- Email: [email protected]
Third-generation DASH helmet
T Pinhas Gilboa and Sasson Abraham
DASH is a monocular, visor-projected, helmet-mounted display for daytime missions. It has more than 20 ° field of display and undisturbed see-through vision. Previous similar systems were based upon non-spherical, mostly parabolic, tilted combiners. These optical projecting systems suffered from relatively large keystone distortion. This paper presents the third-generation DASH. This new optical concept is relatively free from distortion. The system is based on a spherical surface as an optical combiner, coupled to a complementary aberrative focal relay system. The relay includes at least one reflecting surface. Keywords: helmet-mounted display, field of view
The helmet-mounted display (HMD) is an apparatus attached to the helmet of a pilot, for projecting an image generated by a video source, superimposed on the field of view (FOV). A wide variety of HMDs have been developed during the last decade for applications such as night vision and daytime weapon delivery, for rotary wing and fixed wing airplanes. Safety considerations play a significant role in developing HMDs for combat pilots. For high g manoeuvring, the helmet should be lightweight and well balanced. The outer contours of the helmet should be kept compact. The construction must withstand wind blast of 450-600 KEAS (knot equivalent air speed) with no harm to the pilot or to the helmet structure. For daytime mission HMDs, no obstructing elements are allowed in the pilot's FOV. The HMD also should have large enough field of display (FOD), usually more than 15 °, and a minimum of 12 mm exit pupil size. Such an apparatus has a semitransparent reflective power element, serving as an optical combiner. Two methods are in common use. The simplest method is to place the combiner axially relative to the relay optics. Such systems were presented by Ellis ~ and Fournier and colleagues 2. A flat semitransparent mirror is located between the eye of the observer and the combiner, serving as a folding beamsplitter. Light emitted perpendicular to the FOD is folded by the Elbit, Airborne Systems Division, Advanced Technology Center, POB 539, Haifa 31053, Israel Paper received: January 1994; revised: May 1994
106
mirror towards the combiner, from which it reflects back, collimated, to the eye. In this arrangement the light travels twice through the mirror, which lowers the optical transmission. Improvement of the optical transmission is achieved in the tilted cat HMD, reported by Droessler and Rotier 3. In this method the combiner is slightly tilted, and the folding mirror is selectively coated, having different reflectance for different angles, with the purpose of increasing the transmission of the reflected light from the combiner. In both methods, the folding mirror and the combiner are coupled to an eyepiece in front of the pilot's eye, so obstruction of at least part of the FOV is unavoidable, and the risk of injury under crash conditions is increased. A much better solution is to use a portion of the visor of the helmet as a combiner. In this method, light is projected from an optical relay onto the visor, from where it reflects back directly to the eye, without any intervening elements between the eye and the visor. The orientation of the combiner portion of the visor is designed so as to reflect the light at large off-axis angles of at least 55 ° , in order to keep the visor close to the face, while the optical elements, which may obstruct the see-through vision, are kept outside the pilot's FOV. In early versions of visor-projected HMDs, where the FOD had been restricted to angles of up to 10 °, the image source was placed at the focal plane of the combiner. To eliminate the initial aberrations introduced by the off-axis reflection, a parabolic combiner surface was used by Vizenor 4 in the early 1970s. A similar approach, where a toric element replaces the parabolic surface, was used by Bosserman et al. 5 a few years later. Using a spherical surface as a combiner at large folding angles introduces unacceptable aberrations. Elements to compensate for the aberrations must be added to the system. Heller et a l ) suggested such a system in the late 1970s. In this system the focal length of the spherical mirror was selected to be large enough to allow addition of a prism in the optical path. The prism was designed to correct some of the spherical aberration and coma, and by introducing a cylindrical element also to compensate for the astigmatic aberration. The above systems are limited by the FOD they provide. As the FOD of these displays is increased, the aberrations also increase, unless the exit pupil size is limited to an
(3141-9382/94/02/0106-04 © 1994 Butterworth-Heinemann Ltd Displays Volume 15 Number 2 1994
Third-generation DASH helmet: P Gilboa and S Abraham
unacceptably small size. One of the fruitful concepts for a visor-projected HMD of a wider FOD was described by Mostrom 7, and a similar design with a different geometric layout by Evans et al. 8 uses a confocal parabolic visor as an afocal relay. The concept is to cancel the aberrations introduced by the high 55 ° fold angles by using a single parabolic surface and a flat mirror to create double complementary reflections (Figure 1). Aberrations still exist for off-centre projection and must be compensated for. However, geometric distortion created by the powered tilted combiner is the major issue9. It increases nearly as the cube of the fold angle. For a fold angle of 55°, the distortion is as high as 45 %. Electronic correction is required and resolution degradation is expected. For an ordinary reflecting coating an aspherical visor surface is the norm. A spheroid surface is not the natural choice as a combiner for large FOD, since significant geometry distortion, spherical and astigmatic aberrations, are introduced when off-axis projection at large angles is involved. On the other hand, a visor with a spherical surface has some advantages. It is the simplest rotationally symmetric shape and is easy to manufacture. It allows for adjustment of the exit pupil position by rotation of the optical apparatus around any axis that intersects the sphere centre, hence simplifying the overall mechanical structure.
introducing complementary distortion to the system is used. In a similar manner to the parabolic powered afocal relay combiner, a simple solution is achieved by introducing a second complementary reflecting surface to the system. While in the former distortion is not compensated, here, by using the second reflecting surface as an element of a focal relay optics, distortion is well compensated. An unfolded drawing of the optical path is shown in Figure 2. Mirror 1 is coupled to lens 2 in order to create an aberrated focal relay system. Lenses 3 and 4 are field lenses and mirror 5 is a partially reflecting semitransparent spherical combiner. Lens 4 is placed at the tangential focus of surface 5, hence acting as a field lens for the tangential and as a power element for the sagittal ray fans, and vice versa for lens 3. This arrangement, combined with the aherrated relay, allows for separate adjustments of the focal length of the two axes, as well as for the control of the overall dimensions of the apparatus. The orientation of mirror 1, together with the tilts and decentres of the other elements, is designed to have a distortion that is complementary to that of the combiner. Lens 2 also plays a significant role in correcting coma, spherical and chromatic aberrations. On the basis of the described concept, the Elbit thirdgeneration DASH helmet was designed and built. Figure 3
TILTED SPHERICAL COMBINER Compensation for the astigmatic and spherical aberrations, introduced by the tilted combiner of a spherical surface, may be achieved using an astigmatic relay such as cylindrical lenses. However, at wide FOD such a system will still suffer from significant distortion, unless a means of 2
3 4
EXIT PUPIL
Figure 1
Parabolic afocal relay combiner
Figure 2 Tiltedspherical combiner
Displays Volume 15 Number 2 1994
107
Third-generation DASH helmet: P Gilboa and S Abraham
Table
2 Astigmatic aberration Aberration (dioptres)
FOD (degrees)
0
2.5
5
7.5
10
0.13 0.12 0.09 0.06 0.03 0.01
0.10 0.08 0.05 0.03 0.00
-
5
0.00
0.00
0.01 0.01 0.00 0.00 0.02
- 7.5 - 10
0.01 0,06
0.02
0.07 0.05 0.03 0.02 0.00 0.01 0.03
0.02
-
10 7.5 5 2.5 0 2.5
+ + + +
Table
3 Field curvature Curvature (dioptres)
FOD (degrees) + + + +
10 7.5 5 2.5 0 - 2.5 - 5 - 7.5 10
Figure
3 The Elbit DASH helmet-mounted display
Table
0
2.5
5
7.5
10
0.00 0.06 0.10 0.10 0.08 0.06 0.02 0.02 0.08
0.06 0.10 0.10 0.08 0.06 0.02 0.02
0.06 0.10 0.10 0.08 0.06 0.02 0.00
0.08 0.10 0.08 0.06 0.04
0.08
4 Distortion Distortion (mrad)
shows a prototype model of the helmet. The overall helmet dimensions were optimized to cater for possible variations in visor radii and eye relief, as well as the projection direction onto the visor and the amount of obstruction caused by the optical assembly ~°. A configuration with maximum compactness and minimum obstruction was achieved. Based upon these configuration parameters, a monocular display has been designed, having 22 ° FOD for an exit pupil larger than 12 mm, when coupled with a l/2in CRT. The optical elements were carefully selected and the placement was optimally designed for manufacturability. The crisp sharp display achieved is described in Tables 1 to 4 which show typical display parameters for an eye pupil of 4 mm. For a larger eye pupil the results are not significantly different. Table 1 shows the spot size, as calculated for 50% of the peak brightness. Relative to a V2in CRT line width, which is approximately 1 mrad, the effect of the optics on the resulting line width is negligible. Table 2 shows the astigmatic distortion. Since the eye is unable to notice an astigmatism of less than ~/~ dioptres, the Table
1 Spot size Spot size (mrad)
FOD (degrees) + + + +
0
-
10 7.5 5 2.5 0 2.5
0.2 0.4 0.4 0.4 0.4 0.2
-
5
0.2
-
7.5
0.3
-
10
0.4
108
2.5
5
0.4 0.4 0.4 0.4 0.4 0.2 0.2
0.6 0.4 0.4 0.4 0.2 0.2 0.2
7.5
10
FOD (degrees) +10 +6.6 +3.3 0 -3.3 -6.6 -10
0
3.3
6.6
10
1.4 0.1 1.0 1.3 1.0 0.3 0.5
0.2 0.8 1.0 0.6 0.3
0.7 0.2 0.2 0.4 1.4
1.0
r e s u l t s m o r e t h a n satisfy the r e q u i r e m e n t . T h e s a m e quality is a c h i e v e d for field c u r v a t u r e , as s h o w n in Table 3. T h e e x t r e m e l y l o w d i s t o r t i o n is t h e c h a r a c t e r i s t i c w h i c h d i s t i n g u i s h e s the t h i r d - g e n e r a t i o n D A S H H M D f r o m p r i o r d e s i g n s . W h i l e t h e p a r a b o l i c a f o c a l relay c o m b i n e r suffers f r o m l a r g e g e o m e t r i c d i s t o r t i o n , in this s y s t e m , as c a n b e s e e n in Table 4, t h e m a x i m u m d e v i a t i o n is 1.4 m r a d , w h i c h is b a r e l y n o t i c e a b l e . H e n c e , n o e l e c t r o n i c dist o r t i o n c o r r e c t i o n is r e q u i r e d , a p a r t f r o m offset a n d gain compensation. A l t h o u g h the a p p a r a t u s c o n s i s t s o f o n l y five optical e l e m e n t s , t h e r e s u l t i n g p e r f o r m a n c e is i m p r e s s i v e . T h e h e l m e t h a s b e e n t e s t e d in flight w i t h results that m a y b e s u m m a r i z e d w i t h t h e w o r d s o f o n e o f t h e test pilots: ' T h i s is t h e first d e s i g n t h a t h a s n o t o n l y a n e x c e l l e n t d i s p l a y , b u t also l o o k s like a h e l m e t a n d feels like a helmet'.
0.6 0.6 0.4 0.3 0.2
Displays Volume 15 Number 2 1994
0.6
ACKNOWLEDGEMENT The authors thank Dr Alexander Gold for his significant contribution to this project.
Third-generation DASH helmet: P Gilboa and S Abraham
REFERENCES 1 Ellis, S M 'Helmet display system'. US Patent 4 902 116 (1990) 2 Fournier, J T, Smith, H A, McLean, W E, Smith, S J and McKinley, H R 'Helmet mounted display having dual interchangeable optical eyepieces'. US Patent 4 969 714 (1990) 3 Droessler, J G and Rotier, D J 'Tilted cat HMD'. Opt. Eng. 1990, 29, 849-854 4 Vizenor, R P 'Inside helmet sight apparatus'. US Patent 3 787 109 (1974)
5 Bosserman, X et al. 'Toric reflector display'. US Patent 4 026 641 (1977) 6 Heller, F P and Ellis, S M 'Headgear with spherical semi-reflecting surface'. US Patent 4 081 209 (1978) 7 Mostrom, X 'Head mounted display'. US Patent 3 923 370 (1975) 8 Evans, C D, Tirums, A T, Larkin, E W and Melzer, I E 'Compact helmet mounted display'. US Patent 4 761 056 (1988) 9 Rotier, D J 'Optical approach to helmet mounted display'. SPIE 1989, 1116-01, 14-18 10 Gilboa, P 'Designing the right visor'. SPIE 1991, 1456-18, 154-163
Displays Volume 15 Number 2 1994
109
DASH is a monocular, visor-projected, helmet-mounted display for daytime missions. It has more than 20 ° field of display and undisturbed see-through vision. Previous similar systems were based upon non-spherical, mostly parabolic, tilted combiners. These optical projecting systems suffered from relatively large keystone distortion. This paper presents the third-generation DASH. This new optical concept is relatively free from distortion. The system is based on a spherical surface as an optical combiner, coupled to a complementary aberrative focal relay system. The relay includes at least one reflecting surface. Keywords: helmet-mounted display, field of view
The helmet-mounted display (HMD) is an apparatus attached to the helmet of a pilot, for projecting an image generated by a video source, superimposed on the field of view (FOV). A wide variety of HMDs have been developed during the last decade for applications such as night vision and daytime weapon delivery, for rotary wing and fixed wing airplanes. Safety considerations play a significant role in developing HMDs for combat pilots. For high g manoeuvring, the helmet should be lightweight and well balanced. The outer contours of the helmet should be kept compact. The construction must withstand wind blast of 450-600 KEAS (knot equivalent air speed) with no harm to the pilot or to the helmet structure. For daytime mission HMDs, no obstructing elements are allowed in the pilot's FOV. The HMD also should have large enough field of display (FOD), usually more than 15 °, and a minimum of 12 mm exit pupil size. Such an apparatus has a semitransparent reflective power element, serving as an optical combiner. Two methods are in common use. The simplest method is to place the combiner axially relative to the relay optics. Such systems were presented by Ellis ~ and Fournier and colleagues 2. A flat semitransparent mirror is located between the eye of the observer and the combiner, serving as a folding beamsplitter. Light emitted perpendicular to the FOD is folded by the Elbit, Airborne Systems Division, Advanced Technology Center, POB 539, Haifa 31053, Israel Paper received: January 1994; revised: May 1994
