Transmittance & Reflectance Measurement Guide (2026)
Transmittance and reflectance (T/R) are the two most fundamental optical measurements of a thin film. Together they describe how much light passes through a coating and how much bounces back. The difference goes into absorption, which is what determines a film's optical function. This guide explains what T/R measurement is, where it is used, and which instruments cover each case.
What transmittance and reflectance actually measure
When light hits a thin-film surface, it is split into three components:
- Transmittance (T): the fraction of incident light that passes through the film and substrate.
- Reflectance (R): the fraction reflected back from the surface and interfaces.
- Absorbance (A): the remaining fraction absorbed by the material, where A = 1 - T - R.
For a single-wavelength measurement these are simple ratios. Real instruments sweep a wavelength range to produce a T/R spectrum, which reveals how the film behaves across UV, visible and infrared. This is the basis for deriving optical constants (n, k) and film thickness through optical modeling.
Why T/R measurement matters across industries
| Application | What T/R reveals |
|---|---|
| Optical coatings | Anti-reflection (AR) and high-reflection (HR) coatings are defined by their reflectance spectrum |
| Display panels | Transmittance of barrier and electrode films directly affects brightness and efficiency |
| Photovoltaics | Front-glass and anti-reflection layers maximize light reaching the cell |
| Food & pharma packaging | Barrier coatings are verified by their transmission in UV/IR bands |
Wide-band vs single-band T/R: what the range changes
- Narrow-band systems measure a fixed wavelength and are fast but limited.
- Wide-band systems sweep from UV (350 nm) through visible to NIR (2400 nm), revealing where the film transmits, reflects, or absorbs across the full spectrum. This is essential for optical design and coating QC.
Which TDM instruments measure transmittance and reflectance
- SE100 Multi-functional Thin Film Measurement: benchtop instrument delivering thickness, n/k, T&R, band gap and PL across a 5-1200 nm film range, for R&D labs and production sampling.
- SE600X Inline Perovskite Metrology: production-line system measuring inline thickness, PL and T&R at 10 Hz with 31 measurement points and no sample prep, for continuous coating monitoring.
- SE31-FM Wide-Band Optical Film Monitor: an in-situ monitor covering 350-2400 nm T/R optical-film monitoring with sub-0.2 nm wavelength resolution and PLC integration, for real-time coating control.
Important: A product page describing T/R capability does not by itself establish calibration for every substrate or wavelength condition. Measurement range and repeatability are defined per model on each product page.
A practical note on choosing offline, inline, or in-situ
- Offline (SE100): highest flexibility for multi-parameter characterization of samples.
- Inline (SE600X): high-throughput T/R at production speed for 100% inspection.
- In-situ (SE31-FM): real-time monitoring during deposition, closing the loop on coating control.
Frequently asked questions
What is the difference between transmittance and reflectance?
Transmittance is the fraction of light passing through a film; reflectance is the fraction reflected back. Absorption is the remainder.
Why measure T/R over a wavelength range instead of one wavelength?
A spectrum reveals how the film behaves across UV/visible/NIR, which is required to derive optical constants and to verify coating performance.
Can T/R measurement determine film thickness?
Yes. Combined with optical modeling, T/R spectra are a standard way to extract film thickness and refractive index (n/k).
Related guides and products
- Thin Film Thickness Guide
- Optical Coatings solutions
- SE100 Thin Film Measurement
- SE600X Inline Perovskite Metrology
- SE31-FM Wide-Band Optical Monitor
