Download Night Vision and Night Vision Devices

Night Vision and Night Vision Devices
By Cristian R. Martinez, TSgt, USAF, MBA, CPOT
Is seeing at night just a matter of our ability to navigate in the darkness? Does eating carrots make a
huge difference in seeing better in the dark, or are there other factors that influence our night
vision?
Understanding Light
In order to have a better understanding of how night vision works, it is important to first understand
something about light. The amount of energy in a light wave is related to its wavelength: Shorter
wavelengths have higher energy. Of the visible light, violet has the most energy, and red has the
least amount of energy. Just next to the visible light spectrum, on the red end, is the infrared
spectrum. Infrared light can be split into three categories:


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Near-infrared (near-IR) - Closest to visible light, near-IR has wavelengths that range from
0.7 to 1.3 microns, or 700 billionths to 1,300 billionths of a meter.
Mid-infrared (mid-IR) - Mid-IR has wavelengths ranging from 1.3 to 3 microns. Both nearIR and mid-IR are used by a variety of electronic devices, including remote controls.
Thermal-infrared (thermal-IR) - Inhabiting the largest part of the infrared spectrum, thermalIR has wavelengths ranging from 3 microns to over 30 microns”. 1 (J. Tyson, 2011)
The key difference between thermal-IR and the other two is that thermal-IR is emitted by an object
instead of reflected off of it. Infrared light is emitted by an object because of what is happening
with that object at the atomic level.
Night Vision and Where it Occurs
Starting with the layers in the eye where night vision occurs, night vision occurs in the
photoreceptor layer. This layer has the highly specialized cells known as the photoreceptors (rods
and cones), which are responsible for converting the light striking them into electrochemical nerve
pulses. The rods and cones process electromagnetic radiation with wavelengths in the 750
nanometers (nm) (red) to 400 nm (violet) range, which is what we consider the visible spectrum of
light. (See image 1)
(Image 1)
Rods are specialized cells in the retina responsible for the discrimination of motion and night vision.
There are approximately 125 million rods in the retina. Rods are primarily for night vision, which
is also called scotopic vision. (See image 2) Most rods are in the periphery of the retina, so they
handle most if not all of our peripheral vision. The visual pigment found in the rods is called
rhodopsin; it’s this pigment that helps the rods convert light into the electrochemical nerve impulses
sent to the brain.
(Image 2)
The Retina: Cones
There are approximately six million cones in the retina. Cones can be found throughout the retina,
but they are predominantly found in the macula and fovea. There are no rods in the fovea; this area
is exclusively for cones. This explains the fovea’s extremely detailed clarity and super color vision
qualities. This is also why we see poorly at night; we need rods for low-light vision and the fovea
has none. The cones function best under photopic or fully illuminated conditions, such as sunlight.
(see image 3)
(Image 3)
Cones provide our color vision, and they do this the same way your HDTV works. The
TV projects only three colors––red, green, and blue. Yet you see all the colors of the rainbow on
TV. This is because mixing red, green, and blue can create any other color. Your retina has at least
three different types of cones, each sensitive to a different color. The cone sensitive to red has a
visual pigment called erythrolabe (erythro is Greek for red). The green-sensitive cone has a
pigment called chlorolabe (you can remember this by thinking of green plants producing
chlorophyll). The last type of cone is sensitive to blue color; which contains a visual pigment called
cyanolabe (cyan being the color blue). By stimulating the red-, green-, and blue-sensitive cones in
various amounts, the brain receives electrochemical messages from each and interprets the actual
color seen. Cones are cool because they allow us to see color.2 (W.P.Muse,2013)
Factors Influencing Visual Acuity (VA)
Many factors influence visual acuity. Primary factors are the region of the retina stimulated,
illumination, spectral quality of light, contrast, pupil size, time of exposure, patient’s age, condition
of the ocular media, presence of ametropias, and individual variations.
