Spectrophotometry

Spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. While relatively simple in concept, determining the reflectance or transmittance involves careful consideration of the geometrical and spectral conditions of the measurement. We maintain the national scales for reflectance and transmittance in the ultraviolet, visible, and near-infrared spectral regions (250 nm to 2500 nm).

Measurement services are available for dissemination of the scales of reflectance and transmittance maintained by the reference instruments. The primary purpose is to provide traceability to the scales, not for research on particular test items. These services are available as either calibrations or standard reference materials. For a calibration service the customer generally supplies the test item, which is measured under the spectral and geometrical conditions specified by the customer. Standard reference materials are available for immediate purchase, and are calibrated under specific spectral and geometrical conditions.

Calibrations

Spectral Transmittance Filters (38010C-38040C) – These standard filters, designated as cobalt blue (38010C), copper green (38020C), carbon yellow (38030C), and selenium orange (38040C), are used for checking the photometric scale of spectrophotometers. They are 2 mm to 3 mm thick and available as either 30-mm polished glass disks or 51-mm polished glass squares. The relative expanded uncertainty of the filters ranges from approximately 0.2 % to 0.3 % of the value. The calibration report includes the values of transmittance from 380 nm to 770 nm at 10 nm intervals at 25 °C, the estimated uncertainty of each value, and the temperature dependence of the transmittance at each wavelength.

Wavelength Standards (38050C-38051C) – Based on NIST’s extensive experience with holmium oxide glass and its long-term stability, NIST no longer sells or recertifies this standard. These standards have shown insignificant spectral variation from batch to batch and from one manufacturer to another. More information can be found in the paper, Holmium Oxide Glass Wavelength Standards. Transmittance and absorbance data from this paper can be download as an Excel file (404 kB).

Special Tests of Spectral Reflectance (38060S) – Measurements of spectral reflectance can be made for the wavelength region 250 nm to 2500 nm for non-fluorescent submitted test items. This service includes both specular and diffuse reflectance measurements taken at angles, wavelengths, and polarizations specified by the customer. For the directional-hemispherical geometry incident angles of 0° and 6° are possible, and the later can be measured with the specular component either included or excluded. In general, measurements are performed at room temperature with test items (2 cm to 30 cm wide) placed perpendicular to the incident beam. The calibration report includes reflectance measurements and uncertainty estimates based on the optical characteristics of the submitted test item and the instrument used to perform the measurement. Arrangements for these measurements on submitted test items must be made before shipment. NIST will decide whether and how to perform the measurements. Test items not accepted for measurement will be returned.

Special Tests of Spectral Transmittance (38061S) – Measurements of spectral transmittance can be made at wavelengths from 250 nm to 2500 nm for non-fluorescent submitted test items. Measurements are performed at room temperature with the incident beam normal to the front surface of the test item. Items with dimensions from 1 cm to 5.1 cm are routinely accommodated. However, other sizes and measurement conditions are possible in consultation with NIST staff. The calibration report includes transmittance measurements and uncertainty estimates based on the optical characteristics of the submitted test item and the instrument used to perform the measurement. Arrangements for these measurements on submitted test items must be made before shipment. NIST will decide whether and how to perform the measurements. Test items not accepted for measurement will be returned.

Reported uncertainties will follow, to the extent possible, the ISO Guide to the Expression of Uncertainty in Measurement (International Organization for Standardization, Geneva, Switzerland, 1993). Since 1994, the NIST policy has been conforming to this guide when reporting its activities, using an expanded uncertainty coverage factor (as defined in the Guide) of k = 2. See Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results for a detailed explanation of the NIST policy.

Current fees can be found at NIST Calibration Services’ Optical Properties of Materials Measurements. The minimum price for Special Tests is $5,000. Pricing is based on actual costs, labor and time, the number of test items, and the number of data points. Contact Catherine Cooksey for a quote.