Effective Evaluation of the Noise Characteristics of Solar-Blind Cs2te

Optik 143 (2017) 14–18

Contents lists available at ScienceDirect

Optik

j ournal homepage: www.elsevier.de/ijleo

Original research article

Effective evaluation of the noise characteristics of solar-blind

Cs2Te ultraviolet image intensiﬁers

a,b,c,∗ a a a b,c

Honggang Wang , Wenju Zhou , Hongguang Li , Jun Yue , Enze Zhang ,

b,c a

Yan Wang , Xinhua Chang

School of Information and Electrical Engineering, Ludong University, Yantai 264025 Shandong, PR China

Ministerial Key Laboratory of JGMT, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu, PR China

School of Electronic and Optical Engineering, Nanjing university of science and technology, Nanjing 210094, Jiangsu, PR China

a r t i c l e i n f o a b s t r a c t

Article history: In practice, for a higher signal to noise ratio of the solar-blind ultraviolet (UV) image inten-

Received 18 March 2017

siﬁer, one of the most meaningful work is to effectively evaluate its noise characteristics

Accepted 15 June 2017

and to obtain suitable work voltages. In this paper, we have proposed an evaluation method

and then implemented an effective evaluation of the noise characteristics of a typical solar-

Keywords:

blind Cs2Te UV image intensiﬁer, through the measurement of signal to noise ratio at the

Solar-blind

output end (SNRout) of this image intensiﬁer in different work conditions. The evaluation

Ultraviolet image intensiﬁers

results show that the SNRout of a Cs2Te UV image intensiﬁer increases as the photocath-

Signal to noise ratio

ode voltage and the voltage across MCP increase within an appropriate range. Additionally,

the voltage across phosphor screen has almost no impact on the SNRout. More speciﬁcally,

the suitable values of Cs2Te photocathode voltage, the voltage across MCP and the voltage

across phosphor screen are −300 V, 800 V, and 4500 V, respectively.

1. .Introduction

Many characteristics must be considered in the evaluation of any imaging system[1–5]. The apparently dominant char-

acteristic for a solar-blind ultraviolet (UV) image intensiﬁer is the noisy appearance of the intensiﬁed image [6,7]. This image

is made up of numerous scintillations which are each the result of one photon being detected at the primary photocathode.

In particular, for an actual solar-blind UV image intensiﬁer, the noise characteristics varies when it is in different work

conditions such as the applied voltage. Therefore, to obtain a suitable value of the applied voltage, the effective evaluation

of this noise characteristics is needed.

In theory, the noise characteristics of a solar-blind UV image intensiﬁer should be described by its noise factor [8,9].

However, in fact, the signal-to-noise ratio at the input end (SNRin) of this image intensiﬁer is closely related to the radiation

illumination of incident light. Accordingly, the signal-to-noise ratio at the output end (SNRout) of the image intensiﬁer is

introduced to represent the noise characteristics of a solar-blind UV image intensiﬁer [10]. More importantly, the SNRout is

a key parameter to describe the imaging performance of a solar-blind UV image intensiﬁer. Unfortunately, most research

into the noise characteristics of image intensiﬁers has focused on low-light-level image intensiﬁers, and relatively little

attention has been paid to solar-blind UV image intensiﬁers [11–14]. Thus, in this paper, the measurement method of SNRout

∗

Corresponding author.

E-mail address: [email protected] (H. Wang).

http://dx.doi.org/10.1016/j.ijleo.2017.06.051

H. Wang et al. / Optik 143 (2017) 14–18 15

Fig. 1. Schematic diagram of SNRout measurement system for Cs2Te UV image intensiﬁers.

of solar-blind UV image intensiﬁers is proposed in detail. Subsequently, by adjusting the photocathode voltage, the voltage

across microchannel plate (MCP), and the voltage across phosphor screen, we have measured the SNRout of a solar-blind

Cs2Te UV image intensiﬁer, and thus its noise characteristics have been effectively evaluated.

2. Measurement method

The value of SNRout of a solar-blind Cs2Te UV image intensiﬁer is affected by the measurement conditions, and these

conditions mainly include the incident radiation illumination, the diameter of light spot, and a bandwidth of the system [7].