106
mirror towards the combiner, from which it reflects back, collimated, to the eye. In this arrangement the light travels twice through the mirror, which lowers the optical transmission. Improvement of the optical transmission is achieved in the tilted cat HMD, reported by Droessler and Rotier 3. In this method the combiner is slightly tilted, and the folding mirror is selectively coated, having different reflectance for different angles, with the purpose of increasing the transmission of the reflected light from the combiner. In both methods, the folding mirror and the combiner are coupled to an eyepiece in front of the pilot's eye, so obstruction of at least part of the FOV is unavoidable, and the risk of injury under crash conditions is increased. A much better solution is to use a portion of the visor of the helmet as a combiner. In this method, light is projected from an optical relay onto the visor, from where it reflects back directly to the eye, without any intervening elements between the eye and the visor. The orientation of the combiner portion of the visor is designed so as to reflect the light at large off-axis angles of at least 55 ° , in order to keep the visor close to the face, while the optical elements, which may obstruct the see-through vision, are kept outside the pilot's FOV. In early versions of visor-projected HMDs, where the FOD had been restricted to angles of up to 10 °, the image source was placed at the focal plane of the combiner. To eliminate the initial aberrations introduced by the off-axis reflection, a parabolic combiner surface was used by Vizenor 4 in the early 1970s. A similar approach, where a toric element replaces the parabolic surface, was used by Bosserman et al. 5 a few years later. Using a spherical surface as a combiner at large folding angles introduces unacceptable aberrations. Elements to compensate for the aberrations must be added to the system. Heller et a l ) suggested such a system in the late 1970s. In this system the focal length of the spherical mirror was selected to be large enough to allow addition of a prism in the optical path. The prism was designed to correct some of the spherical aberration and coma, and by introducing a cylindrical element also to compensate for the astigmatic aberration. The above systems are limited by the FOD they provide. As the FOD of these displays is increased, the aberrations also increase, unless the exit pupil size is limited to an
(3141-9382/94/02/0106-04 © 1994 Butterworth-Heinemann Ltd Displays Volume 15 Number 2 1994
Third-generation DASH helmet: P Gilboa and S Abraham
unacceptably small size. One of the fruitful concepts for a visor-projected HMD of a wider FOD was described by Mostrom 7, and a similar design with a different geometric layout by Evans et al. 8 uses a confocal parabolic visor as an afocal relay. The concept is to cancel the aberrations introduced by the high 55 ° fold angles by using a single parabolic surface and a flat mirror to create double complementary reflections (Figure 1). Aberrations still exist for off-centre projection and must be compensated for. However, geometric distortion created by the powered tilted combiner is the major issue9. It increases nearly as the cube of the fold angle. For a fold angle of 55°, the distortion is as high as 45 %. Electronic correction is required and resolution degradation is expected. For an ordinary reflecting coating an aspherical visor surface is the norm. A spheroid surface is not the natural choice as a combiner for large FOD, since significant geometry distortion, spherical and astigmatic aberrations, are introduced when off-axis projection at large angles is involved. On the other hand, a visor with a spherical surface has some advantages. It is the simplest rotationally symmetric shape and is easy to manufacture. It allows for adjustment of the exit pupil position by rotation of the optical apparatus around any axis that intersects the sphere centre, hence simplifying the overall mechanical structure.
