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  • The aim of this chapter is to provide a guide for the correct reading of a detector specifications, and for the identification of those not declared parameters important to evaluate its possible use for the scheduled experiment . See our Camera & detector product catalog.

    At the end of this chapter you will find a bibliography of the available texts dealing with the single devices and the scientific and technical issues associated with them.

    The detector is the device that converts the incident radiation on its surfaceinto electrical signals.

    The electrical signal generated can be a function of the power of the incident radiation (see thermal detectors for infrared radiation as microbolometers or pyroelectrics) or to the photons number (quantum detectors). This is the case of the most common devices currently used to detect the visible and near infrared (NIR) radiation.

    It is important to notice that, regardless of any other cause, as a consequence of the detection mechanism their sensitivity is proportional to the wavelength.

    The diagrams and specifications provided may be referred to the power or to the incident number of photons per second; in order to evaluate the detectors, it must be immediately realized which data are reported.

    In the following paragraphs will be dealt, in a general way, with the most common characteristics of the detectors; it is useful to remark that in the case that laser sources are used, their characteristicshave to be taken into account. The maximum power applied on the detector will be in fact very different, depending on if the laser emits continuous waves (CW) or pulses. A detector suitable for measuring (or monitoring) of a CW laser, can be damaged by the high power pulse peak, even if the average power is below the declared damage threshold (for CW signals). Moreover in the case of pulsed laser is important to assess whether the detector has to return the laser pulse shape and provide data on the single pulse (peak power or energy per pulse), or it has to integrate the pulses within a second and to provide average values.

    A further distinction regards the usage of the laser, i.e. if it is used forthe beam monitoring (in this case it will receive only a small portion of the intensity of the laser beam) or for measuring the energy or the power of the laser. Ramen Spectroscopy can also help to stabilize a laser source 


    Since each amplification chain can only worsen the output signal-to-noise ratio (S/N), in order to make negligible the noise contribution in the following stages, it is important to couple the detectors with initial amplification stages that produce low noise and enough gain.

    A further improvement in this direction is offered by integrated amplification mechanisms, thanks to which the signal amplitude of the detector will result enough largeto make negligible the noise added afterward.

    This mechanism is usually an avalanche effect that occurs when a photoelectron (i.e. an electron released by a revealed photon) interacts, within the device, with other electrons excited by a strong electric field. This interaction will release an avalanche of electrons, which accelerated in the direction of the electric field, willcontribute to the electrical output signal. The number of the generated electrons per single photoelectron is the device gain.

    Devices with such characteristics are the photomultipliers, the MCP (Micro Channel Plate), the avalanche photodiodes, the electron bombardment CCD and the multiplication gain CCD - EMCCD and ICCD.


    The list below provides the most important specifications of single and bi-dimensional array detectors:

    Responsivity the electrical signal (current, voltage or charge) generated by the collected light radiation [I / W, V / W, Q / W].

    - NEP needed power to generate a ratio S/N = 1 [W], keeping in mind that the noise level is proportional to the square root of the electrical band, this will be normally expressed in W/Hz1/2.

    - Detectivity (D *) relates the responsivity, the noise and the area of the detectors. It allows to compare different detectors irrespective of their surfaces:

    (Responsivity (detector area ·electrical bandwidth ) ½/current noise [cm Hz ½ W -1]).

    It may be noted that:

    - QE the photons percentage converted into electrons, it is function of λ, it is identified with the device spectral response, therefore:

    - Linearity is the variation of the Responsivity (R) with respect to the intensity of the collected radiation.

    - Dynamics is the maximum possible signal (before the occurrence of non-linear effects) divided by the NEP. In the CCD is often referred to the Well Capacity.

    -Well Capacity is the total number of electrons available in a single pixels (for a certain conditions of work and reading speed).

    - Dark current is the current present in the detector even in the absence of signal. In the CCD can reach extremely low values (e·pixel/hour).

    - Electrical Band [Response time] is the bandwidth of the detectorand/or of the electronics for the signal processing (Hz) , in the arrays it is often identified with the speed of the multiplexer and of the A/D converter and it is typically expressed in Pixel/sec or frame/sec (for interline CCD).

    - Filling factor is the ratio between the pixels active area and the pitch (center-center), for scientific CCD is typically equal to 1. In the interline CCD is about 0.4 (see next paragraph). This parameter is important in order to assess the effective flow intercepted by the detector, it has to be taken into account for example in assessing the QE of different detectors.


    The following list indicates the most used detectors and furnishes some guidelines of their most peculiar characteristic and/or application:

    1 Microbolometers  andPyroelectrics, are thermaldetectors, they show a wide spectral band limited by the surface treatment. The typical application field is the thermal infrared (from 1 to beyond 14 µm). Though with a similar behavior, the pyroelectrics are sensitive only to signal variation. Hence in case of continuous signals will be necessary tomodulate the signal, for example by using electro mechanic modulators as rotating mirrors o chopper (modulator of rotating disc).

