Detectivity limit of very small objects by video-enhanced microscopy

Detectivity limit of very small objects by video-enhancedmicroscopy

Yoshihiko Mizushima

Video-enhanced optical microscope imaging has recently achieved considerable improvements in the detecti-vity of very small objects, even exceeding Rayleigh's resolution criterion. Improvements in the signal-to-noise ratio have been attained utilizing digital image processing by reducing background and fixed patternnoise. Thus the video-enhanced optical microscope is a modern version of a traditional dark-field ultrami-croscope for detecting scattered light. Theoretical calculations presented in this paper predict that theultimate detectable size is several nanometers, 1/100 of the wavelength. This is in agreement with recentexperimental reports. Effects of microscope structure factors are discussed.

1. Introduction

As is widely accepted, spatial resolution of an opticalmicroscope is conveniently expressed by Rayleigh'scriterion. The formula for the resolution limit (d) is

d = 0.61 (1)N.A.

where X is the wavelength and N.A. is the numericalaperture.

A powerful set of objective and condenser lenses ofhigh N.A. (e.g., 1.4) yields a resolution of 230 nm for X =450 nm. Even if we assume another criterion such asSparrow's, the limit is only slightly improved to two-thirds of the above.' However, it has recently beendemonstrated that a video-enhanced digital imageprocessing technique yields a much improved image ona monitor screen over that obtained by direct observa-tion. Utilizing video enhancement with digital imageprocessing, several important results have been pub-lished, mainly in the area of cell biology,2-4 where verysmall structural features of biological cells have beendiscovered.

These reports claim that features as small as -20 nmare observable. These structures correspond to thoseobserved with an electron microscope. However, theelectron microscope measurements are made with

The author is with Hamamatsu Photonics, K.K., 1126 Ichono-cho,Hamamatsu City 435, Japan.

Received 2 October 1987.0003-6935/88/122587-08$02.00/0.(©) 1988 Optical Society of America.

nonliving specimens, while the optical observationscan be performed without damaging a living cell if nostain and low levels of ultraviolet irradiation are used.Therefore this technique is of vital importance in bio-logical as well as in medical sciences. Also, there is arecent report indicating detection of colloidal gold par-ticles as small as 5 nm in a biological cell when in astationary state and 10 nm when in motion.5 This isdiscussed in detail.

Light-scattering particle counters, able to countsmall particles of <100 nm, are utilized in the semicon-ductor industry for dust or aerosol monitoring. In thiscase, no quantitative estimation of the detectivity limithas been made.

II. Video-Enhanced Microscopy

A schematic of a typical video-enhanced microscopysystem (Hamamatsu C-1966) is shown in Fig. 1. Inthis system it is important that a low dark-currentimage pickup tube be employed to maximize the sig-nal-to-noise ratio. Images of 512 X 512 pixels aredigitized to a depth of 8 bits. The images are stored inframe memories and numerical operations are madebetween the input and stored images by algorithmsstored in ROM. The digital operations stored in ROMinclude contrast enhancement, bias offset, integration,rolling averaging, jumping averaging, subtraction ofbackground noise, canceling of fixed pattern noisesuch as a mottle image. Even when direct observationby the human eye gives no definite image because thefield is too bright or the contrast is too low, a clearimage is obtained on a monitor screen through properadjustment of the electronics and choice of digitaloperations.

Allen was the first to explain the image enhance-

15 June 1988 / Vol. 27, No. 12 / APPLIED OPTICS 2587

I MICROSCOPE1

IMAG E PICKUP TUBEI

E~~ ~~ MEORES

Fig. 1. Schematic of a video-enhanced microscopy system.

mert and the improved resolution obtained with avideo microscope. He demonstrated that the advan-tage of electronic bias offset on an image obtained froma differential interference contrast (DIC) microscopeis due to an enhancement of the contrast by electronicbackground adjustment along with optical phase shiftbias.2 In this way, a resolution several times the Ray-leigh limit was possible. However, for a resolutionapparently exceeding the limit by several orders ofmagnitude, no quantitative investigation has ap-peared.

