Technology

■Development Background

In recent years, the performance of various optical filters—including the edge filters and notch filters used in Raman spectroscopy—has improved dramatically.
While filters offering superior blocking capabilities and steep transmission/rejection characteristics were being developed in rapid succession, there were few measurement instruments capable of accurately evaluating their performance. It was against this backdrop that we received an inquiry from a customer.

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“We want to evaluate the performance of the filter we’ve developed, but commercially available spectrophotometers have measurement limitations.”
Conventional spectrophotometers are typically limited to measuring optical densities (OD) in the range of OD 4 to OD 6.
However, the filter the customer was developing possessed light-blocking capabilities that exceeded this range.

“Are we truly measuring the filter’s performance accurately?”
“Are we underestimating its actual performance due to the limitations of the measurement equipment?”

These were the challenges the customer faced.

Furthermore, evaluating high-performance filters requires more than just the ability to measure high OD levels.
For edge filters and notch filters, high wavelength purity (narrow spectral bandwidth) is essential to accurately evaluate the steep transmission and blocking characteristics near the cutoff wavelength.

However, narrowing the slit width to improve wavelength purity reduces the amount of light illuminating the sample, resulting in an extremely weak signal reaching the detector. This leads to increased measurement noise, making high-precision measurement difficult.

In short, the two requirements—
・Measuring high OD levels
・Achieving high wavelength resolution

—presented conflicting conditions.

We considered solving this problem by designing a measurement system from scratch, rather than simply extending the capabilities of commercially available equipment.

■Development Process

First, we adopted a double-monochromator configuration to minimize stray light to the absolute limit.
To ensure stable measurements even with weak light, we built a detection system that automatically switches between lock-in detection and photon counting methods based on the light intensity.

We also achieved a wide dynamic range and high measurement precision by combining photomultiplier tube (PMT) preamplifier gain switching, applied voltage control, and lock-in amplifier range switching.
Through these innovations, we completed a dedicated system capable of simultaneously achieving high wavelength purity and high-OD measurement.

■Equipment Description

The OD-10 spectrophotometer for high-optical-density measurement, developed in this manner, enables the measurement of spectral transmittance up to an optical density (OD) of 10—a task that was difficult with conventional spectrophotometers.
It allows for the high-precision evaluation of samples with extremely low transmittance, such as multilayer filters, edge filters, notch filters, high-density filters, light-blocking films, and high-concentration solutions.

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The system measures transmittance across a wavelength range of 350 to 700 nm, targeting plate-shaped optical elements.
It employs a unique optical configuration with spectrometers positioned both before and after the sample; this design minimizes the impact of fluorescence, enabling accurate transmittance measurements even for fluorescent samples.
Furthermore, by irradiating the sample with quasi-collimated light and reducing instrument-induced interference, the system achieves high measurement accuracy and reproducibility.

Transmittance is calculated based on the ratio between sample measurement data and background data acquired without a sample in place. The measurement range spans OD 0 to 8, with display and measurement capabilities extending up to OD 10.
We have verified that the system can measure up to OD 8 using lock-in detection and up to OD 10 using photon counting, all while maintaining a high wavelength resolution of approximately 0.15 nm.
We have also demonstrated the system’s ability to measure high blocking performance and steep transmission characteristics with high reproducibility when evaluating both commercially available filters and those under development. A dedicated PC handles instrument control, data acquisition, and analysis, making the system suitable for a wide range of applications, from R&D to quality assessment.

■Summary

This instrument was not developed with the goal of achieving “world-leading specifications.”

Our sole objective was:
“To accurately measure the true performance of filters developed by our customers.”
That was our only aim.

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Rather than simply modifying off-the-shelf equipment, we designed every component—from the light source, spectrometer, and optical system to the detector and control system—from scratch to create a measurement system perfectly tailored to the customer’s specific application.
We continue to offer custom systems that allow for the flexible combination of measurement wavelength ranges, light sources, spectrometer configurations, and detection methods to suit each customer’s specific samples and evaluation conditions.

If you are facing measurement challenges that cannot be resolved with standard commercial products, please feel free to consult us.
We propose “one-of-a-kind” spectroscopic measurement systems designed to support your research, development, and quality evaluation needs.