    2 Photodiodes,are the most commonly used detectors; depending on the material, are available detectors for UV, NIR and thermal Infrared (3-14 µm) radiation. They offer high sensitivity and high rate response time (i.e. a wide electrical frequency bandwidth). See our InGaAs APDs


    They can be used both in photovoltaic or in photoconductive configuration. The former generally offers the best noise performance but it is worse in frequency response and dynamic.

    3 CCD, quantum detectors, are characterizedby very low noise and dark current, hence they are particularly suitable for long exposure times. Typical applications are in spectroscopy and fluorescent microscopy. There exist a specific type called Back Illuminated, obtained by means thinning the detector substrate side (i.e. theback side of the device) and hence, collecting the signal from the back and not from the frontal surface of the device. In this way we can obtain higher values of QE and a wider spectral response toward the UV range.

    4 C-MOS, quantum detectors, have generally very high noise, compensated by a very wide electrical bandwidth. They are used for high rate imaging (500-100.000 frame/sec).

    5 Interline CCD - ICCD, thestructure of these detectors generally consists of an arrays of small photodiodes with a low filling ratio, and of one or more CCD arrays used as memory support (from which the name of CCD, even if the photodiodes are the real detectors). In order to enlarge the collection area of each pixel, it is used an arrays of immersion lenses (small semi-spheres), which multiply the useful area forthe refractive index. Such a solution is only usable for not too bright optics (F#>>1).The lenses arrays can be formed by RGB (Red, Green, Blu) filters and then used for colored images. This kind of CCD permits very wide electrical bandwidth (20 Mpixel/sec, 50-100 frame/sec) with noise levels suitable for fluorescent microscopy, they are in the middle between CCD and C-MOS.

    6 Photomultipliers are available either individually or in small arrays. They offer a high collection area and gain, they have good electrical bandwidth and dynamic, by contrast, they requires high bias voltages. As a result an entire system (photomultiplier + housing + power + amplifier) can be expensive and delicate to use. The spectral response depends by the photocathode material (front surface). The current availability of photodiodes with performances very similar to those of photomultiplier tubes, but much easier to use, it is limiting the use of photomultiplier only in cases in which their sensitivity it is an essential requirement.

    7 MCP, are two-dimensional arrays of microtubes (of glass) with the faces coated by a metallic layer. The structure is shown inFig.31. These devices have high gain and the capability to keep the spatial information of the input signal. Although the MCP can give an electric output signal,it is normally usedin output a phosphorus (see fig. 33), where it is formed again an image with an enhanced intensity . Usually,the Phosphorus is coupled to a CCD by meanslensesor FAP (Fiber Optic Plate) eventually able to vary the size of the image (Taper). The MCP areparticularly suitable for low level signals. By contrast, the noise does not allow the use for long exposures times (> 1 sec).Depending on the material and on the size of the microchannels we can distinguish between II, III or IV generation (Gallium Arsenide with microchannels of 6-7 µm, particularly suitable for near-IR). In the section related to the noise, specific comments on the use of these detectors will be discussed.

    Fig. 33– Section of an MCP


    The MCP and CCD can be used for the X-ray detection.

    In the case of CCD, the ability to detect X-rays is a direct consequence of the physical and thermodynamic properties of the pixels structure constituting the CCD. When a photon X reaches a CCD pixel, this photon X produces, by means photoelectric absorption, an amount of electrons proportional to the photon X energy, hν.

    For example, a photonX of 1KeV generates roughly 275 electrons, while a photon in the visible region, at themost, generates one electron. Given the X-rays high penetration power in the pixel active region, the generated electron cloud can involve several pixels at the same time for each single X event : we can speak of "split-pixel event". It is always possible to determine the initial impact point (single pixel) by measuring the center of gravity of the generated electron cloud in the pixels involved. The following figure (Fig. 34) shows the quantum efficiency curve of a CCDin the energy region between 1EV and 10.000eV.

    Fig. 34– CCD quantum efficiency curve

    The sensitivity region can be extended up to energies of 100.000eV by depositing suitable films of"sparkling" materialon the surface of a particular device aimed to the optical coupling between the scintillator and the CCD, called "fiberoptic taper" (Fig. 35), this device consists of an optical fiber array of optical fibers deformed in such a way to make the diameter of the array and of the single fiber different on the two sides with consequent enlargement or reduction of the transposed image.

    Fig. 35 - Fiber optic taper


    In what follows we furnish some practical considerations on the detector noise in relation to their applications, referring to the texts in the bibliography for a detailed discussion of the topic.

    The noise in the detectors is a statistical phenomenon due to temperature and to the "corpuscolar" properties of the light signal and to the device internal current.