The fovea centralis is the area where best vision (under photopic conditions) occurs. The fovea
contains only cones, which produce the clearest images. VA progressively decreases the farther an
image strikes the retina from the fovea. This is because the concentration of cones is greatest in the
fovea and decreases toward the retina’s periphery.
Good illumination (photopic) conditions allow the visual system to use the cones in the fovea to
process light stimuli. Dim light (mesopic) conditions force people to use a mixture of rods and
cones to see adequately. This causes a loss of clarity, as rods do not provide images as sharp as
cones.
When illumination is very poor (scotopic), the visual system becomes almost completely dependent
on the rods for any vision. Rods, while very good at picking up visual images under low light
conditions, do not produce very sharp and clear vision.
Under scotopic conditions, vision is best when images are placed just outside the fovea. This allows
a mixture of rods and cones to process what visual images can be seen. (It’s interesting to note the
greatest number of rods per area of retina exists just outside of the fovea.) For example, a very dim
star in the night sky may only be seen when you look slightly away from it. This allows the rods to
pick up its image. If you look straight at it, it disappears, as the cones are not sensitive enough to
process the minute amount of light coming from the galaxy far, far away.
The spectral quality of light refers to its color or wavelength. The eye can generally see wavelengths
between 400 and 750 nm. White light contains all the colors of the rainbow. Some lights are white,
but have a reddish or bluish tinge to them. Look at the fluorescent lights in your building and you
may notice this. Some of the lights will look different from others.
The clarity of vision can change due to variations in the spectral quality of the light being seen.
Some light has more blue in it; some has more red, and so on. This variation, though subtle, does
have an effect on VA and efficiency. Some people are sensitive to fluorescent lighting, but do fine
under incandescent lighting. This is most probably related to their sensitivity to changes in the
spectral quality of the light.3 (K. Cooper, 2010)
The Aging Eye
Diminished night vision is a common problem as the eye ages. Some of the reasons for diminished
night vision are attributed to:
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
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Light being blocked by clouded, yellowed lenses
The number of light sensitive rods in the retina decreasing
Less light getting to the retina as the pupil gets smaller
Car accidents and falls in the home may be attributed to the inability for older eye muscles to react
to darkness changes. Dark adaptation is the ability for vision to adjust back and forth between light
that’s suddenly very bright and then dim again. It occurs in situations such as driving at night and
with the onset of bright headlights. Decreased dark adaptation may be due to fewer rods and to a
reduction in the regeneration rate for rhodopsin, the light-sensitive pigment in the rod cells, as we
age. Carrots contain Vitamin A which aids in the regeneration of rhodopsin in the rod cells. From
this stems the belief that eating carrots improves our night vision.4
Workable solutions to help with night vision loss
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Eat a good overall diet that's rich in fruits and vegetables
and low in saturated fat
Open the shades and use bright lights as dusk approaches
Scatter night lights throughout the home to illuminate the
path in the evenings
Limit or stop driving at night
Night Vision Devices: How do they work?
The ability to be able to see in the dark has many applications other than with our day to day
activities which prompted the development of night vision goggles. Night vision goggles are not
just toys or props used in spy movies. Night vision goggles have several useful applications.
Below is a list of several common uses for night vision goggles:
 Military operation
 Law enforcement
 Hunting
 Wildlife observation
 Surveillance
 Security
 Navigation
 Hidden-object detection
Law enforcement officials use night vision goggles for nighttime operations to include training and
to enhance mission readiness. With the appropriate night-vision equipment they can see an
individual standing over 200 yards (183 m) away on a moonless, cloudy night. Night vision can
work in two very different ways, depending on the technology used.
Image Enhancement Technology
Image enhancement works by accumulating small amounts of light, comprised of the lower end of
the infrared light spectrum. This may be present, but may be unnoticeable to our eyes. This is then
amplified to a point that we can easily detect the image. Image-enhancement technology is what
most people think of when you talk about night vision. In fact, image-enhancement systems are
normally called night-vision devices (NVD’s). NVD’s rely on a special tube, called an imageintensifier tube, to collect and amplify infrared and visible light.