In this paper, a quantitative calculation of light scat-tering by small particles or rods is performed and theresultant detectivity limit is discussed. Since achiev-ing the detectivity limit has been established empiri-cally in the biological sciences, the present theory as-sumes an optical transmission type microscope withbiological samples, such as living cells embedded in atransparent medium. Here, biological samples arenearly transparent, and the light attenuation is negli-gible, so that the contrast is caused by the refractive-index difference. The refractive index of the particleis assumed to be n, while that of the medium is no.Further, scattering from a metallic particle in a trans-parent medium is also estimated to show that themetallic scattering has a larger cross section than thatfrom a dielectric particle. MKS units are used in thispaper.

11. Scattering Cross Section

Since we consider an object that is much smallerthan the wavelength of light, the majority of the signalcomes from the scattering. First, we consider scatter-ing from a small dielectric sphere with a refractiveindex n. According to the textbooks,6 the incident andradiated power density (wo and w) are the time aver-ages of the Poynting vectors:

U)11 = 2 , r/pE2, (2a)

U) = 2 \ c/pE (2b)

where E denotes the electric field and e and p are thedielectric constant and permittivity, respectively.

Integrating w over a sphere, we obtain a total scat-tered power (I) in all directions:

1 k4 cp2

47rCz, 3I = 27r/N,

dS

Fig. 2. Scattering ray trajectory with respect to an objective lens.

where p is the dipole moment of the object and c is thelight velocity:

p = aeOEefr, a2 = 12a2 + m2 a2 + n2 a2,

Eeff = E + P (4)3co

Here 1, m, and n are the direction cosines of the Evector with respect to the three main axes of the polar-izability tensor a. For simplicity, the polarizability isassumed to be isotropic:

a] = a2 = a3 = - (4a)

From the Clausius-Mossotti formula for continuousmedia consisting of N dipoles per unit volume thepolarizability is

aN = 3(n2- 1)

n2 + 2(5)

Effective a for scattering from a sphere (radius a) is4 3 3(n2 -1)

a = 3ra (6)

There are several scattering theories that differ slight-ly from one another. But since the object is very small,the following calculation is mostly based on Rayleigh-Gans theory for a dielectric sphere' and the approxi-mation is based on Mie scattering from a metallicsphere.

When the polarizability is isotropic, the scatteredintensity (I) from a dielectric sphere at an angle (0) inFig. 2 is

2588 APPLIED OPTICS / Vol. 27, No. 12 / 15 June 1988

!

E

Fig. 3. Polarization orientation of the incident ray with respect tothe incident angle.

Equation (a) gives a larger cross section than Eq.(lib), but the latter result differs only slightly. Forrandomly distributed curved fibers, a rigorous expres-sion is complicated, therefore a simplified assumptionis made for the integration of Eq. (la). It is assumedthat the scattering is isotropic and that an angle-aver-aged value is used.

IV. Microscope Structure Factor

The amount of light accepted by the objective lens isdependent on the solid angle due to the microscopestructure factor. The scattering cross section is multi-plied by this factor to obtain the total eficiency, asshown in Fig. 2. The objective lens is defined by anaperture angle y. Degree of acceptance is determinedby an angle X, which is the intersection angle of the twocones -y and 0, whereby is an incident angle. Aftersome manipulating

cos = 2(cos- cos6 cosO)2_ 1 + 2 coS20.

(1 + Cos°2OM4

a2

2r2(7)

where r is the distance from the scatterer.The effective signal is calculated as the difference of

the scattering from the object and that without it,namely, scattering from the same volume but filledwith the surrounding medium. The effective scat-tered signal is then

I(0) = I(O,n) - I(6,no). (8)

If the incident light is linearly polarized as in apolarization microscope, the scattered intensity distri-bution is, referring to Fig. 3,

I(0) =(,1- sin2X sin20)k4 a2

1(0) ~2r2 (9

(12)

When 0 = 0, the two cones are tangential to each other.Acceptance of the scattered flux is limited by 0 withinthe y cone. Here k is proportional to I, where

cost = 1 1- cos 2 (cosy-cos cos0)2 _ 1sin2 0 sin2 3 sin2 0

(13)

So that for a given 6, the amount of scattered light thatis collected, I(6) is obtained by integrating I(0).