Thus, it is necessary to specify the conditions for the measurement. Until recently, the standard measurement conditions

for solar-blind UV image intensiﬁers have not yet been publicly given. After many effective measurements, the suitable

−9 2

conditions are determined as an incident radiation illumination of 1 × 10 W/cm at Cs2Te photocathode surface, a light

spot of 0.2 mm diameter, and a bandwidth of the system of 10 Hz [14]. The measurement method is as follows:

Firstly, for a Cs2Te UV image intensiﬁer in normal operation, the UV light at 200 ∼ 400 nm from a deuterium lamp passes

through a ﬁlter and becomes a monochromatic light at 254 nm. This monochromatic light passes through an integrating

−9 2

sphere and then through a pinhole of 0.2 mm, the UV light with an incident radiation illumination of 1 × 10 W/cm can

be formed within a circular area of 0.2 mm on the Cs2Te photocathode input surface. After intensifying, a circular spot is

formed on the phosphor screen. The diameter of this circular spot is deﬁned as a multiplication of the diameter of incident

light spot and the magniﬁcation of this image intensiﬁer.

Secondly, the output light is detected by a low-dark-current photomultiplier. After suitable ampliﬁcation and ﬁltering,

the alternating current (AC) component (root mean square) and DC component (averages) of the signal with the input light

are determined, respectively. Similarly, the AC component and DC component of the background brightness signal without

the input light are also measured, respectively.

Finally, the SNRout can then be obtained from the following expression [14]: − Sdl Sd SNRout = (1)

2 2 −

Nal Na

where Sdl, Nal represent the DC component and AC component of the output signal with the input light, respectively, Sd,

Nadenote the DC component and AC component of the output signal without the input light, respectively.

3. Measurement system

Fig. 1 shows the schematic diagram of SNRout measurement system for Cs2Te UV image intensiﬁers, it consists mainly

of light source, ﬁlter, integrating sphere, optical system, power supply module, signal acquisition and processing module,

industrial computer, and corresponding testing software. The workﬂow of this measurement system is as follows:

1) The monochromatic UV light at 254 nm from the light source module which is made up of deuterium lamp, ﬁlter, and

aperture enters into the integrating sphere, and then after multiple diffuse reﬂections, the uniform UV light is generated.

Subsequently, this uniform UV light enters into the test box and irradiates the input surface of Cs2Te photocathode

through an aperture of 0.2 mm diameter and conjugate lens. Consequently, the required incident radiation illumination

−9 2

of 1 × 10 W/cm can be obtained.

2) The image on the input surface of Cs2Te photocathode is intensiﬁed by the UV image intensiﬁer, and its diameter is

dominated by the multiplication of this image intensiﬁer.

3) By using a low-dark-current photomultiplier and the signal acquisition and processing units, the output signal is detected

and then processed. Ultimately, according to expression (1) and with the help of the corresponding test software, the

SNRout of a solar-blind Cs2Te UV image intensiﬁer can be obtained.

16 H. Wang et al. / Optik 143 (2017) 14–18

Table 1

Values of SNRout with different Cs2Te photocathode voltages.

sample of solar-blind UV image intensiﬁer Cs2Te photocathode voltage (V) SNRout

−150 13.20 − 200 15.23 − 250 17.36 SAIIIBO27 −300 18.22 −350 18.50

−400 18.56

17 t ou 16 SNR 15

-100 -150 -200 -250 -300 -350 -400 -450

pho tocathode voltage (V)

Fig. 2. Effect of Cs2Te photocathode voltage on SNRout.

4. Results and discussion

−9 2

In our measurement, the conditions are taken as an incident radiation illumination of 1 × 10 W/cm , a light spot of

0.2 mm diameter, and a bandwidth of 10 Hz. The sample of a solar-blind UV image intensiﬁer is a widely used Cs2Te image

intensiﬁer (SAIIIBO27). Additionally, it is necessary to use a naked image intensiﬁer with which the power supply is not

packaged. By varying the photocathode voltage, the voltage across MCP, and the voltage across phosphor screen, respectively,

the effect of the variation of these voltages on the SNRout of the UV image intensiﬁer SAIIIBO27 can be acquired as follows.

4.1. Effect of Cs2Te photocathode voltage on SNRout

It is assumed that the voltage across MCP is 800 V, the voltage across the phosphor screen is 4500 V. By varying Cs2Te

photocathode voltage from −150 V to −400 V, the SNRout of the solar-blind UV image intensiﬁer SAIIIBO27 is measured and

shown in Table 1. Moreover, the corresponding curve is shown in Fig. 2.

