BRDF & BTDF Scatterometers

An Overview of the BRDF & BTDF Scatterometers from The Scatter Works

Scatter data is an important input to optical design and ray-tracing software. Rather than assuming that a material is a perfect mirror or an ideal diffuser, BRDF or BTDF data sets can be read into commercial optical design software so as to greatly improve their modelling accuracy.

A scatterometer is an instrument which characterises how diffuse or how specular an optical material is. The metrics that are commonly used are BRDF for reflectance and BTDF for transmittance. BRDF (bidirectional reflective distribution function) is the ratio of incident irradiance to reflected radiance for a given angle of illumination and for a defined angle of view. The respective planes of illumination and collection must also be specified. Similarly, BTDF (bidirectional transmissive distribution function) is the ratio of incident irradiance to transmitted radiance, again for defined angles (and planes) or illumination and view. The summation of scatter over a hemisphere is called the total integrated scatter (TIS). The term “BSDF” is the generic name given to scatter measurements and the acronym stands for Bidirectional Scatter Distribution Function.

SphereOptics supplies and supports the scatterometers made by The Scatterworks (TSW), a company operated by the worldwide guru on scatterometry, John Stover. TSW’s reflective & transmissive scatterometers provide measurements of BRDF and BTDF from the UV to the mid IR, the actual measurement wavelength depending on which laser/s the customer specifies with their system.

The MicroScan Scatterometer

The MicroScan is a compact, affordable, handheld BRDF reflectance scatterometer that employs a single laser diode (red wavelength as standard) and three photodiodes. The system measures specular reflectance at an incident angle of 25° and scattered light at angles of 0° and -50°. Assuming that the surface is optically smooth, isotropic values of the r.m.s. roughness from 0.01 to 1.0µm^-1 are reported in addition to the associated TIS. The noise floor is about 10^-5/sr. The MicroScan has been used extensively to monitor the ageing of large laser optics in very hostile environments.

The ScatterScope 4 Scatterometer

The ScatterScope 4 is a table-top scatterometer capable of performing both reflective BRDF and transmissive BTDF scatter measurements. It uses multiple silicon detectors that scan 180 degrees to measure hemispherical and incident plane scatter in both reflection and transmission. It can be equipped with as many as three laser diode sources. It does not measure close to the specular beam and the noise floor is limited to the 10^-5/sr to 10^-6/sr range. Its huge advantages are speed (1520 seconds per scan) and ease of use. Operators can be trained in just a few minutes. It is an ideal instrument for routine QA testing of optical materials.

The CASI Scatterometer

The CASI Scatterometer is a research-grade instrument that can be configured with multiple laser sources ranging from the near UV to the mid-IR. It uses programmable collection apertures and step sizes to allow measurements within 0.1° to the specular beam and it has a noise floor of about 10^-8/sr in the visible, which is limited by scatter from air molecules. It takes a linear incident plane scan at a programmable incident angle. It requires a laboratory with an optical table (not supplied) and benefits from HEPA filtered air flow for some applications. Operator training takes about a week. The CASI (“Complete Angle Scatter Instrument”) provides the level of accuracy required for measurements in demanding space science and high energy laser applications.

The TASC Scatterometer

The TASC scatterometer is essentially a CASI that takes hemispherical measurements with manually changed apertures. UV, visible, mid-IR and broadband systems have been built.

Need a Measurement?

SphereOptics operates a measurement service using the ScatterScope 4 in our ISO 17025 certified laboratory in Munich, Germany.