Mini-spectrometer

How are spectrophotometric measurements made in a mini-spectrometer?

Light incident on a mini-spectrometer is spectrally separated by the grating and arrayed one-dimensionally by wavelength. This light then enters the image sensor and the wavelength information is obtained by reading out the electrical signals that are output at each incident light position on the image sensor. The image sensor consists of an array of photoelectric conversion elements and a circuit for transferring the output charges to an external device. We call mini-spectrometers containing only these components “head type.”

We also provide mini-spectrometers called “module type,“ which includes a head type mini-spectrometer configuration, a driver circuit, and an interface circuit for connecting to a PC. For head type mini-spectrometers, we offer an evaluation circuit (sold separately) designed to connect to a PC.

 

What types of image sensors are incorporated in mini-spectrometers?

Linear image sensors with different spectral response ranges and sensitivity, etc. are used according to each type of mini-spectrometers.

 

Mini-spectrometer (example) Type Image sensor Spectral response range Features
TG series
(C9404CA, etc.)
TM series
(C10082CA, etc.)
Module type Back-thinned CCD image sensor 200 to 1100 nm High sensitivity
TM series
(C10082MD, etc.)
Module type CMOS image sensor 200 to 1100 nm Wide dynamic range
TM series
(C10082MD, etc.)
RC series
MS series
Micro-spectrometer
Head type CMOS image sensor 200 to 1100 nm Wide dynamic range
TG series
(C11482GA)
Module type Non-cooled InGaAs
image sensor
900 to 1700 nm Power supply for cooling is not required.
TG series
(C9914GB, etc.)
Module type Thermoelectrically cooled
InGaAs image sensor
900 to 2550 nm Low noise

 

What is the integration time?

The integration time is the time for accumulating the electrical charge generated in an image sensor by input of light. The amount of the accumulated electrical charge is proportional to this integration time, so if the incident light level is low, a sufficient amount of charge can be obtained by making the integration time longer. Note, however, that the image sensor dark output also increases in proportion to the integration time.

 

What should I know about setting the integration time?

The integration time can be set in 1 μs or 1 ms steps. (The integration time for head type mini-spectrometers is set by the timing of image sensor drive signals.)

 

How are mini-spectrometers driven?

Module type mini-spectrometers have a USB interface and so can be driven by connecting to a Windows PC using the supplied USB cable. Some mini-spectrometers operate only on USB bus power, and others require an external power supply. When using a mini-spectrometer requiring an external power supply, use the power supply connector that comes with the product.

USB bus power only C10082MD, C10083MD, C11697MB
C11482GA, C11007MA, C11008MA, C13053MA
USB bus power + external
power supply
C10082CA, C10082CAH, C10083CA
C10083CAH, C9404CA, C9404CAH
C9405CB, C11713CA, C11714CA
C9913GC, C9914GB, C11118GA

Note 1:

To operate a head type mini-spectrometer, please prepare a driver circuit that meets the conditions needed for installation into equipment. We provide an evaluation circuit (sold separately) for simple evaluation of head type mini-spectrometer characteristics.

 

Note 2:

Precautions for USB bus power

  • USB bus power has an upper limit in terms of current consumption. Caution is therefore needed when using a bus-powered type hub that draws power through a connected port or when used along with another USB bus powered device or when connected to multiple mini-spectrometers.
  • On some PC (especially notebook PC) depending on their settings, the power supplied from the USB bus may shut off when the PC shifts to power-saving mode. Communication problems with the mini-spectrometer may also occur when recovering from power-saving mode. So power-saving mode should be disabled.

 

Is software for operating a mini-spectrometer available?

Mini-spectrometers (except for head type mini-spectrometers) come supplied with evaluation software. This evaluation software includes the functions (setting measurement conditions, acquiring and storing data, and displaying graphs, etc.) needed for making basic measurements.

 

What systems are required to operate the evaluation software for mini-spectrometers?

Operation of the evaluation software that comes supplied with a mini-spectrometer was verified with the system shown below. We do not guarantee operation on other systems or environments.

OS Microsoft Windows 7 Ultimate SP1 (32/64-bit)
Monitor XGA (1024 × 768) or higher resolution

Using a PC equipped with a high performance CPU and memory is recommended.

 

What else can the evaluation software do besides basic functions?

The evaluation software can store measurement results as a CSV file (wavelength or pixel is selectable with the button). This file can be loaded on other software for further processing. The evaluation software is also supplied with a DLL that is usable with Visual C++, Visual Basic, etc., allowing users to develop their own measurement programs. However, the DLL function specifications of the evaluation circuit for head type mini-spectrometers are not available to users.

 

I need to convert each pixel data of the image sensor into wavelength data. How can I do it?

The relation between each image sensor pixel and the wavelength can be calculated by using the following 5-order approximate expression.

 

Wavelength [nm] = a0 + a1pix1 + a2pix2 + a3pix3 + a4pix4 + a5pix5

a0~a5:
wavelength conversion factors listed on final inspection sheet
These wavelength conversion factors are internally stored in mini-spectrometers (other than head type mini-spectrometers).
pix:
any pixel number (1 to last pixel) of image sensor

 

The evaluation software can use these factors to display data converted into wavelengths. Please be aware that the values calculated with this approximate expression may show a slight difference when comparing with the known wavelengths of spectral lines.