For a sphere, from Eq. (7)

I(O) =I I (O)dO

= f I + Cos 2 k4 a2 w02irr2 sinOdS.(14)

where x is an angle between the E vector of the inci-dent light and a vector OT. Also a similar equation isobtained for the case of incident natural light and areceiving polarizer at the detector.

For a metal sphere, an approximate calculation us-ing Ref. 6 yields the following:

I(O) = 4 2r2 1- 5 cosO + cos20)w0. (10)

A tiny value for small 0 implies dominant backwardscattering, because the signal scattered from the objectis weaker than from the medium itself. Thus theeffective signal is negative.

For scattering by a very thin dielectric circular cylin-der (fiber), lying perpendicular to the incident ray, thefollowing expressions are given.6 Although some fi-brous structures exhibit double refraction, they makea minor contribution, as mentioned later. Scatteredflux from a cylinder both at perpendicular incidenceand the radiation is given per unit length:

Outside of the intersection (the limit of integration y +3 0 _ :y - 61), Eq. (14) gives

I(6) = 0. (14a)

A similar equation for polarized light or metallic parti-cle scattering can also be obtained.

Total flux intake due to incident flux intensity wo isestimated by the condenser lens aperture, as shown inFig. 4. Here, for simplicity, the image magnificationratio of the condenser is assumed to be unity (symmet-rical arrangement with respect to condenser lensplane), so that the incident angle is the same as theradiating angle. Although the condenser can be ad-justed, the result is not strongly dependent on theadjustment; in this calculation a simple configurationis assumed. In this respect, the K6hler and criticalilluminations are not so different; the former is usuallyadopted, because of its low shading property.

The flux, dF, radiating from a horizontal small areadS in Fig. 4 is

I = k a (n2- 1)2Wo, Ell axis,

I7rk 3 a4 In 21 \2

2 n2

+ 1 ) Wo, Elaxis.

dF = 2rLdF sinb cosb.(Ila) (15)

where L is the radiance of the source in W/m2 . The(llb) incident power density w0 at an angle 6 through the

condenser is


Il

(9)

f(PQ) = | (1 + cos2 0)t sinOdO sinb cosbdb.

V. Noise Considerations

The object image is dispersed by diffraction into anAiry diffraction area, so we estimate the signal-to-noise ratio within this area.

The residual shot noise cannot be canceled out.Thermal noise is negligible, and the fixed pattern noiseis canceled by the image processing. The image signalis biased to cancel out the background intensity level,or to operate on another image, such as fixed patternnoise. Mottled subtraction is made by utilizing abackground image containing no object obtained froma defocused image. Therefore an ideal dark-field ul-tramicroscope with a minimum background level isachieved by video image processing.

In an optical regime where h v > kT (h is Planck'sconstant, v is the frequency, and k is Boltzmann'sconstant), shot noise is the dominant factor. Opticalpower is

w = Nh v, (19)

where N is an average incident photon number persecond striking the detector. An average number ofphotoelectrons released from the photocathode (Ne) is

TV = NT t7,

QK p

Fig. 4. Ray trajectory with respect to condenser lens geometryindicating a dark-field arrangement.

wo(b) = 2rL sinb cosb(dS/dS') = 2rL sinb cosb.

(20)

where T is the measuring time and 77 is the quantumefficiency. If it is assumed that the fluctuation ofelectrons is caused by the fluctuation of the number ofincoming photons at each pixel,

(AN)2 (Ne)2 2 = NT . (21)

Due to the Poissonian nature of photons, the minimumdetectable signal is of the order of the fluctuation,which is the noise-equivalent power (NEP):

(16)

Here dS = dS' because of unity magnification. It isassumed that the transmission loss of the lenses isnegligible.