introducing complementary distortion to the system is used. In a similar manner to the parabolic powered afocal relay combiner, a simple solution is achieved by introducing a second complementary reflecting surface to the system. While in the former distortion is not compensated, here, by using the second reflecting surface as an element of a focal relay optics, distortion is well compensated. An unfolded drawing of the optical path is shown in Figure 2. Mirror 1 is coupled to lens 2 in order to create an aberrated focal relay system. Lenses 3 and 4 are field lenses and mirror 5 is a partially reflecting semitransparent spherical combiner. Lens 4 is placed at the tangential focus of surface 5, hence acting as a field lens for the tangential and as a power element for the sagittal ray fans, and vice versa for lens 3. This arrangement, combined with the aherrated relay, allows for separate adjustments of the focal length of the two axes, as well as for the control of the overall dimensions of the apparatus. The orientation of mirror 1, together with the tilts and decentres of the other elements, is designed to have a distortion that is complementary to that of the combiner. Lens 2 also plays a significant role in correcting coma, spherical and chromatic aberrations. On the basis of the described concept, the Elbit thirdgeneration DASH helmet was designed and built. Figure 3
TILTED SPHERICAL COMBINER Compensation for the astigmatic and spherical aberrations, introduced by the tilted combiner of a spherical surface, may be achieved using an astigmatic relay such as cylindrical lenses. However, at wide FOD such a system will still suffer from significant distortion, unless a means of 2
3 4
EXIT PUPIL
Figure 1
Parabolic afocal relay combiner
Figure 2 Tiltedspherical combiner
Displays Volume 15 Number 2 1994
107
Third-generation DASH helmet: P Gilboa and S Abraham
Table
2 Astigmatic aberration Aberration (dioptres)
FOD (degrees)
0
2.5
5
7.5
10
0.13 0.12 0.09 0.06 0.03 0.01
0.10 0.08 0.05 0.03 0.00
-
5
0.00
0.00
0.01 0.01 0.00 0.00 0.02
- 7.5 - 10
0.01 0,06
0.02
0.07 0.05 0.03 0.02 0.00 0.01 0.03
0.02
-
10 7.5 5 2.5 0 2.5
+ + + +
Table
3 Field curvature Curvature (dioptres)
FOD (degrees) + + + +
10 7.5 5 2.5 0 - 2.5 - 5 - 7.5 10
Figure
3 The Elbit DASH helmet-mounted display
Table
0
2.5
5
7.5
10
0.00 0.06 0.10 0.10 0.08 0.06 0.02 0.02 0.08
0.06 0.10 0.10 0.08 0.06 0.02 0.02
0.06 0.10 0.10 0.08 0.06 0.02 0.00
0.08 0.10 0.08 0.06 0.04
0.08
4 Distortion Distortion (mrad)
shows a prototype model of the helmet. The overall helmet dimensions were optimized to cater for possible variations in visor radii and eye relief, as well as the projection direction onto the visor and the amount of obstruction caused by the optical assembly ~°. A configuration with maximum compactness and minimum obstruction was achieved. Based upon these configuration parameters, a monocular display has been designed, having 22 ° FOD for an exit pupil larger than 12 mm, when coupled with a l/2in CRT. The optical elements were carefully selected and the placement was optimally designed for manufacturability. The crisp sharp display achieved is described in Tables 1 to 4 which show typical display parameters for an eye pupil of 4 mm. For a larger eye pupil the results are not significantly different. Table 1 shows the spot size, as calculated for 50% of the peak brightness. Relative to a V2in CRT line width, which is approximately 1 mrad, the effect of the optics on the resulting line width is negligible. Table 2 shows the astigmatic distortion. Since the eye is unable to notice an astigmatism of less than ~/~ dioptres, the Table
