Why HID headlights bother older drivers

Abstract

Driving requires effective coordination of visual, motor, and cognitive skills. Visual skills are pushed to their limit at night by decreased illumination and by disabling glare from oncoming headlights. High intensity discharge (HID) headlamps project light farther down roads, improving their owner's driving safety by increasing the time available for reaction to potential problems. Glare is proportional to headlamp brightness, however, so increasing headlamp brightness also increases potential glare for oncoming drivers, particularly on curving two lane roads. This problem is worse for older drivers because of their increased intraocular light scattering, glare sensitivity, and photostress recovery time. An analysis of automobile headlights, intraocular stray light, glare, and night driving shows that brightness rather than blueness is the primary reason for the visual problems that HID headlights can cause for older drivers who confront them. The increased light projected by HID headlights is potentially valuable, but serious questions remain regarding how and where it should be projected.

Full text

PERSPECTIVE
Why HID headlights bother older drivers
M A Mainster, G T Timberlake
.............................................................................................................................
Br J Ophthalmol
2003;87:113–117
Driving requires effective coordination of visual, motor,
and cognitive skills. Visual skills are pushed to their limit
at night by decreased illumination and by disabling
glare from oncoming headlights. High intensity
discharge (HID) headlamps project light farther down
roads, improving their owner’s driving safety by
increasing the time available for reaction to potential
problems. Glare is proportional to headlamp brightness,
however, so increasing headlamp brightness also
increases potential glare for oncoming drivers,
particularly on curving two lane roads. This problem is
worse for older drivers because of their increased
intraocular light scattering, glare sensitivity, and
photostress recovery time. An analysis of automobile
headlights, intraocular stray light, glare, and night
driving shows that brightness rather than blueness is the
primary reason for the visual problems that HID
headlights can cause for older drivers who confront
them. The increased light projected by HID headlights is
potentially valuable, but serious questions remain
regarding how and where it should be projected.
..........................................................................
H
igh intensity discharge (HID) headlights
are brighter, more energy efficient, and
longer lasting than conventional incandes-
cent headlights. They allow owners to detect road
hazards, signage, and pedestrians at greater
distances. They are standard equipment on grow-
ing numbers of more expensive automobiles.
HID headlights probably improve the safety of
night driving for their users. Unfortunately, they
can dazzle viewers on two lane highways, making
it more difficult for approaching drivers to
identify pedestrians, road hazards, and curves in
the road. Night-time driving is difficult for older
individuals. Confronting HID headlights makes it
even more difficult. To understand why older
drivers may complain about their encounters
with HID headlights, it is useful to understand
automotive headlight design and how ageing
affects intraocular light scattering, glare, and
automobile driving.
HEADLIGHTS
Conventional headlights use incandescent light
bulbs. An incandescent bulb consists of a tung-
sten filament mounted in a sealed glass container.
The bulb is evacuated or filled with gases to
prevent filament oxidation. The filament glows
when it is heated by an electrical current. Higher
filament temperatures produce brighter, bluer
light. They also shorten a filament’s lifetime.
A tungsten-halogen (halogen) bulb provides
brighter light at higher filament temperatures,
using a regeneration cycle to lengthen the
tungsten filament’s lifetime.
1
A halogen gas such
as iodine is added to the bulb’s atmosphere. When
tungsten evaporates from a heated filament, it
collects on relatively cool bulb surfaces where it
combines with iodine. Volatile tungsten-halide
diffuses back to the heated filament, dissociating
and redepositing tungsten on the filament.
Irregularities in the redeposition process eventu-
ally cause filament thinning and bulb failure.
Halogen bulbs are used widely in automotive
headlights and also in ophthalmic instruments.
The luminous power output of halogen headlights
increases with increasing wavelength across the
visible spectrum.
2–4
HID lamps overcome many of the limitations of
incandescent bulbs. A discharge lamp consists of
two electrodes in a quartz container filled with a
high pressure gas. An electronic starter initiates
an electric discharge between the electrodes, pro-
ducing an ionised gas (a plasma) that generates a
continuous spectrum of light as well as narrow
spectral lines. Mercury arc highway illumination
lamps are HID devices. Xenon HID headlights
produce some continuous spectrum white light,
but much of their luminous power is generated as
broadened spectral lines, including blue lines at
405, 435, and 475 nm.
4
HID headlamps produce
two to three times more luminous power (flux)
than halogen bulbs.
56
The colour temperature of a light source is the
temperature of a black body that emits radiation
of the same subjective colour as the light
source.