How Image Enhancement Works:
1. A conventional lens, called the objective lens, captures ambient light and some near-infrared
light.
2. The gathered light is sent to the image-intensifier tube. In most NVDs, the power supply for
the image-intensifier tube collects power from two N-Cell or two "AA" batteries. The tube
outputs a high voltage, about 5,000 volts, to the image-tube components.
3. The image-intensifier tube has a photocathode, which is used to transform the photons of
light energy into electrons.
4. As the electrons pass through the tube, similar electrons are released from atoms in the tube,
multiplying the original number of electrons by a factor of thousands through the use of a
micro-channel plate (MCP) in the tube. An MCP is a tiny glass disc that has millions of
microscopic holes (micro-channels) in it, made using fiber-optic technology. The MCP is
contained in a vacuum and has metal electrodes on either side of the disc. Each channel is
about 45 times longer than it is wide, and it works as an electron multiplier. When the
electrons from the photo cathode hit the first electrode of the MCP, they are accelerated into
the glass micro-channels by the 5,000-V bursts being sent between the electrode pair.
As electrons pass through the micro-channels, they cause thousands of other electrons to be
released in each channel using a process called cascaded secondary emission. Basically, the
original electrons collide with the side of the channel, exciting atoms and causing other
electrons to be released.
These new electrons also collide with other atoms, creating a chain reaction that results in
thousands of electrons leaving the channel where only a few entered. An interesting fact is
that the micro-channels in the MCP are created at a slight angle (about a 5-degree to 8degree bias) to encourage electron collisions and reduce both ion and direct-light feedback
from the phosphors on the output side.
5. At the end of the image-intensifier tube, the electrons hit a screen coated with phosphors.
These electrons maintain their position in relation to the channel they passed through, which
provides a perfect image since the electrons stay in the same alignment as the original
photons. The energy of the electrons causes the phosphors to reach an excited state and
release photons. These phosphors create the green image on the screen that has come to
characterize night vision.
6. The green phosphor image is viewed through another lens, called the ocular lens, which
allows you to magnify and focus the image. The NVD may be connected to an electronic
display, such as a monitor, or the image may be viewed directly through the ocular lens.5
(J. Tyson, 2001)
Thermal Imaging Technology
Thermal imaging technology functions by capturing the upper portion of the infrared light
spectrum. It is emitted as heat by objects instead of simply reflected as light. Hotter objects such as
warm bodies, emit more of this light than cooler objects like trees or buildings.
Below you will find a detailed explanation of how the thermal imaging works.
How Thermal Imaging Works:
1. A special lens focuses the infrared light emitted by all of the objects in view.
2. The focused light is scanned by a phased array of infrared-detector elements. The detector
elements create a very detailed temperature pattern called a thermogram. It only takes about
one-thirtieth of a second for the detector array to obtain the temperature information to make
the thermogram. This information is obtained from several thousand points in the field of
view of the detector array.
3. The thermogram created by the detector elements is translated into electric impulses.
4. The impulses are sent to a signal-processing unit, a circuit board with a dedicated chip that
translates the information from the elements into data for the display.
The signal-processing unit sends the information to the display, where it appears as various
colors depending on the intensity of the infrared emission. The combination of all the
impulses from all of the elements creates the image.6 (J. Tyson, 2001)
Reason for Night Vision Devices
The original purpose of night vision goggles was to locate enemy targets at night. It is still used
extensively by the military for that purpose, as well as for night navigation, surveillance and
targeting. Police and security often use both thermal imaging and image-enhancement technology,
particularly for surveillance. Hunters and nature enthusiasts use NVD’s to maneuver through the
woods at night. Detectives and private investigators use night vision devices to watch people they
are assigned to track. Many businesses have permanently mounted cameras equipped with night
vision components to monitor the surroundings of their facilities.
