INSTRUMENT DISTRIBUTION

FOR OF

MEASURING LIGHT

V. K. Goncharov

and

THE

SPATIAL

ENERGY A.

N.

Loparev

UDC 621.396.6:535.2

The p r o b l e m of m e a s u r i ~ g r e f l e c t i o n and s c a t t e r i n g i n d i c a t r i c e s a r i s e s in connection with c e r t a i n studies a s s o c i a t e d with illumination of v a r i o u s objects with s h o r t light p u l s e s . Short light p u l s e s a r e usually r e c o r d e d with the aid of v a r i o u s o s c i l l o s c o p e s . Plotting i n d i c a t r i c e s f r o m one o r two points (depending on the n u m b e r of o s c i l l o s c o p e b e a m s ) p e r one light pulse [1] r e q u i r e s illumination of the studied object by many p u l s e s until one c o m p l e t e indieatrix can be obtained. H o w e v e r , since the r e p r o d u c i b i l i t y of the e n e r g y p a r a m e t e r s of s o u r c e s of s h o r t light pulses is as a rule quite low, plotting i n d i c a t r i c e s by such methods l e a d s to c o n s i d e r a b l e e x p e r i m e n t a l difficulties. M o r e o v e r , the d e s c r i b e d method cannot be used at all in the study with rapidly changing c h a r a c t e r i s t i c s . H e r e we d e s c r i b e an i n s t r u m e n t (Fig. 1) for m e a s u r i n g the spatial distribution of r e f l e c t e d o r s c a t t e r e d e n e r g y of light p u l s e s in the n a n o - and m i c r o s e c o n d range. The m e a s u r i n g principle is as follows. Light e n e r g y is incident on p h o t o d e t e c t o r s located at points at which the light intensity is to be m e a s u r e d . E l e c t r i c a l s i g n a l s produced by p h o t o d e t e c t o r s a r e s t o r e d and a f t e r a c e r t a i n time a s a m p l i n g s y s t e m d i s p l a y s i n f o r m a t i o n about the intensity of the light signal incident on each photodetector. T h i s information, in the f o r m of r e c t a n g u l a r p u l s e s , is d i s p l a y e d on an o s c i l l o s c o p e . The amplitude of each pulse is p r o p o r t i o n a l to the level of light e n e r g y incident on the given photodetector. The i n s t r u m e n t block d i a g r a m is shown in Fig. 2. The i n s t r u m e n t o p e r a t e s as follows. A light signal incident on the active a r e a of the photodetector 4 is c o n v e r t e d into an e l e c t r i c a l signal which is applied to the s t o r a g e network 5. To p r o l o n g the s t o r a g e t i m e , the s t o r a g e network is followed by a de a m p l i f i e r with a high input i m p e d a n c e 6. The output voltage of network 5 is applied through a m p l i f i e r 6 to a pulse m o d u l a t o r 7 for the entire s t o r a g e t i m e duration. An e l e c t r i c a l pulse is applied to the i n s t r u m e n t input (trigger c i r c u i t 1) s i m u l t a n e o u s l y with the light pulse o r a f t e r a s m a l l delay. The t r i g g e r c i r c u i t c o n v e r t s a pulse of any shape into a positive r e c t a n g u l a r pulse which is applied to input c i r c u i t 2, The input c i r c u i t g e n e r a t e s a negative pulse d e l a y e d with r e s p e c t to the input pulse f o r a c e r t a i n time which, in the final a n a l y s i s , depends on the light pulse duration. The output signal of the input c i r c u i t is applied to the input of r e c t a n g u l a r pulse g e n e r a t o r 3. F o r each t r i g g e r pulse, g e n e r a t o r 3 p r o d u c e s exactly one positive r e c t a n g u l a r pulse. T h i s pulse is applied to pulse m o d u l a t o r 7 which, as a r e s u l t of interaction of two voltages (the pulse f r o m g e n e r a t o r 3 and the output voltage of a m p l i f i e r 6), p r o d u c e s a positive r e c t a n g u l a r pulse whose amplitude is proportional to the level of light e n e r g y incident on photodetector 4. This pulse is applied to o s c i l l o s c o p e 8. After a delay equal to its width, the pulse produced by g e n e r a t o r 3 is also applied to the input c i r c u i t of the second channel. This p r o d u c e s two consecutive pulses on the o s c i l l o s c o p e s c r e e n ; the i n f o r m a tion about the level of light e n e r g y incident on each photod e t e c t o r is thus consecutively s a m p l e d and is displayed on a single-beam oscilloscope.