78
Higher colour temperature sources have
a bluer appearance. Conventional incandescent
bulbs, halogen incandescent bulbs, xenon HID
bulbs, and the sun’s disc at midday when viewed
from ground level have colour temperatures
around 2800, 3200, 4200, and 5600°K,
respectively.
18
Thus, HID bulbs are not as blue as
ordinary sunlight, but they are brighter and bluer
than conventional halogen bulbs.
A HID headlight system consists of a discharge
lamp, its electronics, and a reflector. Reflector
design determines the illumination pattern pro-
jected ahead and adjacent to a vehicle.
3
It is also a
key factor in determining how bright a headlight
appears to an observer (that is, the headlight’s
luminance in candles/m
2
, where a candle is lumi-
nous intensity in lumens/steradian) or how
brightly the headlight lights up a surface at the
observer’s location (that is, the surface illumi-
nance that the headlight produces in lumens/m
2
or lux).
39
Automobiles have high beam and low beam
illumination systems. High beam illumination is
See end of article for
authors’ affiliations
.......................
Martin A Mainster, PhD,
MD, Department of
Ophthalmology, University
of Kansas Medical Center,
3901 Rainbow Boulevard,
Kansas City, KS
66160-7379, USA;
mmainste@kumc.edu
Accepted for publication
1 September 2002
.......................
113
www.bjophthalmol.com

aimed parallel to the road surface and not intended for use
with oncoming traffic. Low beam headlights are aimed slightly
downward to reduce glare for oncoming drivers. The brightest
central area (“hot spot”) in HID headlight illumination
patterns is positioned away from the centre of the road so as
to reduce an oncoming driver’s light exposure. Compared to
halogen low beam headlights, HID headlights have a larger
hot spot, project light farther down the road, and project light
farther to the right and left of their optical axis.
Low beam HID headlight systems in Europe and the United
States have comparatively sharp and soft horizontal illumina-
tion cutoffs, respectively.
51011
Luminous intensity drops off
abruptly (sharp cutoff) above the height of European
headlights, but declines more slowly (soft cutoff) above the
height of American headlights. Each of these illumination
patterns has its own advantages and disadvantages. European
HID headlights provide more light close to driver and less light
farther down the road than American HID headlights. The
sharp cutoff of luminous intensity above the horizontal height
of European headlights protects oncoming drivers from glare,
but limits visibility distance. The sharp cutoff also causes
“flashing” on bumpy roads when the horizontal cutoff
bounces up and down, into and out of an oncoming driver’s
field of view. Conversely, the softer American horizontal cutoff
causes more glare for oncoming drivers but provides better
overhead sign visibility, less flashing, and longer visibility dis-
tances. Adaptive headlights that adjust to changing driving
conditions could offer the advantages of both American and
European illumination systems.
5
LIGHT SCATTERING
Scattered light from oncoming headlights makes night
driving more difficult, in part because the human eye is an
imperfect optical device. Light from the visual environment
enters the eye through the pupil and is imaged on the retina.
Additional light enters the eye by transillumination through
the iris and sclera. Some light is absorbed in photoreceptor
photopigments or other pigments such as melanin, haemo-
globin, xanthophyll, and lipofuscin. Some of it is deflected by
light scattering in ocular tissues.
Light that has been scattered in the eye is termed “stray”
light. Stray light reaching the fovea decreases the contrast of
foveal images, producing disability glare. Light scattering,
reflection, and absorption determine the spectrum of in-
traocular stray light.
12–16
The directionality and spectrum of scattered light depends
on the density and size of the particles that scatter the light.
Particle density determines the intensity of light scattering.
7
Particle size determines its directionality and wavelength
dependence.
17 18
Small particle or Rayleigh scattering has no preferential
direction, but small particles scatter shorter wavelengths more
efficiently than longer wavelengths.
71719
Daylight sky is blue
because light reaching an observer from any direction other
than directly out of the sun is scattered by atmospheric parti-
cles that are small in comparison to visible light wavelengths.
Conversely, light coming directly from the setting sun appears
red because shorter wavelength light is scattered out of the
sun’s image during atmospheric transit.
Large particle or Mie scattering is not wavelength depend-
ent, but light is preferentially scattered in the forward
direction.
717
Mie scattering by retinal pigment epithelial
melanin granules, roughly 1000 nm in diameter,
20 21
improves
retinal image contrast by suppressing side scattered light.
22
Fog doesn’t change the apparent colour of automobile
headlights because large fog droplets scatter all visible
wavelengths equally effectively.
