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Internal Shutter Improves Accuracy in Color Measurement

QE Pro with internal shutterThe high-sensitivity QE Pro spectrometer is now available with an internal shutter for more effectively managing dark measurements in experiment setups. The shutter is useful for applications where light intensities change — LED color measurement, for example — requiring users to dynamically adjust integration times and renormalize the spectrometer.

The QE Pro internal shutter removes the need for an external shutter, which improves signal throughput in the setup and allows for shorter integration times for an equivalent level of signal (see Case Study below).

Shutter Operation and Specifications

The QE Pro internal shutter is an optional optical bench accessory installed just behind the aperture of the slit. The shutter is controlled in OceanView spectroscopy software by activating one of the spectrometer GPIOs, and attenuates 100% of the signal when closed. Operation is accomplished in a few simple steps.

QE Pro Internal Shutter Specifications
Actuation time: 11 ms
Signal attenuation: 0 dB (100% attenuated)
Lifetime: >2 x 106 cycles (as observed in extensive testing)
Feature type: Accessory option (additional charge; installed at time of spectrometer assembly)
Retrofit option: QE Pro models only (additional component/service upgrade charge)

Benefits of the QE Pro Internal Shutter

Spectrometer accessories such as shutters allow users to block light from reaching the detector. With an external shuttering option – either as part of a light source or as a discrete inline accessory that puts the shutter between two optical fibers – there are performance trade-offs. The QE Pro internal shutter helps to eliminate several of these performance trade-offs:

  1. Light loss associated with inline shutters. External shuttering devices such as inline shutters are subject to significant loss of light throughput at each optical interface. With an internal shutter, more light can be collected in a shorter period of time. This translates into a shorter spectrometer integration time, which in turn allows users to collect more data.
  2. Light leakage into the optical bench. The optimum dark measurement eliminates all light inside the optical bench. With an external shutter, the chance of extra signal reaching the bench as a source of unwanted noise is greater than with an internal shutter.
  3. Measurements where installing a shutter is not feasible. For measurements where external shuttering is not an option – in a probe-based application, for example, or with emissive measurements or a flowing process stream setup – having the shutter internal to the spectrometer bench simplifies the setup.

Case Study: Evaluating the QE Pro Internal Shutter versus an External Shutter Option for Color Application

CCD-array spectrometers are an excellent option for real-time characterization of LEDs, light sources and light source systems. In the case of LED measurements, especially for testing and binning processes in high-volume LED manufacturing operations, measurement speed is critical. Even a design decision as seemingly minor as the choice of shutter can have significant impact.

An internal shutter is especially useful in applications where the light intensities being measured are rapidly changing and spectrometer integration times are adjusted accordingly.  Also, for applications such as emissive color measurement and irradiance calculations, an internal shutter offers a more convenient and cost effective way to re-normalize the spectrometer without any impact on stray light.

For Labsphere (North Sutton, N.H.), a leading provider of hardware and software for light measurement, criteria such as system performance, speed and flexibility are critical for its line of CDS 1100 spectrometers. Labsphere is an Ocean Optics OEM customer and, like Ocean Optics, part of the Halma P.L.C. group of companies.

Labsphere recently evaluated the QE Pro with internal shutter as an option for its CDS 1100 spectrometers and generously shared its results. This material has been edited for length and style considerations, and with a focus on the shutter performance. Testing was conducted by Labsphere’s Joe Jablonski, Director of Engineering, and Mason Jin, Engineering Manager at the company’s Shanghai, China, facility.

The purpose of the testing was to evaluate alternative spectrometer options for the Labsphere CDS-1100/2100 series spectrometers, specifically the Ocean Optics QE Pro outfitted with an internal shutter. (Editor’s note: The CDS-1100 today uses an earlier-generation Ocean Optics back-thinned CCD array spectrometer and an external shutter.)

System Setup

An LMS-100 (10-inch integrating sphere), ICS-050 lamp, Agilent 3634 power supply and spectral line source were set up to measure sensitivity, noise and spectral bandwidth (FWHM).

QE Pro - Labsphere System Setup

Internal versus External Shutter Setup

This diagram depicts the external shutter mechanism as it is implemented in the CDS-1100/2100 spectrometers. The external fiber optic brings light into the external shutter assembly. Light from the fiber is collimated and projected through the shutter mechanism and then refocused onto another fiber optic in the CDS-1100/2100 assembly. The fiber is then introduced to the spectrometer.

There are significant optical losses in this setup. The primary loss is at the external shutter assembly itself, due to the critical alignment of the fibers on either side of the shutter assembly.

External vs Internal Shutter

The QE Pro shutter configuration is shown below. It uses an internal shutter, which avoids the losses incurred by the external shutter configuration.

QE Pro Shutter Configuration

Internal versus External Shutter Results

The internal shutter is not a test but a feature, but is important because the alternative is an external shutter that causes a substantial loss of the input signal when utilized. This is evident in the sensitivity differences between the CDS-1100 and QE Pro since both use similar optical benches. For purposes of testing, sensitivity is defined as the amount of time it takes for the spectrometer to reach 75% of its saturation level while viewing an ICS-050 (2750K, 4.4W) lamp in a 10” Spectraflect integrating sphere looking through an opal glass diffuser.

  CDS-1100 QE Pro
Internal shutter: No Yes
Sensitivity (75% saturation): 700 ms 290 ms
% noise at 75% saturation: 0.10% 0.09%
Active scan time to reach 0.09%: 7.00 seconds 2.90 seconds
Average resolution (FWHM): 2.1 nm 2.0 nm

As mentioned above, sensitivity is the amount of time the spectrometer needs to reach 75% saturation in the system setup. As demonstrated, the QE Pro reached saturation nearly 2.5x faster than the CDS-1100.

What is presented here is the optical performance as viewed from a signal to noise and speed of measurement viewpoint. From this viewpoint it is clear that the QE Pro would be an improvement over the current CDS-1100/2100 spectrometer assemblies.