As can be seen fromTab.1 and Fig. 2, the SNRout of the Cs2Te UV image intensiﬁer generally rises with increasing Cs2Te

photocathode voltage. When this photocathode voltage is low (–150 V), the value of SNRout is small. It is mainly because that

the lower ﬁeld-assisted voltage inhibits the energies of photoelectrons emitted from Cs2Te photocathode. As the photocath-

ode voltage increases step by step in the range of −150 V to −300 V, the SNRout upgrades signiﬁcantly. It is due to the fact that

the higher ﬁeld-assisted voltage will result in higher energies of photoelectrons, and thereby obviously upgrade the SNRout.

However, it should be noted that the SNRout inclines to saturation when the photocathode voltage reaches a certain value

(such as −350 V). Moreover, an overhigh photocathode voltage has a negative effect on the performance of photocathode.

Therefore, to upgrade the SNRout of a solar-blind UV image intensiﬁer, the selection of an appropriate photocathode voltage

is necessary. Concretely, the typical value of photocathode voltage is −300 V for a solar-blind Cs2Te UV image intensiﬁer.

4.2. Effect of the voltage across MCP on SNRout

Assuming that the photocathode voltage is −300 V and the voltage across the phosphor screen is 4500 V, we have mea-

sured the SNRout of the UV image intensiﬁer SAIIIBO27 as the voltage across MCP varies from 700 V to 950 V. Table 2 and

Fig. 3 show that the value of SNRout and the corresponding curve of relations between the voltage across MCP and SNRout,

respectively.

Both Table 2 and Fig. 3 show that the value of SNRout increases rapidly as the voltage across MCP ranges from 700 V to

800 V. This can mainly be explained by the impact of this voltage on the gain of MCP. Speciﬁcally, with raising the voltage

across MCP from 700 V to 800 V, the gain of MCP increases exponentially. At the same time, in spite of the fact that both the

useful signal and nosie signal are multiplied, however, the SNRout increases signiﬁcantly since the amplitude of muliplication

of the useful signal clearly surpasses that of the noise. Moreover, when the voltage across MCP reaches 800 V, the maximum

H. Wang et al. / Optik 143 (2017) 14–18 17

Table 2

Values of SNRout with different voltages across MCP.

sample of solar-blind UV image intensiﬁer voltage across MCP (V) SNRout

700 15.01

750 16.39

800 18.23 SAIIIBO27

850 18.12

900 17.28

950 15.59

17 out

SNR 16

650 700 750 800 850 900 950 1000

Volt age across MCP (V)

Fig. 3. Effect of the voltage across MCP on SNRout.

Table 3

Values of SNRout with different voltages across phosphor screen.

sample of solar-blind UV image intensiﬁer voltage across phosphor screen (V) SNRout

4000 17.39

4100 17.82

4200 17.97 SAIIIBO27

4300 18.09

4400 18.17

4500 18.21

value of SNRout is achieved, which indicates that the optimal operation condition of MCP is performed. Provided that the

voltage further increases, MCP will be operated in a saturation condition and thus its gain does not increase, which leads to

a lower value of SNRout. Hence, in order to obtain a higher SNRout, it is necessary to determine a suitable voltage for MCP.

For solar-blind Cs2Te UV image intensiﬁers, this voltage across MCP of 800 V is satisfactory.

4.3. Effect of the voltage across phosphor screen on SNRout

For the case when the photocathode voltage is −300 V, and the voltage across MCP is 800 V, respectively, we have

measured the SNRout of this UV image intensiﬁer SAIIIBO27 with increasing the voltage across phosphor screen from 4000 V

to 4500 V. And then the values of SNRout of SAIIIBO27 are shown in Table 3 and corresponding curves are shown in Fig. 4.

As shown in Table 3 and Fig. 4, it is clear that the SNRout of solar-blind Cs2Te UV image intensiﬁer SAIIIBO27 increases

slowly as the voltage across phosphor screen increases from 4000 V to 4500 V, even almost retains a steady value between

4300 V and 4500 V. This fact indicates that the voltage across phosphor screen has a small or even negligible effect on

upgrading the SNRout of solar-blind Cs2Te UV image intensiﬁers, provided that this phosphor screen operates in normal

condition. In other words, to obtain a higher SNRout, an increase in the voltage across phosphor screen should not be

considered as an effective means. Moreover, a higher voltage across phosphor screen gives a stronger negative effect on the

lifetime of this phosphor screen. After many measurements, a typical value of the voltage across phosphor screen is 4500 V.

