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Runway visibility
that the c e n t r a l axial r a y s for each i m a g e should be n o r m a l to the common image plane and consequently p a r a l l e l to each other. One method of achieving this a x i a l p a r a l l e l i s m is to use a p r i s m a t i c light s p l i t t i n g arrangement. Crossed dichroic reflectors are used at a n g l e s of incidence o f 45 ° to the p r i n c i p a l a x i s of the incoming light flux and to deflect t h e s e c o m ponents at an angle of 90 d e g r e e s to the p r i n c i p a l axis. Second r e f l e c t o r s in the r e d and blue paths, again u s e d at 45 d e g r e e s , s e r v e to r e e s t a b l i s h the p r i n c i p a l a x e s of these components p a r a l l e l to, but d i s p l a c e d from, the p r i n c i p a l a x i s of the r e m a i n i n g undeflected g r e e n component. In o r d e r to p r o v i d e equal path lengths through g l a s s for all s p e c t r a l components, the p r i s m configuration must be as shown. All i m a g e s can be brought to focus at the output f a c e s of the p r i s m a s s e m b l y , by m e a n s of a lens. In the optical s y s t e m i m p l e m e n t e d by the authors, c o p l a n a r i t y of i m a g e s is attained through use of f i b r e optic s e c t o r s , equal in length to the l i n e a r d i s p l a c e m e n t of image p l a n e s which b r i n g the i m a g e s into the s a m e focal plane as the g r e e n i m a g e . The p r o b l e m s t i l l r e m a i n s of moving this i m a g e plane into the plane of the p h o t o s e n s i t i v e s u r f a c e . Although the l e n s can be r e p o s i t i o n e d so as to r e f o c u s the g r e e n i m a g e , through the homogeneous g l a s s p o r t i o n s of the p r i s m s and faceplate, the r e d and blue image f o r m i n g light will be s e v e r e l y s c a t t e r e d within the f a c e p l a t e . The p r o b l e m was solved by r e m o v i n g the f i b r e optics f r o m the p r i s m a s s e m b l y , together with o n e - h a l f the length of the homogeneous g l a s s in the g r e e n image path, and combining these as a plane p a r a l l e l f a c e p l a t e for the vidicon. The configuration of the c o m p o s i t e f a c e p l a t e and its use in conjunction with the s p e c t r a l s e p a r a t i o n p r i s m is shown in Fig. 3. The v e r t i c a l image configuration a s s o c i a t e d with field sequential scanning was chosen, m a i n l y b e c a u s e the d e s i g n of deflection c i r c u i t s of the n e c e s s a r y d e g r e e of l i n e a r i t y could be m o r e r e a d i l y a c c o m p l i s h e d working with the v e r t i c a l d e f l e c tion s y s t e m . It was felt that the m u l t i p l e - i m a g e / s i n g l e - v i d i c o n concept could be p r o v e n to be f e a s i b l e (or not) r e g a r d l e s s of the c o m p o s i t e image o r i e n t a tion. R. L. E i l e n b e r g e r , F. W. K a m m e r e r & J. F. Muller, Bell Telephone L a b o r a t o r i e s , I n c o r p o r a t e d Holmdel, New J e r s e y , USA
Target ring
Vidicon envelope
rJJJJffff
f J]:
.~--,.\\\\\\'~
I Fig. 3.
Optical s y s t e m giving c o p l a n a r identical images.
to the i n t e r n a l and e x t e r n a l b e a m s . A p r e - a m p l i f i e r is used for the r e c e i v i n g s i g n a l s and the two c o m ponents a r e then s e p a r a t e d by n a r r o w b a n d f i l t e r s . A n u m b e r of t r a n s m i s s o m e t e r s a r e s i t u a t e d along a runway, each site m e a s u r i n g a t m o s p h e r i c t r a n s m i s s i o n s and at l e a s t one m e a s u r i n g background luminance. All m e a s u r e m e n t s a r e d i g i t i z e d and p a s s e d in s e r i a l f o r m to a c e n t r a l p r o c e s s o r which a l s o r e c e i v e s i n f o r m a t i o n on the state of runway lighting, the d i r e c t i o n of runway in use and the t i m e r e f e r e n c e for the a i r p o r t . This in turn is connected to a i r t r a f f i c control c o n s o l e s w h e r e all the information is c l e a r l y d i s p l a y e d . D. C. T h o m a s , G E C - A E I ( E l e c t r o n i c s ) Ltd, L e i c e s t e r
RUNWAY VISIBILITY
CHEAPER OPTICAL COMPONENTS
AIR T R A F F I C SAFETY d e m a n d s a method of e s t a b lishing whether v i s i b i l i t y on the runway is above the l i m i t set f o r piloted landings and t a k e - o f f s . The I n t e r n a t i o n a l Civil Aviation O r g a n i s a t i o n has made r e c o m m e n d a t i o n s which make the r e q u i r e d a s s e s s ment of the p i l o t ' s v i s u a l range i m p r a c t i c a b l e without r e c o u r s e to an a u t o m a t i c s y s t e m . D.C. T h o m a s has outlined the t h e o r y and p r a c t i c e of the solution developed by G E C - A E I ( E l e c t r o n i c s ) Ltd. at L e i c e s t e r which was s e l e c t e d by the B o a r d of T r a d e for i n s t a l lation in the p r i n c i p a l civil a i r p o r t s in the United Kingdom.