A really amazing ability of thermal imaging is that it reveals whether an area has been disturbed -- it
can show that the ground has been dug up to bury something, even if there is no obvious sign to the
naked eye. Law enforcement agents can use thermal imaging technology to try to discover if items
have been hidden by a criminal, such as money, drugs and/or bodies. Recent changes to areas such
as walls can be seen using thermal imaging, which has provided important clues in several cases.
Many people are beginning to discover the unique world that can be found after darkness falls. If
you are often out camping or hunting, chances are that night-vision devices can be useful to you -just be sure to get the right type for your needs. Night vision goggles are a great asset to have if you
are performing special operations but for more practical purposes you should stick to regular
spectacles.
Conclusion
Aside from cataract surgery, there is not a medical solution to fix declining night vision. As the
millions of Baby Boomers continue to age, there might be more opportunities for technology to
develop the use of night vision devices for every day purposes, or should I say for every night
purposes.
__________________________________________________________
Cristian R. Martinez, TSgt, USAF, MBA, CPOT is an optometry Flight Chief for the United States
Air force. He has worked in both optometry and ophthalmology. After getting the experience and
necessary training Sergeant Martinez became the Non-Commissioned Officer in Charge (NCOIC)
of Ophthalmology surgical services. This led to Sergeant Martinez working as a surgical trainer,
supply custodian and equipment custodian. As a supply custodian he ensured all surgeries ran
smoothly and had all the required supplies. As equipment custodian he ensured all the surgeons
had the latest and greatest technology in place for all surgical procedures whenever possible. After
6 years of dedicated service at Eglin AFB, Florida he was then required to move to Japan where he
now works as the Flight Chief of the optometry clinic and works closely with two technicians and
two Optometrists.
No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means
(electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of the publisher.
Copyright 2015 American Optometric Association
Reference page
1,5,6
Tyson, Jeff. "How Night Vision Works" 27 April 2001. HowStuffWorks.com.
2
Muse, W. P. (2013). Ophthalmic Journeyman: Ocular Anatomy/Physiology, Disorders, and Pharmacology. Vol 2.
Maxwell, AFB, Al: Air Unversity.
3
Kim Cooper, CJ Kazilek. (2010, January 6). Seeing Color. ASU - Ask A Biologist. Retrieved March 29, 2014 from
http://askabiologist.asu.edu/rods-and-cones
DoD. Jun 2008. US Night Vision: Enhanced and Ubiquitous. http://www.soldiermod.com/summer-08/features-nightvision.html. retrieved 31 March 2014.
4
http://electronics.howstuffworks.com/gadgets/high-tech gadgets/nightvision.htm. 31 March 2014.
http://www.health.harvard.edu/newsletters/Harvard_Womens_Health_Watch/2007/June/Blinded-by-the-night
Night Vision and Night Vision Devices Quiz
To receive two hours of continuing education credit, those taking the quiz must be an AOA Associate
member and answer 14 of the 20 questions correctly. This exam consists of multiple-choice questions
designed to measure the level of understanding of the material covered in the continuing education article
“Night Vision and Night Vision Devices.”
This article is worth two hours of continuing education credit from the Commission on Paraoptometric
Certification. Expiration date: December of the current year
If you are renewing a CPO, CPOA or CPOT certification this year, proof of 18 earned credits and your
$95 renewal payment will be due to the CPC by November 1. All quizzes submitted by October 1 will be
graded and CE verification forms will be emailed to the paraoptometric by October 15 so that the
November 1 renewal deadline can be met. Please note that quizzes submitted after October 1 will
not be processed in time for you to meet the November 1 deadline.
Quizzes and order forms can be mailed to AOA-PRC, 243 North Lindbergh Blvd., St. Louis, MO 63141;
faxed to 314-991-4101; or scanned and emailed to [email protected].