5. Conclusion

In summary, varying the photocathode voltage, the voltage across MCP, and the voltage across phosphor screen, respec-

tively, we have measured the SNRout of a typical solar-blind Cs2Te UV image intensiﬁer (SAIIIBO27), and then the suitable

values of above voltages for this UV image intensiﬁer are determined. In consequence, the effective evaluation of the noise

characteristics of solar-blind Cs2Te UV image intensiﬁers has been achieved. The evaluation results show that, the SNRout

18 H. Wang et al. / Optik 143 (2017) 14–18

18 out 17 SNR

3900 4000 4100 4200 4300 4400 4500 4600

Voltage across phosphor screen (V)

Fig. 4. Effect of the voltage across phosphor screen on SNRout.

of a Cs2Te UV image intensiﬁer increases with increasing the photocathode voltage and the voltage across MCP within an

appropriate range. Additionally, the voltage across phosphor screen has almost no impact on the SNRout. Therefore, from a

practical standpoint, an effective approach to upgrading the SNRout of Cs2Te UV image intensiﬁers is the selection of suitable

values of the photocathode voltage and the voltage across MCP. More speciﬁcally, the suitable values of Cs2Te photocath-

ode voltage, the voltage across MCP and the voltage across phosphor screen are −300 V, 800 V, and 4500 V, respectively.

Ultimately, this evaluation of the noise characteristics of solar-blind Cs2Te UV image intensiﬁers will provide an effective

evaluation means and experimental support for developing low noise solar-blind UV image intensiﬁers.

Acknowledgements

The authors thank Jian Liu and Xiaoyu Zhou for their useful discussions. This work is supported by the National Natural

Foundation of China (Grant No. 61405025), by the National Natural Foundation of China (Grant No. 61472172), by the

Fundamental Research funds for the Central Universities (Grant No. 30920130129625), and by the Talents Introduction

Scheme of Ludong University (Grant No. LB2016016).

References

[1] T. Xie, M.R. Hasan, B.T. Qiu, E.S. Arinze, N.V. Nguyen, A. Motayed, S.M. Thon, R. Debnath, High-performance visible-blind photodetectors based on

SnO2/CuO nanoheterojunctions, Appl. Phys. Lett. 107 (2015) 241108.

[2] J.F. Carbary, E.H. Darlington, T.J. Harris, P.J. McEvaddy, M.J. Mayr, K. Peacock, C.I. Meng, Ultroviolet and visible imaging and spectrographic imaging

instrument, Appl. Opt. 33 (1994) 4201–4213.

[3] J.K. Li, W.Q. Jin, X. Wang, X. Zhang, MRGC performance evaluation model of gas leak infrared imaging detection system, Opt. Express 22 (2014)

A1701–A1712.

[4] R.L. Bell, Noise ﬁgure of the MCP image intensiﬁer tube, IEEE T, Electron Dev. ED 22 (1975) 821–829.

[5] X.H. Wang, B.K. Chang, Y.J. Du, J.L. Qiao, Quantum efﬁciency of GaN photocathode under different illumination, Appl. Phys. Lett. 99 (2011) 042102.

[6] H.G. Wang, Y. Du, Y. Feng, X.M. Hu, Y.S. Qian, Effective evaluation of the noise factor of mcirochannel plate, Adv. OptoE 2015 (2015) 781327.

[7] R.T. Holmshaw, P. Holton, Signal-to-noise measurements on channel image intensiﬁers, Proc. SPIE 274 (1981) 159–164.

[8] H.K. Pollehn, Evaluation of image intensiﬁers, Opt. Eng. 21 (1982) 034–037.

[9] L. Liu, T. Huang, Y.S. Qian, Measurement device for noise factor of microchannle plate, Appl. Opt. 51 (2012) 883–887.

[10] I.P. Csorba, Image Tubes, Howard W. Sams &Co., Indianapolis USA, 1985.

[11] J.C. Lupo, The theory and measurement of the signal-to-noise ratio of second generation image intensiﬁers, Opt, Acta 19 (1972) 651–655.

[12] R.J. Hertel, Signal and noise properties of proximity focused image tubes, Proc. SPIE 1155 (1989) 332–343.

[13] G. Clamberlini, P.L. Ramazza, S. Residori, New approach to oise factor measurement of imaging devices, Opt, Eng. 33 (1994) 845–849.

[14] Y.S. Qian, B.K. Chang, Q.H. Zhan, M.Y. Tong, L. Liu, Development of Signal-to-noise Ratio Tester for LLL Image Intensiﬁer, 22, 2002, pp. 389–391 (in Chinese).