    We can express the total noise as the quadratic sum of several components depending by the device type (CCD, photodiode or other), which we can describe as follow:

    • (4KTB/R)½ (thermal noise)

    • (2eIB)½ (junction diode current noise)

    • (Ni•QE)½ (shot noise of the incident radiation)

    The formula below furnishes a general estimation of the noise in a detector for an exposure duration t (sec). For exact calculations of the noise of a detector,we refer to the literature, our aim here is to show the effect of factors such as the temperature, the material resistivity, the presence or absence of a junction and the value of the bias current.


    where In= total noise current, K = Boltzmann constant, T= temperature in Kelvin, B = electrical noise bandwidth, I = thermal or junction bias current (photovoltaic or photoconductive configuration), e = electron charge, Ni = photon number/sec, QE = Quantum Efficiency, Nr = readout noise where present (CCD).


    Looking at the formula above we can see how the electrical band appears at the numerator, hence the acquisition rate (electrical band) shall always be the minimum compatible with the experiment.

    Even the temperature appears in the formula (thermal noise, dark current, etc...), the cooling of the detector and of the first amplification stage is the best technique in order to improve the signal-to-noise ratio of a measurement, especially when long exposure times are required.
    However it should always verify the consequences on the other parameters, such as the quantum efficiency, the spectral response and the dynamics which can be strongly affected by the temperature.

    The use of bi-dimensional arrays leads to not statistical noise due to permanent inhomogeneitiesamong the electrical and geometrical features of the individual elements (pixels). We do not deal with this issue in the present work, as it is widely discussed in several texts, and because nowadays this noise is "easily" compensateby sophisticated analysis software.

    Shot noise is always associated to the signal (√n photons) and to thebackground. In some cases (BLIP detectors, background limited), it represents the real sensitivity limit of the devices, since it can be dominant with respect to the intrinsic noise of the detector (eg. in thermal IR detectors cooled to cryogenic temperatures). The shot noise is independent by the detector, but some detectors have detection mechanisms or gain that can increase its amount.

    The MCP, for example, worsen the shot noise by a factor "K" that only in the best cases is close to 2. Therefore it is necessary, especially in the choice of intensified detectors, to verify their behavior with respect to the shot noise.

    Dark currentis not a real noise but the its cause, since to it is associated with a shot noise
    equal to the squared root of its value. The dark current is important in applications requiring long exposures, not only for the associated noise, but also for the saturation and non-linear effects possible in the detector and in the processing electronic.

    RMS values and Peak-Peak, the producers provide the noise data (eg. readout noise) in terms of RMS (Root Mean Square) values. This datum is useful for statistical evaluation and it is usually the most easily measurable value. However in single acquisitions is necessary to take into account that the value of the singlel peak noise can greatly vary. A general rule in measurements by electronic devices is that the value of the peak-peak noise in average is about 6 times the RMS value.

    By assessing the effects of theduring the image acquisition, we must remember the effect of the spatial integration over the image. This will make the real value of thedependent on the size of the observed object. In fact if the object (even with a complex structure) covers many pixels, it is possible to recognize and make measurements even if the electric S/N signal-to-noiseratio is less than 1.

    Noise 1/f this component of the noise, is mainly due to charges generated or accumulated on the detector surface and that show themselves as an inversely proportional function of the electrical frequency. The general formulation is Y/fα. The formula identifies the knee (i.e. thefrequency at which the noise becomes significant with respect to the other components) and the slope.

    In some detectors it is very important and makes their use inconvenient at low frequency. In the case ofinterline CCD, this noise prevents the useA/Dconverter at low-frequency (50-100 KHz instead of the typical 10-20 MHz), since the advantage in terms of noise due to the reduction of the electrical band is deleted by the presence of the 1/f noise.

    MCP, particular remark, as mentioned above the Micro Channel Plate are the elected detectors when it is necessary to detect low intensity signals and, in particular, when nonlinear threshold techniques are employed (e.g. Photon Counting). In this case it is necessary that the signal generated by each single photon is widely larger than the noise level of the processing electronic, but is not required a linear response in the event that several electrons reach simultaneously (i.e. in the same unitary time interval of measure ) the same pixel. In the event that the MCP is used for linear measurements it is necessary to verify that the conditions for thesaturation of the microchannels are avoid.

    This can typically occurs in the following cases:

    • the charging time of the MCP is long compared to the frequency of the events;
    • too much intense signals in a given area, slow down the charge time of the involved microchannels (if the signal is particularly intense, a charge inversion occurs that further slow down the recharging process);
    • moreover an intense signal can temporary reduce the gain even in adjacent channels and this can be very critical for very weak signals occurring (spatially) close tointense signals;
    • in spectral measurements, the sensitivity variation of the photocatode can generate non-linearity at the wavelengths of greater sensitivity. It is important to observethat such effects occur while dynamically using of the device, therefore these nonlinear effects may not appear during the preliminary tests.

    As a consequence:

    Checks on the linearity at the specific measurement conditions must be carried out with all the intensified devices, including multiplication gain or electron bombardment CCD.

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