts that snap together with perfect alignment. The results a timeless scientific tool reimagined with the precision and accessibility of modern 3D printing.
| Quantity | Item | Notes |
|---|---|---|
| 2 | M2 × 12 mm screws | Pan-head or socket-head |
| 2 | M2 nuts | Nylock nuts preferred for secure hold |
| 2 | Razor blades | Standard stainless steel, single or double edge |
- Remove all printing supports and debris.
- Align the base plate and back plate using the interlocking kigumi joints.
- Press them together firmly until they fit flush and tight.
- This Japanese joinery system ensures perfect alignment without glue or extra hardware.
- Lay the two razor blades flat on the shim area of the back plate, on both sides of the central recess.
- The sharp edges should face inward, toward the small central gap where the cover slip will rest.
- Handle blades carefully
- Position the two orange blade-holder plates over the blades.
- Their tapered tips should meet neatly at the recess, forming a precise “tweezer-like” edge.
- Ensure all holes are clear and smooth — test-fit one M2 screw to confirm easy passage.
- Insert the 2 M2 screws through the holes in the blade holders and back plate.
- Secure each with an M2 nut on the opposite side.
- Tighten the screws evenly — firm enough to clamp the blades securely, but not so tight that plastic parts deform.
- Inspect the alignment: both blade tips should be parallel and evenly spaced around the recess.
- The distance between them MUST BE 0.17 mm.
The OpenScad code file is open source well written and clear, find it and make it yours !
Shillaber, C. P. (1944). Photomicrography in Theory and Practice. Wiley.
Inoué, S., & Spring, K. R. (1997). Video Microscopy. Plenum Press.
Inoué, S. (1951). Cover-Slip Thickness Gauge. In Collected Works of Shinya Inoué.
Pawley, J. B. (Ed.) (2006). Handbook of Biological Confocal Microscopy. Springer.
Born & Wolf (1999). Principles of Optics. Cambridge Univ. Press — for spherical aberration derivation.