The total power into the objective lens is then calcu-lated, including a case for the dark-field microscope,where the condenser lens or mirror has a concentricstructure with an opening at the center (doughnutshape), as shown in Fig. 4, so that the incident radia-tion comes from a ring with an angle ( between P(maximum entrance angle) and Q (minimum entranceangle). The total signal (P,Q) is obtained by integrat-ing I(6) from Q to P for a given set of condenser lensparameters,

I(P,Q) = J I(6)db. (17)

For the example of a dielectric sphere, Eq. (14) yields

I(PQ) = 167rik4a

6L [(n ) - ( )2 x f(P,Q), (18)

where f(P,Q) is a structure-dependent term,

NmnTjT = NminT7,

Nmii = 1/T,7.(22)

If T . 1/B, where B is the bandwidth, the minimumdetectable power, Wmin, is expressed by the well-estab-lished photon counting limit:

7lWmiii = h77B. (23)

Since this relation is also deduced from the uncer-tainty principle, Wmin should be compared with thequantum limitation, which has almost been reached inphoton counting experiments. This is based on re-duced dark current of the image pickup tubes. Forexample, the dark counts correspond to 30 electrons/sper Airy area, which is 10 nA/cm2 . This can easily beachieved.

For spatial fluctuation component (Aq), from theradiation Hamiltonian,

(24a)


IdS/

d

l

H = 1/2) (P2 + 2q2),I I

I

4

3

2

40 60 8 S 200 40 60

(a) (b)

Fig. 5. Examples of the angle-dependent functionI(6) in Eq. (18) for a dielectric sphere: I(6) = S (1 +

t cos2

0) t sinO sin6 cos6dO; (a) as a function of 6 and (b) asa function of y.

and if p (momentum) and q are equally uncertain, Aqfollows as a zero-point criterion, which is

Aq = !hP2v. (24b)

It is very much smaller than the pixel size, therefore itcan be neglected. Thus, fluctuation noise is mostlyconsidered in a temporal scale at a definite point, suchas within the imaging pixel size.

Similarly, the minimum detectable scattered powercan be deducted as (in units of photoelectron number)

[I(P,Q)11/2 _ I(P,Q)

L hvB J hvB

When the signal-to-noise ratio is unity,

I(P,Q) = hvB.

(25)

f'(PQ) = J J [5(l - coso + cos 20)-167r2

(1 + cos20)

4n- \2

X ( -- 1k sinOdO sinb cosbd6.

For a dielectric cylinder (fiber) the unit length elementis simply assumed to be the Rayleigh resolution limitas in Eq. (1). From Eq. (1a),

ad= 16 hvB }1/4

0.61 (X/N.A.)k 3'iL [(n2- 1)2 _ (no 1)2] f"(P Q)

where

f"(P,Q) = J J t sinOdO sinb cosbd6. (29)

(26)

In addition to this widely accepted way of analysis,the least significant bit may contribute to the back-ground level noise. For 8-bits depth, 2- 8N + <N = N.So that

I(P,Q) = hvB(l + 2-7). (26a)

This does not essentially alter Eq. (26).

VI. Minimum Detectable Size

Since I(P,Q) is a function of the object size, theminimum detectable size (ad) can be computed. As anexample, the detectivity measurement (ad) is obtainedfor a dielectric sphere, as from Eqs. (17) and (18),

ri (/ X\~4[(n2 1\2 2 nO- 1\21- hvB ]1/6ad = j167r3 k2ir, Lkn2 + 2 n-2/ Lf(P,Q)J

(27)

Also there is a similar equation for polarized light. Formetallic particles, ad is obtained from Eq. (10):

ad = [7r(X)4 1Lf'(PQ)] (28)

where

VII. Numerical Results on Structure Factors and theMinimum Detectable Size

Some examples of numerical computations of I(6)are shown in Fig. 5. The factor is then integratedwithin P to Q to get f(P,Q) for Figs. 6-9.

I(6) is linearly dependent on -y and the f(P,Q) termsare slightly sublinearly proportional to P, and super-linearly proportional to -y, so that a wider objective lensis advantageous. The advantage of polarization isfound to be relatively weak, since the polarizationmethod utilizes only a fraction of the incoming light.Here, it should be mentioned that the various geomet-rical structure factors are not strongly affected by theobject, except for metallic spheres (Fig. 8).

For the dark-field effect, the decrease in scatteredlight due to the structure factor is nearly proportionalto P - Q. However, small Q is not affected very much.For a typical dark-field arrangement, P = 75° and Q =40°, the signal decrease is about one-half compared tothe bright-field case. By electronic image processing,the background as well as stray light can be thoroughlycompensated. This is an ideal tool to achieve theclassical concept of the ultramicroscope.


4

3

2

(a) (b) ( )Fig. 6. f(P,Q) for adielectric sphere: (a) as afunction of P, y,and Qas parameters; (b) as afunction of y,P, and Qas parameters; and (c) as a

function of Q, P, and -y as parameters.

2

1

80

7

~ L= 60'- - - 9 0

260 406 6C? 80'

(a ) (b)Fig. 7. f(P,Q) for polarized light for a dielectric sphere as a function of P, Q, and 'y as

parameters: (a) x = 0 and 30 and (b) x =600 and 90'.

-20 -20

T= 80=* - P=900/0

60 ~~~~~~~70:40 30 - -.. 40 50 / 3

-70 - --

-1007 0

200 4Q0 600 0 20" 4 O' O6" 80'

( a) (b)Fig. 8. f(P,Q)for armetallic sphere: (a) as afunction of P, y,and Qas parameters; (b) as

a function of -y, P, and Q as parameters. Note the sign is negative.


3 P".900 "

30 /2 X

1 46 8

20' 40". 6O" 80e

(a) (b)

The final numerical results on ad are, as computedfor typical parameters, appropriately estimated ormeasured on practical devices:

X= 450nm, N.A. = 1.4, = 0.2, n = 1.5, n = 1.3,

T = 1/B = 30 ms, L = 107-8 W/m 2

[108 W/M2 = 1 W/(0.1 mm)2 ].

A liquid immersion type lens should be converted tothe respective acceptance angle y.

Computed ad (in nanometers) is listed in Table I.Here, relatively wide and narrow lenses are compared,to show that there is little difference. For other deviceparameters (y,6,P,Q), the results can be easily convert-ed according to Figs. 6-9.

Vil. Discussions

For a dielectric sphere and a metallic ball, the calcu-lated best detectivity for L = 108 W/m2 in Table I lies at-7- and 3.5-nm (radius) for moving and static states,respectively. Even if a point image spreads out ac-cording to diffraction, it is still detectable. Althoughthis situation does not correspond to the rigorous reso-lution criterion, the detectivity can be defined by ad.It should be borne in mind that two neighboring parti-cles are not separately identified.

According to Ref. 5, gold particles as small as 5-nm(diameter) can be experimentally detected by digitalvideo microscopy in a stationary state, with temporalintegration of 64 frames. When in motion, 10-nmparticles are detected without integration. This ob-servation is in crude agreement with the theoreticalprediction.

Also it has been reported that the metallic particleimage appears dark,5 implying that the scattered sig-nal from a metallic particle is a negative signal. This isin agreement with the above theory. In such a case,the background subtraction should be made to leave asmall residual signal.

In Table I, the effect of averaging by integration isobvious. Integration of 64 frames is practical for de-tectivity improvement. Further extended integrationis possible with an imaging tube of lower dark current.

To further improve the detectivity, the exponent 1/6or 1/4 in the above equations indicate an insignificantcontribution of such parameters as P,Q, -y, 6, N.A., or L.If L is assumed to be 120 times as high, which is

Fig. 9. f" (P,Q) for a dielectric fiber: (a) as a functionof P, y, and Q as parameters, (b) as a function of y, P.

and Q as parameters.

Table 1. Minimum Detectable Size (Radius) in Nanometers

I.SMI LINS DI2LAICTOIC METAIIAC D"lllilC1rICIM1E AP'ElUREi SP'llilliR, a SIIEl1Ii,a, 11ER, a,

UNI'OIl II) I'ILAR I ZEDX = 90 X = :l0

1, 1' 1 I 1. 64 F. I 1. 64 l:. I . 1 F. F. 4. 14. I F. (1 F.(w/nl2 ) (1)I1X) (Ii) (nn) (Cml) (rml) (raln)

3) 0 30 1:3.6 6.9 16.; 8.3 16.0 8.0 15.5 7.8 26.4 l1.27( 10.8 5.4 13.9 7.0 12.0 6.0 12.3 6.2 17.7 8.9

10 7 5 12.1 6.1 14.5 7.2 13.2 6.6 13.1 6.5 111.6 13.871 11.4 4.0 11.7 5.8 10.8 5.4 10.2 5.1 131.1 6.5

30 5() 11.0 6.5 11.6 (6. 12.2 6.1 11.7 5.8 16.4 8.270 9.6 4.8 12.4 6.2 11.) 5.5 111.5 5.2 14.1 7.1

30 () :() 9.8 4.9 11.3 5.7 11.) O 5.5 10.6 5.3 10.8 5.47() 7.0 3.9 9.5 4.7 8.5 4.3 8.4 4.2 7.1 :.6

10 Ia 30 8.2 4.1 9.9 4.11 9.0 4.5 8.971 6.4 31.2 7.3 4.91 7.1 3.5 7.0

7()31 51 7.5 3.7 9.2 4 . 8.1

71 10.6 :1.2 8.5 4.2 7.3

4.4 0.0 4.03.5 5.4 2.7

4.1 8.0 4.0 6.7 :1.4.7 7.1 :1.(; 5.8 2.9

practically the highest brightness, the detection radiusbecomes only 0.68 of the indicated value, so that 7 nmis a reasonable detectable diameter. Since the param-eters weakly affect the detected diameter, the estimat-ed ad can rather be regarded as a general criterion forthe detectivity, almost insensitive to the microscopestructure. Therefore, the crude agreement with theexperiment implies the legitimacy of this theoreticalanalysis and gives evidence that the present video-enhanced microscopy has almost attained the ultimatedetectivity limit. This theory treats an idealized situ-ation, but the insensitivity to real parameters makesthis theory valid for practical devices.

Although a classical design of optics with high N.A.is not always necessary, the use of modern electronicsis more important for obtaining higher detectivity.Expensive optics can be partly replaced by electronicsif a crucial resolution is not needed. In addition, digi-tal processing offers other advantages, such as quanti-tative data evaluation and storage.

IX. Conclusion

Video-enhanced microscopy has revealed a remark-able possibility to obtain improved detectivity of veryfine objects. Particles as small as several nanometerscan be detected by an optical microscope equippedwith electronic image processing. Considerable ratio


_ _ _ _ _ _

improvements in signal to noise are mostly due tosuppression of the background level and noise. A darkcurrent-free imaging device is also important. As aresult, full utilization and accumulation of the scat-tered signal makes 5-10-nm detectivity possible. Thevideo-enhanced microscopy is an optimum concept ofthe dark-field ultramicroscope.

Although this technology can be used in variousareas of science and technology, it has recently attract-ed much attention in the biological sciences, such asimmunocytometry; biological substructures such asmicrotubules and DNA aggregates are possibly becom-ing observable in vivo without damage, permitting ob-servation of their movement.

The author wishes to acknowledge valuable discus-sions with T. Hiruma, S. Fujiwake, S. Miyaki, and K.Kaufmann. The research was stimulated by the lateR. D. Allen of Dartmouth College. Assistance with thenumerical calculations by T. Kato is gratefully ac-knowledged.

References1. T. Asakura, "On the Sparrow's Resolution Criterion," Oyo But-

suri (Jpn. J. Appl. Phys.) 31, 709 (1962).2. For example, R. D. Allen and J. L. Travis, "Video-Enhanced

Contrast, Differential Interference Contrast (AVEC-DIC) Mi-croscopy," Cell Motility 1, 291 (1981); "Video-Enhanced Con-trast Polarization (AVEC-POL) Microscopy," Cell Motility 1,275 (1981); R. D. Allen and N. S. Allen, "Video-Enhanced Micros-copy with a Computer Frame Memory," J. Microsc. 129,3 (1983).

