240
T. Okura
Fig. 10.8 Spectra measured
using the slow single scan
and FRS methods
10.1.3 Noise Reduction in a Linear Array Spectrometer
Many NIR linear array spectrometers containing a linear array and grating spectrometer have been widely used for onsite ingredient measurements. A linear array
spectrometer measures all the wavelengths simultaneously to ensure low noise levels.
Most of the noise sources in a linear array spectrometer are intrinsic to the sensor.
The photoelectrons produced by light in a pixel of the linear array sensor are
accumulated in a capacitor connected to each pixel. The number of accumulated
photoelectrons in the capacitor is proportional to the incident light intensity. The
noise in the linear array includes the circuit and quantum noises. Quantum noise is
caused by the photoelectric effect. The number of photoelectrons produced by the
incident light is proportional to the light intensity, and the noise is
√
N when the
averaged photoelectron number is N . The signal-to-noise ratio (SNR) (Eq. 10.2) can
then be defined as follows [5].
SNR =
N
√
N
=
√
N
(10.2)
Although the noise
√
N increases at large N, the SNR improves. This indicates
that high intensity light is required to achieve a good SNR in a NIR spectrometer.
The maximum number of photoelectrons is limited by the size of the capacitor.
This is called saturation exposure, which is expressed in terms of the number of
electrons (Me-). As shown in Fig. 10.9, when the saturation exposure is large, the
capacitance of the linear array is large along with an improved SNR. A linear array
with a large saturation exposure is called a “deep well.”
The saturation exposure ranges from 0.03 × 10
6 to 1000 × 10
6 (Table 10.1). In
a NIR spectrometer, a saturation exposure of 500–1000 × 10
6 is preferred to obtain
a good SNR. As shown in Table 10.1, the saturation exposure of a CCD linear array
is smaller, while that of the NMOS linear arrays is larger. Recently, CMOS linear
arrays with large saturation exposures of around 1000 × 10
6 have been developed.
T. Okura
Fig. 10.8 Spectra measured
using the slow single scan
and FRS methods
10.1.3 Noise Reduction in a Linear Array Spectrometer
Many NIR linear array spectrometers containing a linear array and grating spectrometer have been widely used for onsite ingredient measurements. A linear array
spectrometer measures all the wavelengths simultaneously to ensure low noise levels.
Most of the noise sources in a linear array spectrometer are intrinsic to the sensor.
The photoelectrons produced by light in a pixel of the linear array sensor are
accumulated in a capacitor connected to each pixel. The number of accumulated
photoelectrons in the capacitor is proportional to the incident light intensity. The
noise in the linear array includes the circuit and quantum noises. Quantum noise is
caused by the photoelectric effect. The number of photoelectrons produced by the
incident light is proportional to the light intensity, and the noise is
√
N when the
averaged photoelectron number is N . The signal-to-noise ratio (SNR) (Eq. 10.2) can
then be defined as follows [5].
SNR =
N
√
N
=
√
N
(10.2)
Although the noise
√
N increases at large N, the SNR improves. This indicates
that high intensity light is required to achieve a good SNR in a NIR spectrometer.
The maximum number of photoelectrons is limited by the size of the capacitor.
This is called saturation exposure, which is expressed in terms of the number of
electrons (Me-). As shown in Fig. 10.9, when the saturation exposure is large, the
capacitance of the linear array is large along with an improved SNR. A linear array
with a large saturation exposure is called a “deep well.”
The saturation exposure ranges from 0.03 × 10
6 to 1000 × 10
6 (Table 10.1). In
a NIR spectrometer, a saturation exposure of 500–1000 × 10
6 is preferred to obtain
a good SNR. As shown in Table 10.1, the saturation exposure of a CCD linear array
is smaller, while that of the NMOS linear arrays is larger. Recently, CMOS linear
arrays with large saturation exposures of around 1000 × 10
6 have been developed.