Regular reflectance and transmittance

One of the Working Group's fundamental tasks is the determination of regular spectral transmittance and reflectance in the optical spectral region and the further development of measurement procedures for these tasks. Commercial spectral photometers, self-made devices for special tasks (e.g. reflectance close to 100%) and a reference set-up are available for this purpose. The measurements can be performed in the spectral range from 200 nm to 18 µm.

The reference set-up allows transmission standards to be calibrated with a relative uncertainty of Δτ (λ) / τ (λ) = 0,3 ^. (1 - lg (τ (λ))) %, where τ  (λ) is the spectral transmittance at wavelength λ . In the visible spectral range, spectral transmittance values between 1 and 10^-6 can be measured.

The same measuring set-up can be used to determine regular spectral reflectance for angles of incidence between 6° and 85°. The relative uncertainty amounts to at least 0,5%. With extended effort and special techniques appropriate calibration object can also be measured with smaller uncertainty.

Filters with small absorption bands can be calibrated as wavelength standards. The measurement uncertainty depends on the width and shape of the absorption bands. The great versatility and flexibility of the measuring set-up also allows angular dependencies of scattered light and radiance coefficients to be measured in defined geometries.

With increased measurement uncertainty it is also possible to determine spectral transmittance values below 10^-6, in particular at specific laser wavelengths down to 10^-10.