Calibrate wavelength shifts to each pixel on detector array
3.
Linearity correction
Calibrate spectrometer responds linearity to light levels over its full intensity range
4.
Intensity correction
Calibrate detector response consistency at each pixel
5.
Stray light correction
Calibrate false reading of light being scattered / reflected / refracted onto the detector
►Resolution The resolution of the spectrometer is expressed in terms of Full Width at Half Maximum (FWHM), which refers to the wavelength difference of half the maximum value of the peak, which is different from wavelength range, grating density, slit and the pixel number of sensor. Some people will use the corresponding band range per unit pixel to represent (nm / pixels). For example, the measured wavelength range is 350 ~ 1020nm, the slit is selected 25 um, the sensor pixel is 2048 pixels, then the full width at half maximum (FWHM) is 1.2nm, and the corresponding wavelength range per unit pixel is 670nm / 2048 pixels = 0.3nm / pixels.
►Stray Light The definition of the stray light ratio of OtO spectrometer is to observe the stray light signal of 450nm by blocking the halogen lamp with R60 high-pass filter.
►Dark Noise There are three types of noise that mainly affect the output signal: "Light Source Stability", "Electronic Noise", and "CCD Sensor Noise". If we do not consider the influence of external light sources, we can first check the dark noise of the measurement system. The definition of "dark noise" is the voltage output (Vout RMS) within a 1ms integration time in a completely dark environment, so the level of dark noise depends entirely on the electronic read noise and the CCD sensor itself.
►Signal-to-Noise Ratio (SNR) The definition of "signal-to-noise ratio" is the maximum signal (65535) divided by the RMS value. A larger signal-to-noise ratio means that the readout signal is more stable, and it is easier to distinguish the difference in the low signal.
►Integration Time The time it takes to collect an optical signal to generate a spectrogram, the longer the integration time, the stronger the signal, which can be used to improve the signal-to-noise ratio (SNR). Basically, we can customize by user needs (1ms ~ 65sec), but the shortest integration time (<1ms) depends on the characteristics of the CCD / CMOS sensor.
►Dynamic Range The ratio of the maximum light intensity to the minimum light intensity.
►Thermal Stability When the spectrometer at different temperatures (-10 to 50℃). Dislocation or deformation of the internal structure and objects of the spectrometer due to thermal expansion and contraction. It will cause the wavelength shift, the unit is nm / ℃ or nm.
►Trigger Mode OtO spectrometers provide I / O ports to support “trigger mode”. With the trigger mode, users can use external I / O signals to trigger the spectrometer for data acquisition. In this way, users can trigger multiple spectrometers to simultaneously acquire data at the same time, instead of using computer software to command multiple spectrometers through APIs. This can avoid computer un-synchronized to ensure that multiple spectrometers perform the acquisition at the same time.
►Signal Averaging "Signal averaging" can truly reduce the noise affecting each pixel. The more samples the average will be, the better the signal performance will be, but it will require more time to get all the spectrum. When using Signal Averaging, signal-to-noise ratio (SNR) will be the times of square root of the sample number. For example: When the average number of samples is 100, the SNR will become 10 times.
►Boxcar Filter Use adjacent sampling points for averaging to obtain a smooth signal curve, but this method will affact the optical resolution. If you need to obtain a peak signal, this method is not recommended.
►Electronic Dark Correction When the electronic part of the spectrometer system (main circuit board and CCD) is started, there is already a basic current (called dark current). Dark current means that even if no light source enters the spectrometer, there is a basic current magnitude. The magnitude of the dark current will become the basic count value after passing through the analog-to-digital converter (ADC) of the spectrometer. During factory calibration, OtO will set the value of dark current through ADC to be about 1000. Since the dark current is not the actual measured light source intensity, the dark current needs to be deducted first during the measurement. The magnitude of dark current may change due to the use temperature, so OtO has established a correction function to dynamically deduct dark current. Each OtO spectrometer will perform electronic dark correction before leaving the factory, and store the correction data directly in each spectrometer. When SpectraSmart enables electronic dark correction, dark current will be deducted dynamically. The following two figures demonstrate the difference between unenabled and enabled electronic dark value correction:
►Linearity Correction The response of the CCD of the spectrometer to different intensity of the light source is not an ideal linear straight line. Moreover, the linearity curve of the intensity of each CCD is not exactly the same, so each OtO spectrometer will undergo OtO linearity correction before leaving the factory, and store its own proprietary linearity correction table in the spectrometer. The 16-bit analog-to-digital converter (ADC) is used internally in the OTO spectrometer, so the output linear value will be corrected for the interval 0 ~ 65535. When SpectraSmart enables linearity correction, it will correct the value of each pixel according to the value, as shown in the following figure:
▲ Intensity Correction (non-preset, optional) The CCD photosensitive element of OtO spectrometer not only has different response curves to the intensity of the light source, but also the response of its pixels to the wavelength of the light source. Therefore, before the OtO spectrometer leaves the factory, it will undergo intensity correction to correct its response, and store the correction list in the OtO spectrometer. OtO intensity correction uses the standard light source from Taiwan Industrial Research Institute (ITRI), which can provide standard absolute light source intensity, so the wavelength response can be corrected after OtO factory intensity correction, and also includes the absolute light source intensity correction (350nm ~ 900nm). However, if the customer needs to establish an absolute light source measurement with his own system, he needs to re-establish the intensity correction table on the system. The following two figures demonstrate the difference between inactive and active intensity correction:
