Optical Properties of Microfluidic Materials: Transmission and Autofluorescence

Why optical properties matter

The large majority of microfluidic detection is optical. Fluorescence, absorbance and imaging all pass light through the chip wall, so the material is part of the optical system whether you intend it to be or not. Four properties dominate: how much light the material transmits, how much background fluorescence it emits, its refractive index, and any birefringence from moulding. Together these set how faint a signal you can detect and how cleanly you can image a channel.

If you are sizing an absorbance measurement, our Beer–Lambert tool relates path length and concentration to signal, and the fluorescence converter helps with excitation and emission units.

Optical transmission

Transmission is the fraction of light that passes through the material at a given wavelength. It matters most when your fluorophore is excited in the near-UV or when you read absorbance there. The common materials differ noticeably, particularly below about 400 nm.

• COC and COP — transmit very well across the visible and usefully into the near-UV, which suits many common fluorophores.
• PMMA — clear in the visible but transmits more poorly in the UV.
• Polycarbonate (PC) — clear but typically slightly yellow, and it blocks UV.
• Glass and fused silica — glass is good across the visible; fused silica is excellent and transmits deep into the UV.
• PDMS — clear across the visible, which is one reason it is so widely used for imaging.

Autofluorescence and the limit of detection

Autofluorescence is light the material itself emits when illuminated. In a fluorescence measurement this adds to the background, raising noise and worsening the limit of detection. For sensitive assays it is often the deciding factor, because no amount of excitation power helps if the chip glows alongside the sample.

• COC and COP — among the lowest-autofluorescence polymers, a key reason they dominate fluorescence diagnostics.
• PMMA — typically low to moderate.
• Polycarbonate — notably high, so it is often avoided for sensitive fluorescence work.
• PDMS — generally low to moderate.
• Glass and fused silica — the lowest of all, which is why glass and the cyclic olefins are favoured for demanding fluorescence detection.

For a broader comparison of the cyclic olefins against acrylic, see COC vs COP vs PMMA; for the highest optical performance, see our note on glass chips.

Optical properties at a glance

Refractive index and birefringence

Refractive index governs how light bends and reflects at the channel walls. For these polymers it typically sits in the range of about 1.49 to 1.59, and for glasses around 1.46 to 1.52. Mismatches between material, sample and any immersion medium cause reflections and aberrations that can distort an image.

Birefringence is a direction-dependent refractive index that can arise from frozen-in stress during moulding. It can disturb polarisation-sensitive measurements by rotating or scrambling the polarisation state. COC tends to show low birefringence, which is helpful where polarisation matters; an annealed or carefully moulded part also helps. These effects are most relevant in optofluidic designs that integrate optics into the chip.

Surface finish, clarity and scattering

Even a material with excellent bulk transmission will look hazy if its surfaces are rough. Roughness scatters light, reducing clarity and adding stray background. Optical windows therefore need an optical-quality finish, which depends on the tool or mould polish and on the process — see surface roughness for how finish is specified and achieved. We can advise on which surfaces need optical polishing as part of design-for-manufacture review.

Choosing a material by detection method

• Fluorescence — favour low-autofluorescence materials such as COC, COP or glass, and check transmission at your excitation wavelength.
• Absorbance — prioritise transmission at the read wavelength and a defined optical path length; model it with the Beer–Lambert tool.
• Bright-field and general imaging — clarity and a smooth optical surface matter most, which most clear polymers and glass provide.
• UV detection — favour COC, COP or fused silica, and avoid polycarbonate.

These choices feed directly into instrument-side work such as point-of-care readers, where a low-noise window can simplify the optics. The optical parameters for each material sit alongside the rest of the material parameters, and you can upload a design for a manufacturing quote.

Frequently asked questions