1 Spot size Spot size (mrad)
FOD (degrees) + + + +
0
-
10 7.5 5 2.5 0 2.5
0.2 0.4 0.4 0.4 0.4 0.2
-
5
0.2
-
7.5
0.3
-
10
0.4
108
2.5
5
0.4 0.4 0.4 0.4 0.4 0.2 0.2
0.6 0.4 0.4 0.4 0.2 0.2 0.2
7.5
10
FOD (degrees) +10 +6.6 +3.3 0 -3.3 -6.6 -10
0
3.3
6.6
10
1.4 0.1 1.0 1.3 1.0 0.3 0.5
0.2 0.8 1.0 0.6 0.3
0.7 0.2 0.2 0.4 1.4
1.0
r e s u l t s m o r e t h a n satisfy the r e q u i r e m e n t . T h e s a m e quality is a c h i e v e d for field c u r v a t u r e , as s h o w n in Table 3. T h e e x t r e m e l y l o w d i s t o r t i o n is t h e c h a r a c t e r i s t i c w h i c h d i s t i n g u i s h e s the t h i r d - g e n e r a t i o n D A S H H M D f r o m p r i o r d e s i g n s . W h i l e t h e p a r a b o l i c a f o c a l relay c o m b i n e r suffers f r o m l a r g e g e o m e t r i c d i s t o r t i o n , in this s y s t e m , as c a n b e s e e n in Table 4, t h e m a x i m u m d e v i a t i o n is 1.4 m r a d , w h i c h is b a r e l y n o t i c e a b l e . H e n c e , n o e l e c t r o n i c dist o r t i o n c o r r e c t i o n is r e q u i r e d , a p a r t f r o m offset a n d gain compensation. A l t h o u g h the a p p a r a t u s c o n s i s t s o f o n l y five optical e l e m e n t s , t h e r e s u l t i n g p e r f o r m a n c e is i m p r e s s i v e . T h e h e l m e t h a s b e e n t e s t e d in flight w i t h results that m a y b e s u m m a r i z e d w i t h t h e w o r d s o f o n e o f t h e test pilots: ' T h i s is t h e first d e s i g n t h a t h a s n o t o n l y a n e x c e l l e n t d i s p l a y , b u t also l o o k s like a h e l m e t a n d feels like a helmet'.
0.6 0.6 0.4 0.3 0.2
Displays Volume 15 Number 2 1994
0.6
ACKNOWLEDGEMENT The authors thank Dr Alexander Gold for his significant contribution to this project.
Third-generation DASH helmet: P Gilboa and S Abraham
REFERENCES 1 Ellis, S M 'Helmet display system'. US Patent 4 902 116 (1990) 2 Fournier, J T, Smith, H A, McLean, W E, Smith, S J and McKinley, H R 'Helmet mounted display having dual interchangeable optical eyepieces'. US Patent 4 969 714 (1990) 3 Droessler, J G and Rotier, D J 'Tilted cat HMD'. Opt. Eng. 1990, 29, 849-854 4 Vizenor, R P 'Inside helmet sight apparatus'. US Patent 3 787 109 (1974)
5 Bosserman, X et al. 'Toric reflector display'. US Patent 4 026 641 (1977) 6 Heller, F P and Ellis, S M 'Headgear with spherical semi-reflecting surface'. US Patent 4 081 209 (1978) 7 Mostrom, X 'Head mounted display'. US Patent 3 923 370 (1975) 8 Evans, C D, Tirums, A T, Larkin, E W and Melzer, I E 'Compact helmet mounted display'. US Patent 4 761 056 (1988) 9 Rotier, D J 'Optical approach to helmet mounted display'. SPIE 1989, 1116-01, 14-18 10 Gilboa, P 'Designing the right visor'. SPIE 1991, 1456-18, 154-163
Displays Volume 15 Number 2 1994
109