# N

### NEP (noise equivalent power)

NEP is the incident light level equivalent to the noise level of a device. In other words, it is the light level required to obtain a signal-to-noise ratio (S/N) of 1. We define the NEP value at the peak sensitivity wavelength (λp). Since the noise level is proportional to the square root of the frequency bandwidth, the bandwidth is normalized to 1 Hz.$$NEP[\frac{W}{Hz^{\frac{1}{2}}}]=\frac{Noise\ current[\frac{A}{Hz}]}{Photosensitivity \ at\ λp [\frac{A}{W}]}$$

### NGN (next generation network)

This is a next-generation information communication network utilizing IP (internet protocol) technology, and is intended to deliver multimedia services by merging fixed-line phones, mobile phones, data communication, etc. In addition to the reliability and stability provided by conventional telephone networks, NGN will also offer the same flexibility and economical efficiency as IP networks.

### Node sensitivity - CCD

The conversion ratio of electrons to voltage at the output of a CCD, in units of uV/electron.

### Noise - CCD

The conversion ratio of electrons to voltage at the output of a CCD, in units of uV/electron.

There are numerous sources that generate noise in CCDs, including those originating from extrinsic factors, such as cosmic rays. These can be categorized into the following four factors when considering only intrinsic noise in CCD elements:

• 1. Fixed pattern noise (Nf)
• 2. Photon shot noise (Ns)
• 3. Dark shot noise (Nd)
• 4. Readout noise (Nr)

Nt = (Nf 2 + Ns2 + Nd2 + Nr2 ) 1/2 Nt: total noise Note: Nf = 0 at noise calculation of one pixel

The graph shows how these four factors are related to amount of exposure. The dark shot noise resulting from the dark current is always constant regardless of the number of incident photons as long as exposure time is constant. Likewise, the readout noise is independent of the amount of exposure, as it is determined only by the CCD output method.

The performance of a CCD can be enhanced up to its detection limit (readout noise) by operating the CCD under conditions where the dark shot noise is reduced below the readout noise. Cooling the CCD while operating in the MPP mode is most effective in reducing the dark current. The fixed pattern noise at higher exposure levels, and the shot noise at lower exposure levels determine the S/N during operation. The noise factors that affect the detection limit are dark shot noise and readout noise. Since dark shot noise largely depends on the dark current, if sufficiently minimized, readout noise ultimately governs the detection limit or the minimum level of the dynamic range discussed in the next section.

### Noise - NMOS linear image sensor

NMOS image sensor noise is largely divided into fixed pattern noise and random noise.

Fixed pattern noise includes spike noise and dark current. Spike noise is a switching noise occurring on the video line via the drain to gate capacitance of the MOS switch when an address pulse is input. The magnitude of this noise is constant when the readout conditions are specified, so they can be subtracted from each pixel on signal processing software. In contrast, random noise is traceable to erroneous fluctuations of voltage, current or electrical charge which are caused in the signal output process. This random noise may occur inside the image sensor and also in the readout circuit. When the fixed pattern noise is subtracted by an external circuit, random noise determines the lower limit of light detection of the image sensor, or the lower limit of dynamic range.

Taking performance during actual operation into account, Hamamatsu NMOS image sensors are tested and evaluated by measuring the total random noise derived from the readout circuit, not from the image sensor only.

The noise level is expressed in equivalent input noise or ENI, which is a value converted into input charge units to the image sensor. These units are the root-mean-square value for the number of electrons (electrons r.m.s.).

### Noise - Photodiode

Like other types of light sensors, the lower limits of light detection for photodiodes are determined by the noise characteristics of the device. Noise in photodiodes is the sum of the thermal noise (or Johnson noise) ij of the shunt resistance and the shot noise isD and isL resulting from the dark current and the photocurrent.

### Noise - Photomultiplier tube

Like other types of light sensors, the lower limits of light detection for photomultipliers are determined by the noise characteristics of the device. Noise in PMTs is the result from statistical fluctuation in the dark current and photocurrent known as shot noise. The photomultiplier gain has very little effect on the noise.