 

Can mini-spectrometer A/D-converted values be converted to light levels?

This is not possible because the mini-spectrometer does not have coefficients for converting A/D-converted values to light levels.

 

Are user's manual and technical information also provided?

When the evaluation software is installed in the PC, the user's manual and technical information are also stored in the PC.

To view these items, from the Windows Start menu, select:
[Program]→[Hamamatsuminispectrometer]→[Document]

 

Are optical fibers for connecting to mini-spectrometers available?

The following optical fibers are available (sold separately).

  • *A9762-01: for UV to visible, 600 µm core diameter, 1.5 m long, with SMA connector on both ends
  • *A9762-02: for UV to visible, 800 µm core diameter, 1.5 m long, with SMA connector on both ends
  • *A9763-01: for visible to NIR, 600 µm core diameter, 1.5 m long, with SMA connector on both ends
  • *A9763-02: for visible to NIR, 800 µm core diameter,1.5 m long, with SMA connector on both ends

Note:

MS series and micro-spectrometers do not require an optical fiber since they are designed to measure light that is incident through air.

 

How often is wavelength calibration needed? How can I do it?

Hamamatsu mini-spectrometers have no moving parts and so possess excellent stability. We think there is no need to perform wavelength calibration when used under normal environment such as an indoor area. You can continue using the wavelength conversion factors that are attached at the time of shipment.
Wavelength precision can be checked using calibration lamps that emit already known spectral lines. To reacquire wavelength conversion factors, we recommend using a high-precision monochromator.

 

What type of grating is incorporated in mini-spectrometers.

Our mini-spectrometers use a transmission type grating or reflection type grating.

 

How does the slit size affect detection?

In mini-spectrometers, the slit size is mainly related to resolution and throughput. The smaller the slit size especially along the width, the more the resolution improves. However, making the slit smaller decreases the light level to be measured and also causes the throughput of the mini-spectrometer to drop. The mini-spectrometer slit size is set while taking these factors into account.

 

Can an optical fiber with an FC connector be connected to mini-spectrometers?

Our mini-spectrometers (for optical fiber connection) come supplied with an SMA connector. Optical fibers with an FC connector cannot be coupled to our mini-spectrometers.

Note:

The MS series and micro-spectrometers do not require an optical fiber since they measure light that is incident through air.

 

What should I be careful of when connecting a commercially available fiber to a mini-spectrometer?

Use an optical fiber with an SMA connector and having NA 0.22 and a large core diameter (core diameter of 500 μm or more is recommended). Optical fibers with a small core diameter sometimes affect the measurement accuracy. To reduce noise, select an optical fiber covered with a sheath that is not affected by external light. If measuring ultraviolet light, then select a solarization-resistant optical fiber to prevent loss of transmittance.

 

How is spectral resolution defined?

There are two methods for defining spectral resolution. One defines the resolution by DIN standards using the Rayleigh criterion. Here a numerical value defines to what extent the mini-spectrometer can differentiate the wavelength difference between peaks at the same intensity and adjacent to each other. In this method, the valley between the two peaks must be measurable within 81% or less of the peak value. On the other hand, a more practical method for defining spectral resolution is finding the spectral width by FWHM (full width at half maximum). This method directly defines the broadening of the spectrum at a point that is 50% of the intensity of the spectral peak value. Resolution defined by the half-width (FWHM) is known as about 80% of the resolution value defined by Rayleigh criterion. The spectral resolution of mini-spectrometers is defined by the more practical half-width (FWHM) method.

 

What is the extent of wavelength reproducibility in mini-spectrometers?

Since mini-spectrometers have no mechanically moving parts, their optical precision can be easily maintained so that they provide high wavelength reproducibility. For example, the C9404CA achieves wavelength reproducibility of ±0.1 nm, and the C9405CB and C9406GC offer wavelength reproducibility of ±0.2 nm. The TG series and TM series mini-spectrometers have a compact and robust optical system that minimizes the effects of temperature on wavelength. This allows the above models to attain specifications of 0.02 nm/°C.

 

What is the definition of the stray light level?

There are two methods for defining the stray light level. One method utilizes the white light transmitted through a long-pass filter that passes a specific wavelength as the measurement light. Stray light is in this case defined as the ratio of transmittance in the transmitted wavelength range to the blocked wavelength range. This definition permits measuring the effect of stray light in a wide wavelength range and so is an evaluation method that fits actual applications such as fluorescence measurement. However, the user should be aware that the intensity profile of white light used as the reference light will have effects on the measurement value. The other method utilizes reference light in a narrow wavelength range such as light emitted from a monochromator or a spectral line lamp. In this case, one example for defining the stray light is given by the equation below using a reference light level and the amount of unnecessary light that is output at a position shifted from the peak wavelength of the reference light in a narrow wavelength range. If evaluating stray light by means of reference light in a narrow wavelength range, then the measurement conditions are very simple, so this method offers good reproducibility when making quantitative evaluations.

 

SL=10 × (log IM / IR)

  • SL: stray light by means of reference light in a narrow wavelength range
  • IM: amount of unnecessary light that is output at a position shifted from the peak wavelength of the reference light
  • IR: reference light level

 

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