AT PRESENT a large number of optical components are rejected for defects which would not affect the quality of the image produced by an optical system. The first annual report of the SIRA institute group studying standards for these defects has now been published.
A t r a n s m i s s o m e t e r , was designed to c o m p a r e light r e f l e c t e d over a path i n t e r n a l to the i n s t r u m e n t with light r e f l e c t e d over an e x t e r n a l path through the a t m o s p h e r e to be m e a s u r e d . The light is modulated by a r o t a t i n g d i c s with two c i r c u l a r r o w s of holes, which give c h a r a c t e r i s t i c f r e q u e n c i e s of modulation
The effect of surface defects on optical glare and the optical transfer function are now being examined by the research team who hope to establish a standard for these defects. Such faults usually need to be fairly severe before they sensibly effect optical glare. Computer programmes are to be designed to calculate centring tolerances, giving SIRA group members, and later industry in general, a readily accessible programme, something relatively unobtainable until now. Sira Institute, Chislehurst, Kent.
Optics and L a s e r Technology
229
Target ring
Vidicon envelope
rJJJJffff
f J]:
.~--,.\\\\\\'~
I Fig. 3.
Optical s y s t e m giving c o p l a n a r identical images.
to the i n t e r n a l and e x t e r n a l b e a m s . A p r e - a m p l i f i e r is used for the r e c e i v i n g s i g n a l s and the two c o m ponents a r e then s e p a r a t e d by n a r r o w b a n d f i l t e r s . A n u m b e r of t r a n s m i s s o m e t e r s a r e s i t u a t e d along a runway, each site m e a s u r i n g a t m o s p h e r i c t r a n s m i s s i o n s and at l e a s t one m e a s u r i n g background luminance. All m e a s u r e m e n t s a r e d i g i t i z e d and p a s s e d in s e r i a l f o r m to a c e n t r a l p r o c e s s o r which a l s o r e c e i v e s i n f o r m a t i o n on the state of runway lighting, the d i r e c t i o n of runway in use and the t i m e r e f e r e n c e for the a i r p o r t . This in turn is connected to a i r t r a f f i c control c o n s o l e s w h e r e all the information is c l e a r l y d i s p l a y e d . D. C. T h o m a s , G E C - A E I ( E l e c t r o n i c s ) Ltd, L e i c e s t e r
RUNWAY VISIBILITY
CHEAPER OPTICAL COMPONENTS
AIR T R A F F I C SAFETY d e m a n d s a method of e s t a b lishing whether v i s i b i l i t y on the runway is above the l i m i t set f o r piloted landings and t a k e - o f f s . The I n t e r n a t i o n a l Civil Aviation O r g a n i s a t i o n has made r e c o m m e n d a t i o n s which make the r e q u i r e d a s s e s s ment of the p i l o t ' s v i s u a l range i m p r a c t i c a b l e without r e c o u r s e to an a u t o m a t i c s y s t e m . D.C. T h o m a s has outlined the t h e o r y and p r a c t i c e of the solution developed by G E C - A E I ( E l e c t r o n i c s ) Ltd. at L e i c e s t e r which was s e l e c t e d by the B o a r d of T r a d e for i n s t a l lation in the p r i n c i p a l civil a i r p o r t s in the United Kingdom.
AT PRESENT a large number of optical components are rejected for defects which would not affect the quality of the image produced by an optical system. The first annual report of the SIRA institute group studying standards for these defects has now been published.
A t r a n s m i s s o m e t e r , was designed to c o m p a r e light r e f l e c t e d over a path i n t e r n a l to the i n s t r u m e n t with light r e f l e c t e d over an e x t e r n a l path through the a t m o s p h e r e to be m e a s u r e d . The light is modulated by a r o t a t i n g d i c s with two c i r c u l a r r o w s of holes, which give c h a r a c t e r i s t i c f r e q u e n c i e s of modulation
The effect of surface defects on optical glare and the optical transfer function are now being examined by the research team who hope to establish a standard for these defects. Such faults usually need to be fairly severe before they sensibly effect optical glare. Computer programmes are to be designed to calculate centring tolerances, giving SIRA group members, and later industry in general, a readily accessible programme, something relatively unobtainable until now. Sira Institute, Chislehurst, Kent.
Optics and L a s e r Technology
229