ASTM E387 Standard Test Method for Estimating Stray Radiant Power Ratio of Dispersive Spectrophotometers by the Opaque Filter Method

ASTM E387 Introduction

ASTM E387 specifies a standard method by which the stray radiant power may be estimated, using a set of opaque filters that basically cut off radiation at specific wavelengths. Spectrophotometers are extensively applied in the measurement of optical characteristics of materials, and instrument performance may be affected by stray radiant power (usually referred to as stray light). Stray light will occur due to diffraction, scattering, or internal reflection in the instrumentation and will deform a measurement, especially when strongly absorbed by the sample. This will allow laboratories to check and verify instrument performance and instrument-to-instrument comparison, and ensure data integrity.

ASTM E387 Test Method

ASTM E387 Equipment and Sample Preparation

ASTM E387 Results and Interpretation

The results are reported as a ratio of stray radiant power to incident radiant power on the test wavelength. A smaller ratio corresponds to superior optical performance and to a small measurement error. Laboratories normally perform comparisons of the calculated ratio with an acceptable range as established in instrument specifications or industry requirements. When the ratio is at an intolerable threshold, a spectrophotometer will need some repair or optical alignment or replacement of parts such as gratings or mirrors.

ASTM E387 Related Test Methods

ASTM E1164 guides obtaining spectrometric data for object-color evaluation, ensuring consistency in measurement. ASTM E308 covers the computation of color coordinates from spectral data, linking instrument performance with colorimetric accuracy. 

ASTM E387 Applications and Industry Use

The testing technique is common in laboratories where the characterization of materials is necessary by use of spectrophotometry, such as in the pharmaceutical, chemical, environmental, and food industries. It is used to confirm the efficiency of an instrument by testing the instrument’s performance at a high level of optical research.

ASTM E387 Materials Commonly Tested

The use of spectrophotometers can normally be exempted, but the opaque filter method requires that certain optical filters be made, with the objective being to measure stray radiant power. These filters prevent specific wavelength acceptance so that unwanted stray light at other wavelengths can also be detected. 

ASTM E387 Common Challenges and Troubleshooting

Typical problems in implementing this test method are alignment errors, application of contaminated/damaged filters, and sensitivity limitations of the detections at very low signal levels. Accuracy can also be impaired by stray light and defects in setting the baseline. 

ASTM E387 Safety and Best Practices

The filters must be thoroughly handled to prevent scratches or contamination, as the result could be distorted. The instruments must be well-maintained, free of extrinsic light interference, and used in accordance with the manufacturer’s instructions. Special care should be taken when being exposed to UV radiation, as adequate shielding/protective measures (safety glasses) should be provided.

Importance of ASTM E387 Test Method

This method provides a consistent process to assess stray radiant power in dispersive spectrophotometers that directly affects the accuracy of optical data. The adoption of this standard by laboratories will achieve compliance, reliability of instruments, and comparability of the results of different systems of measurements. This adds to greater confidence in the spectrophotometric assay, which makes the technique a necessity in industries and research where precision is of paramount importance.

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