Stray light from the cornea and lens decreases with
increasing wavelength,
16
showing the influence of small parti-
cle scattering. Stray light from fundus reflectance or transillu-
mination increases with increasing wavelength,
16 23
showing
the influence of the decreased optical absorption of melanin
and haemoglobin in the red end of the visible spectrum.
15
The
net effect is that stray light reaching the fovea has little wave-
length dependence.
16 24 25
Thus, blue-white HID headlights
should produce no more foveal stray light or disability glare
than white headlights of the same luminance.
GLARE AND PHOTOSTRESS
Glare can cause discomfort or disability.
26 27
Discomfort glare
does not impair vision, but it can be startling or distracting to
a driver, and cause blinking, squinting, ocular aversion, and
fatigue. The physiological and psychophysical origins of
discomfort glare remain uncertain.
28 29
Disability glare does impair visual performance. It has been
divided classically into “veiling,” “dazzle,” and “scotomatic”
glare based primarily on the type of glare source.
30
Veiling disability glare occurs when a diffuse light source
reduces the contrast of a visual target by “somewhat
uniformly” superimposing light on the visual target’s retinal
image.
30
Veiling glare makes outdoor reading more difficult in
bright sunshine.
30
It also obscures indoor visualisation of
material between two adjacent windows illuminated by
brilliant sunlight.
30
Dazzle disability glare occurs when a bright glare source is
imaged at an extrafoveal location.
30 31
Ocular transit scatters
some light from the glare source light onto the viewer’s fovea.
This stray light decreases the contrast between the lighter and
darker details of a visual target’s foveal image.
14 32–34
Dazzle
glare from an oncoming vehicle’s headlights makes it more
difficult at night for a driver to identify the edge of a curving
two lane highway.
Scotomatic disability glare occurs when a brilliant light
source decreases visual sensitivity (“puts a retinal area
temporarily out of business”).
30
Sensitivity is diminished
while the visual system rapidly light adapts during glare
exposure and then more slowly dark adapts after glare expo-
sure. This “photostress” may startle and disorient observers,
producing afterimages that interfere with vision.
35
Scotomatic
glare occurs during flash photography and momentary expo-
sure to an inept lecturer’s laser pointer beam.
36
It is caused
primarily by rapid bleaching and subsequent slower regenera-
tion of retinal photoreceptor photopigments.
30 31 37–39
Clinical glare terminology is influenced by its testing meth-
ods. In ophthalmic parlance, clinical disability glare generally
refers to classic veiling and classic dazzle glare
30
produced by
intraocular stray light.
14 34
Clinical photostress refers to classic
scotomatic glare
30 40 41
caused by photopigment bleaching,
regeneration, and associated psychophysical processes.
41–48
Clinical terminology will be used throughout the rest of this
perspective.
Strictly speaking, clinical disability glare (classic veiling and
classic dazzle glare) is an optical process which should resolve
immediately after glare exposure because visual adaptation is
unaffected. Conversely, photostress is a psychophysical process
which should persist after light exposure because dark adap-
tation takes time to restore visual sensitivity to its pre-
exposure level. In reality, most clinical glare tests don’t differ-
entiate between optical and psychophysical processes. Clinical
glare sources are often bright enough to produce photostress
as well as disability glare, accounting for the use of terms such
as “glare recovery time”
49 50
for “photostress recovery
time.”
38 51 52
Light adaptation can be controlled in specialised
disability glare tests,
53
but photostress sources always produce
stray light and disability glare sources can alter visual adapta-
tion.
Further confusion arises because methods other than
disability glare and photostress testing can be used to study
visual perception in suboptimal viewing circumstances.
114 Mainster, Timberlake
www.bjophthalmol.com

Disability glare testing illuminates a patient’s eye with an off-
axis light source to produce stray intraocular light that
decreases the contrast of a foveal target. Visual performance
under adverse conditions can also be studied without a sepa-
rate glare source, however, by decreasing the contrast of an
extraocular target before it is imaged on the retina. For exam-
ple, reduced contrast optotypes and sine wave gratings are
used in variable contrast acuity and contrast sensitivity
testing.
54–57
These studies are not considered to be disability
glare or photostress tests, but they use light sources, so they do
produce intraocular stray light and disability glare.
TESTING GLARE AND PHOTOSTRESS
Discomfort glare can be quantified from numerical estimates
of the extent of discomfort that people experience when they
are exposed to a particular retinal illumination while
performing a specific visual task. Discomfort glare varies at
different times and in different individuals, and it is worse in
older than younger people.