3. S. Inoue, "Video Image Processing Greatly Enhances Contrast,Quality and Speed in Polarization-Based Microscopy," J. CellBiol. 89, 346 (1981); S. Inoue, "Videomicroscopy of Living Cellsand Dynamic Molecular Assemblies," Appl. Opt. 26, 3219 (1987).

4. J. De May, M. Moremans, G. Geuens, R. Nuydens, and M. DeBrabander, "High Resolution Light and Electron MicroscopeLocalization of Tubulin with the IGS Method," Cell BiologyInternational Reports 5, No. 9, 889 (1981).

5. M. De Brabander, R. Nuydens, G. Geuens, M. Moremans, and J.De May, "The Use of Submicroscopic Gold Particles Combinedwith Video Contrast Enhancement as a Simple Molecular Probefor the Living Cell," Cell Motility and Cytoskelton 6, 105 (1986).

6. M. Born and E. Wolf, Principles of Optics (Pergamon, Oxford,1975); H. C. van de Hulst, Light Scattering by Small Particles(Wiley, New York, 1957); K. Ishiguro, Optics (Kyoritsu Publish-ers, Tokyo, 1953), in Japanese.

NASA continued from page 2579

the instrument to observe a meter reading. The user's eyes can keepon the circuit board being tested; there is therefore little risk that theuser would let the tester probes slip from the terminals under testand cause false indications or damage the circuit or components. Inaddition, the current and voltage of the tester are strictly limited. Itcan apply no more than 0.6-V dc and no more than 3 mA through theprobes. It can therefore be used safely on circuit boards in whichsemiconductor components have been installed, and on complemen-tary metal oxide/semiconductor integrated circuits, which are highlysusceptible to damage during testing.

The tester is compact, and its circuit is simple (see Fig. 7). It canbe built from inexpensive standard components available from rqtailoutlets. The tester can be adjusted to indicate continuity below anyresistance value up to 35 Q. For example, if the user sets the tester to30 Q, the unit will emit an audible tone whenever the resistancebetween the probes is 30 S2 or less; if, for example, the resistance is30.2 Q, the unit will remain silent.

This work was done by William B. McAlister of Goddard SpaceFlight Center. Refer to GSC-13102.

Electronic neural-network simulatorA hybrid analog/digital neurocomputer made of standard compo-

nents aids research in neural-network computing. It is used tostudy such neural-network concepts as those described below.

The computer contains a digital random-access memory thatstores binary synaptic information and an array of analog thresholdamplifiers with input and output sample-and-hold circuits thatsimulate neurons. The computer is faster than the software simula-tions previously used to experiment with electronic neural networks.The computer is controlled by two clock signals, Cl and C2 . The rateof clock C2 is N times that of C,, where N is the number of neurons.In Fig. 8, there are 16 neurons, so that in this case, C2 is 16 times asfast as Cl. On each cycle of Cl, a serial shift register selects a neuroninput for updating and sampling by one of the sample-and-holdcircuits in group SHI. The neuron input signal is determined by twofactors: the current neuron output state at the outputs of thesample-and-hold circuits in group SH2 and the synaptic informationat the memory outputs that control pass-transistor array SW2. The

updating process is repeated at each cycle of C1 , each neuron beingchecked in turn as directed by the serial shift registers. On eachcycle of C2 , the neuron outputs-as determined by the updatedinputs and the inhibiting signal-are sampled by the circuits ingroup SH2 , completing one system cycle. Thus, the system through-put and cycle time are set by the update-clock rate and the numberof neurons. To prevent an update from being interrupted, the twoclocks are not permitted to overlap.

"Synapses"

C1 - Serial Shift Register Serial Shift Register

Fig. 8. Serial shift register routes clock pulses C, to neurons insequence. Clock pulses C2 interrogate the neurons. The neuroninterconnection information is stored in the simulated synapses(memory chips). The system contains 16 neurons and 256 binarysynaptic nodes. It can readily be expanded to greater complexity.

continued on page 2602


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Detectivity limit of very small objects by video-enhanced microscopy