Measuring optical densities over 10 Abs on the Agilent Cary 7000 Universal Measurement Spectrophotometer (UMS)

Significance of the topic

High optical density measurements are critical for applications ranging from laser safety eyewear and optical filter production to biophotonics and quantitative analysis of strongly absorbing samples. Reliable determination of absorbance up to and beyond 10 absorbance units (Abs) demands exceptional photometric accuracy, linearity, and dynamic range from the spectrophotometer.

Objectives and overview

This work evaluates the Agilent Cary 7000 Universal Measurement Spectrophotometer (UMS) using the addition-of-filters technique to validate performance up to 10 Abs in the visible region (~640 nm) and over 8 Abs in the near-infrared region (~1500 nm). The study aims to demonstrate photometric range, accuracy, and linearity without relying on expensive calibrated standards.

Methodology and Instrumentation

The addition-of-filters approach measures individual filter spectra and predicts the combined response by summation:

• Visible region: BG25 filter plus mesh attenuator (4.5 Abs rear beam attenuation); data interval 1 nm; bandwidth 5 nm; averaging time 1 s (spectra) or 50 s (blocked beam); double-beam mode.
• NIR region: two UG11 filters (2.2 Abs rear beam attenuation); data interval 4 nm; variable bandwidth; averaging time 1 s or 10 s; double-beam mode; PbSmart lead sulfide (PbS) detector.

The Cary 7000 UMS employs PbSmart technology for superior stray-light rejection, enabling low-noise detection at extreme absorbance levels.

Main results and discussion

In the visible range, measured and predicted spectra align closely, reaching over 10 Abs at 640 nm with excellent signal-to-noise. In the NIR, the combined UG11 filters exhibit over 8 Abs at ~1500 nm, confirming linearity and accuracy across the extended photometric range. These results highlight the Cary 7000 UMS’s ability to balance sample and reference signals via rear beam attenuation and maintain detector performance at very low light intensities.

Benefits and practical applications

• Eliminates need for costly calibrated high-OD standards.
• Extends measurement capability to absorbances beyond conventional limits without sample dilution or destructive modification.
• Enables rapid validation of optical filter manufacturing, laser safety eyewear, and quantitative analysis of highly absorbing media.

Future trends and potential applications

Advances may include fully automated rear beam attenuators for streamlined workflows, integration with hyperspectral imaging, and application to turbid biological samples. Further development of narrow-band detectors, as in the Cary 6000i, will push NIR performance beyond 8 Abs and expand utility in advanced photonics and materials research.

Conclusion

The addition-of-filters technique confirms that the Agilent Cary 7000 UMS delivers photometrically accurate, linear absorbance measurements up to 10 Abs in the visible and over 8 Abs in the NIR. This performance supports a wide range of industrial, research, and QA/QC applications requiring high optical density analysis.

References

• 1. Agilent Technologies, "The Linear Dynamic Range of the New Generation Cary 4000, 5000 and 6000i Spectrophotometers," Data Sheet 5990-7836EN, 2013.
• 2. Josephy D., Logan D., "A Whole Cell Assay for Spectroscopic Measurement of Recombinant Cytochrome P450 Expression in Bacteria," UV-Vis-NIR At Work No. 87.
• 3. Hind A.R., "To Improvements in Spectrophotometry," American Laboratory, vol. 34, no. 24, 2002, p. 32.
• 4. Agilent Technologies, "Photometric Linearity Range of the New Generation Cary 4000/5000/6000i Spectrophotometers," Data Sheet 5990-7843EN, 2013.
• 5. Agilent Technologies, Cary Rear Beam Attenuator accessory description.