426345859
Most disability glare tests measure visual thresholds for
optotype, grating, or acuity targets in the presence or absence
of a glare source.
27 53 60–66
Poor performance on glare sensitivity
testing is often referred to as increased “glare sensitivity” or
decreased “glare resistance.” Stray light that causes glare can
be measured without threshold testing using an annular
flickering glare source that scatters light into the fovea. The
intensity of a foveal target flickering in counterphase with the
glare source is adjusted until flickering from the peripheral
glare source is no longer visible.
67–69
Stray light and disability glare increase with increasing
glare source intensity because there is more light scattered
intraocularly to reduce retinal image contrast.
14 16 23 27 30
Light
scattering increases in people over 50 years of age.
14 16 25 33 70 71
Degenerative changes throughout the visual system may
reduce the efficiency of visual information processing.
38 59
Older individuals perform worse than young ones on standard
disability glare tests, and even worse if they have cataract
formation.
33 59 65 72 73
Disability glare is increased by cataract,
74
intraocular lens implantation,
75–77
and posterior capsule
opacification.
78
Transient glare as occurs in a headlight
encounter is worse than static glare from a light source of the
same luminance, and this effect increases with ageing and
media opacities.
66 79
Most clinical photostress tests measure how long it takes
for visual sensitivity to recover after bright light exposure.
Vision is the most common test end point,
39
but pupillary
80
and
visually evoked
81 82
recovery times can also be measured. Pho-
tostress luminances and exposure durations vary between
protocols. Photostress recovery times have been monitored
with visual acuity test letters, grating targets, and Landolt-C
dark adaptometer targets.
40 51 52
Photostress recovery time increases with photostress source
luminance.
31 35 43 48
It also increases with ageing, although dif-
ferent rates of increase are reported for different testing
methods.
38 49 50 83–85
Macular degeneration and other retinal dis-
orders markedly prolong recovery times.
39 52 86 87
AGEING AND NIGHT DRIVING
The older driver is visually disadvantaged. Almost all self
reported and clinical measures of visual function decline with
ageing.
73 88–91
Eye disorders increase this problem, and their
prevalence also increases with ageing.
88 92 93
Even when high
contrast visual acuity is normal, visual performance decreases
with increasing age on most other sensory tests, including
visual field, glare recovery time, stereopsis, contrast sensitivity,
and low contrast visual acuity with and without glare.
72 89–91 94
Successful driving is a symphony of visual, motor, and cog-
nitive abilities.
95–99
Declining vision increases non-sensory
demands, which also decline with ageing. Older drivers tend
to make less efficient eye movements
100
and slower
decisions
101
in driving situations. Cataract, visual field loss, and
glaucoma are all associated with increased accident risk, as are
reduced mesopic vision and increased glare sensitivity.
94 102–108
Night driving is a mesopic (intermediate brightness vision)
rather than a scotopic (night-time vision) or photopic
(daytime vision) task. It can push even normal visual systems
to their limits. Visual acuity decreases with decreasing target
illumination. This loss is greater in people over 65 years of
age.
109 110
The distance at which night-time highway signs can
be read decreases significantly in older individuals compared
to younger people of the same visual acuity.
111
Headlight glare
sensitivity also increases with ageing.
26
The increased brightness of HID headlights is an advantage
for the older drivers who use them. They enable owners to see
farther down straight roads, providing them with more time
to respond to potential problems. Conversely, disability glare
and photostress recovery time increase with glare source
luminance.
14 23 27 30 31 35 43
Thus, HID headlights can produce
more glare than halogen headlamps for drivers who confront
them on curving or hilly two lane highways.
Older individuals have increased intraocular stray
light, glare sensitivity, and photostress recovery
time.
26 33 38 49 50 72 73 83–85 112
Thus, confronting brilliant HID head-
lights is a greater potential problem for older than younger
drivers. HID headlights also cause more discomfort glare than
conventional halogen headlights of the same photopic or sco-
topic illuminance.
2 4 113
This potential distraction is greater in
61–77 than 20–31 year old drivers.
4
Windshield or spectacle filters decrease night-time visibil-
ity, so they are not a useful solution for headlight glare.
114–116
Some older drivers simply choose to limit their night-
time driving,
107 117 118
but there is still a need to develop
screening techniques to identify drivers at greatest
accident risk.
79 103 106 119 120
Promising techniques have been
examined,
91 104 120 121
but the selection of appropriate methods
for screening older drivers raises complex medical, social, and
legal issues that remain under investigation.