To receive continuing education credit, complete the information below and mail with your $20
processing fee, $10 per hour of CE before December 31st of this year to the:
AOA Paraoptometric Resource Center, 243 N. Lindbergh Blvd, St. Louis, MO 63141-7881
Name
Member ID number
Address
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Card Holder Name
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_Security Code _____
Select the option that best answers the question. 1. The amount of _______________ in a light wave is related to its __________________:
a. Strength, Power
b. Energy, Wavelength
c. Ability, Wavelength
d. Energy, Power
2. Infrared light is emitted by an object because of what is happening with that object at the
________________.
a. Light entrance
b. Atomic level
c. Retina nucleus
d. Cone center
3. The __________________is the area where best vision occurs.
a. Fovea centralis
b. Retina
c. Macula
d. Optic nerve
4. What type of pigment is in a cone sensitive to blue color?
a. Erythrolabe
b. Chlorolabe
c. Electrochemical
d. Cyanolabe
5. The spectral quality of light refers to its color or _________________________.
a. Frequency
b. Amplitude
c. Wavelength
d. Visible light
6. When illumination is very poor the ________________ becomes almost completely
dependent on the _____________ for any vision.
a. Cones, fovea
b. Eyes, sun
c. Visual system, rods
d. Night vision, goggles
7. The eye can generally see wavelengths between ___________ and ___________nm
a. 450, 800
b. 400, 750
c. 350, 600
d. 500, 700
8. Which portion of the eye works like HDTV and how?
a. Cones because mixing red, green, and blue can create any other color.
b. Rods since that is what causes the sharpest vision in the eye
c. Retina because it works like a conductor and creates pictures
d. None of the above
9. Which technology detects heat emitted by objects?
a. Image enhancement technology
b. Micro-channel plate c. Thermal imaging technology d. Image-intensifier tube
10. What was the original purpose of night vision goggles?
a. Used to locate enemy targets at night
b. Used for security surveillance
c. Used for hunting
d. Used for navigation
11. Approximately how many rods are in the retina?
a. 1.25 billion
b. 1.25 million
c. 125 million
d. 12.5 million
12. Good illumination (photopic) conditions allow the visual system to use the cones in the
fovea to process __________________________.
a. Dim light
b. Light stimuli
c. Movement
d. Wavelength
13. Rods are specialized cells in the retina that are responsible for the discrimination of
___________and _________________vision
a. Motion, night
b. Central, color
c. Agility, night
d. Clarity, color
14. What conditions force people to use a mixture of rods and cones to see adequately?
a.
b.
c.
d.
Mesopic
Ametropias
Scotopic
Photopic
15. Why does visual acuity (VA’s) progressively decrease the further an image strikes the
retina from the fovea?
a. This is because the concentration of rods is greatest in the fovea and decreases toward
the retina’s periphery.
b. This is because the concentration of cones is greatest in the retina and decreases
toward the fovea periphery.
c. This is because the concentration of cones is greatest in the fovea and decreases
toward the retina’s periphery.
d. This is because the periphery of cones is greatest in the fovea and decreases toward
the retina’s concentration.
16. With the appropriate night-vision equipment you can see an individual standing over
_____________________ away on a moonless, cloudy night.
a. 200 yards
b. 600 feet
c. 7200 inches
d. All of the above
17. What is the light-sensitive pigment in the rod cells?
a. Melanin
b. Zeaxanthin
c. Rhodopsin
d. Lutein
18. When using thermal imaging technology which of the following emits less light?
a. Human
b. Tree
c. Equal
19. ___ _______________________use night vision to watch people they are assigned to
track.
a. Military, police
b. Police, private investigators
c. Private investigators, hunters
d. Detectives, private investigators
20. There are approximately ________________ cones in the retina?
a. 6.1 million
b. .6 million
c. 6 million
d. 6 billion
No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any
means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of the
publisher. Copyright 2015 American Optometric Association