Fig. 1: G e n e r a l view of i n s t r u m e n t .

The d e s c r i b e d i n s t r u m e n t has sixteen identical channels (any n u m b e r of channels can be used in principle). F i g u r e 3 shows a typical o s c i l l o g r a m showing the

T r a n s l a t e d f r o m Zhurnal P r i k l a d n o i Spektroskopii, Vol. 22, No. 3, pp. 557-560, March, 1975. Original a r t i c l e submitted S e p t e m b e r 26, 1974. 9 76 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. 10011. No part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15.00.

428

Light Is. .

//signal

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'

' ~

L--..aj

[ I

I

_X~aight

~

A

I

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Fig. 2 Fig. 3 Fig. 2. Block d i a g r a m of the instrument. I, II, and XVI denote the channel numbers. Fig. 3.

Typical o s c i l l o g r a m .

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next

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1: 9l&.~-a ~s

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Fig. 4. Circuit d i a g r a m of f i r s t channel and t r i g g e r . D1-D 4 are type D9K diodes.

distribution of light energy reflected f r o m a rough surface. Sharp v e r t i c a l spikes on the o s c i l l o g r a m separate the pulses of adjacent channels. The i n f o r mation about the level of light energy falling on the photodeteetor is provided by the distance between the horizontal line c o r r e s p o n d i n g to a given channel and the base line. Figure 4 shows the c i r c u i t d i a g r a m of the f i r s t channel and t r i g g e r . The t r i g g e r is a driven multiv i b r a t o r using two KT 315G t r a n s i s t o r s T I and T 2. The m u l t i v i b r a t o r is preceded by a diode l i m i t e r (D~, R~, and R3). A differentiating network together with the diode l i m i t e r acts as the input circuit. A blocking g e n e r a t o r consisting of a KT 315G t r a n s i s t o r T 3 p r o d u c e s r e c t a n g u l a r pulses applied to the pulse modulator and the next channel input.

The s t o r a g e c i r c u i t c o n s i s t s of an RC pair. The c a p a c i t o r C4 of this pair is the distributed capacitance of the cable connecting the remote photomultiplier PM with the instrument. P o t e n t i o m e t e r R~ of this pair is used to control the t r a n s f e r constant of the channel and to d i s c h a r g e the storage c a p a c i t o r C4 so it is r e a d y f o r s u b s e q u e n t m e a s u r e m e n t s . A field-effect t r a n s i s t o r KP302A is used as the dc amplifier. It s e r v e s as a buffer stage between the storage pair and the pulse modulator and prevents the storage c a p a c i t o r to be shunted by the low input r e s i s t a n c e of T 5. A KT315G t r a n s i s t o r connected as an e m i t t e r follower acts as the pulse modulator. Rectangular pulses of the blocking o s c i l l a t o r applied through c a p a c i t o r C 5 provide the c o l l e c t o r voltage of T 5. The pulse whose amplitude is proportional to light energy falling on the photomultiplier cathode is taken f r o m the e m i t t e r follower load and is applied to the oscilloscope through decoupling c a p a c t o r C~ and diode D 4. Diode D 4 decouples the signals of different channels f r o m each other. J

The photodetectors were FEU-62 photomultipliers which have a wide spectral range. F o r the sake of convenience we have used an S1-42 oscilloscope with a storage tube, but any o t h e r oscilloscope can be used provided its pass band is sufficient to pass the entire frequency bandwidth of the r e c t a n g u l a r output pulses. The i n s t r u m e n t sensitivity depends on the actual sensitivity of the photodetectors and the light pulse d u r a l tion. With FEU-62 photomultipliers and b e l l - s h a p e d light pulses with a half-width of ~ 50 nsec, the ins t r u m e n t sensitivity w a s 10 -2~ J. The a c c u r a c y of the relative m e a s u r e m e n t s depends on the type of o s c i l l o s c o p e used, and is n e a r l y the same as that of o t h e r oscilloscopic m e a s u r e m e n t s . 429

In conclusion the authors take pleasure in thanking L. Ya. Min'ko for his constant attention and for discussing the "results, and V. I. Nasonov for assisting in the design of the circuit. LITERATURE 1.

430

cITED

V.A. Zhuravlev, G. D. Petrov, and E. F. Yurchuk, Teplofiz. Vys. Temp., 11, No. 4, 874 (1973).