Practical solutions for the highway glare problem have been
studied for decades.
3526
Countermeasures have never been
implemented seriously because of consumer disinterest,
manufacturer resistance, and lack of legislative resolve. HID
headlight glare has now captured the attention of many con-
sumers. There have been improvements in divided highway
design such as wide medians and glare screens separating
opposing lanes. Unfortunately, these construction advances
can’t be implemented on two lane, undivided highways where
headlight glare is worst.
Glare countermeasures include adaptive headlights and
ultraviolet headlight systems. Adaptive headlight systems
monitor and compensate both optically and mechanically for
changing traffic, road, and meteorological conditions. Ultra-
violet headlight systems project invisible ultraviolet radiation,
improving the visibility of fluorescent highway markers, signs,
and objects without increasing glare for oncoming drivers.
Ultraviolet radiation is especially valuable in fog because
highway signs can fluoresce in the visible spectrum when
exposed to ultraviolet radiation, but backscattered ultraviolet
radiation is invisible so it can’t reduce the contrast and legibil-
ity of the signs. This situation is similar to retinal imaging in
patients with asteroid hyalosis. Ordinary reflectance images
have poor detail because back scattered flashlamp light
reduces the contrast and visibility of retinal features. In fluo-
rescein angiography, however, back scattered blue light from
the photographic flash is blocked from reaching the camera by
a barrier filter, and green fundus fluorescence provides good
retinal image contrast.
Polarising headlight systems are perhaps the best solution
to the highway glare problem.
5
A polarising filter is placed in
front of all automobile headlights, with its axis of polarisation
inclined at 45° to the vertical. Another polarising filter known
as an analyser is placed in front of the eyes of all drivers. Ana-
lysers have the same axis of polarisation as polarisers in front
Why HID headlights bother older drivers 115
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of their own headlights, which is perpendicular to that of
polarisers in oncoming vehicles. Thus, analysers block light
from opposing traffic but transmit light scattered from
roadside objects by their own headlights. Even ideal analysers
block approximately 50% of the light reaching them, but HID
headlights provide adequate headlamp brightness for fixed
analyser systems, and there are methods for switching analys-
ers on and off when polarised light is detected. A major draw-
back in the adoption of polarising headlamp systems is the
fact that their cost would be borne by their owners, but the
systems would benefit only oncoming drivers until they
became widely used.
5
CONCLUSION
A HID headlamp manufacturer touting the fact that a “xenon
bulb produces more than twice the amount of light of a halo-
gen bulb,”
6
answered the question “Why does xenon light
sometimes appear to irritate oncoming drivers?”
6
by explain-
ing that “due to the conspicuous colour of xenon light, drivers
are more inclined to look in the headlamps.”
6
Older drivers
encountering HID headlights on a winding two lane highway
know that “photophilia” isn’t the answer.
Night driving can push any visual system to its limits. Con-
ventional clinical glare tests underestimate disability glare
from moving light sources. Disability glare increases with
increasing glare source brightness. HID headlights are
brighter than conventional tungsten-halogen headlights.
Thus, they cause more disability glare under identical viewing
circumstances.
Visual quality declines with ageing, even in individuals with
20/20 visual acuity. Glare due to intraocular light scattering
increases with ageing, prompting many older drivers to curtail
their night driving activities. Increased glare from HID head-
lights is one more visual hurdle for the older driver.
Aircraft landing lights would allow automobile drivers to see
farther down a road. They would also incapacitate oncoming
drivers. Brighter headlights do provide increased visibility for
older drivers who use them. They also cause more glare for older
drivers who confront them. The optimal balance in headlight
brightness between owner visibility and viewer disability
depends on the driving situation, but technologies are available
for more adaptable, less bothersome headlight systems.
Governmental regulations determine which headlights we
encounter. Acceptance or rejection of the current generation of
HID xenon headlights ultimately depends on their record in
traffic and litigation. Xenon bulbs make good headlights. They
also make good glare sources. The additional light they project is
valuable. Questions remain regarding how and where it should
be projected. Until these questions are resolved, for many older
drivers confronting stylish HID headlights on two lane roads,
glare rather than beauty will be in the eye of the beholder.
ACKNOWLEDGEMENT
Supported in part by the Kansas Lions Sight Foundation, Inc.
.....................
Authors’ affiliations
M A Mainster, G T Timberlake, Department of Ophthalmology,
University of Kansas Medical Center, 3901 Rainbow Boulevard, Kansas
City, KS 66160-7379, USA
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