Light in Medicine
Dr. Mustafa Al Musawi
Even though man is now very efficient at making artificial light, the sun is
still the major source of light in the world. The sun is both beneficial and
hazardous to our health. The light is a particular form of the
electromagnetic radiation (EMR).
Electromagnetic radiation consists of an electrical field (E)
which varies in magnitude in a direction perpendicular to the direction
in which the radiation is traveling, and a magnetic field (M) oriented at
right angles to the electrical field. Both these fields travel at the speed
of light (c). Two characteristics of electromagnetic radiation are
particularly important to understand remote sensing. These are the
wavelength and frequency. Electromagnetic radiation (EMR) as an
electromagnetic wave that travels through space at the speed of light
C which is 3x108 meters per second. Fig. 1 gives an overview over the
whole spectrum of electromagnetic waves.
Fig 1. The electromagnetic spectrum
-1-
Light has some interesting properties, many of which are used in
medicine:
1. It consists of bundle of photons. The speed of light changes when it goes from
material into another. The ratio of the speed of light in a vacuum to its speed in
a given material is called the index of refraction. If a light beam meets a
new material at an angle other than perpendicular, it bends, or is refracted.
2. Light behaves both as a wave and as a particle. As a wave it produces
interference and diffraction. As a particle it can be absorbed by a single
molecule. When a light photon is absorbed its energy is used in various ways. It
can cause a chemical change in the molecule that in turn can cause
an electrical change. This is basically what happens when a light photon-is,
absorbed in one of the sensitive cells of the retina. The chemical change in
a particular point of the retina triggers an electrical signal to the brain to inform it
that a light photon has been absorbed at that point.
The energy carried by light
Scientists have observed that light energy can behave like a wave as
it moves through space, or it can behave like a discrete particle with a
discrete amount of energy (quantum) that can be absorbed and emitted. A
beam of light is modelled as a stream of photons, each carrying a welldefined energy that is dependent upon the wavelength of the light. The
energy of a given photon can be calculated by:
Photon energy (E) = hc/λ
Where E is in joules
h = Planck’s constant = 6.6×10-34J•s
c = Speed of light = 2.998 × 108 m/s
-2-
λ = Wavelength of the light in meters
For a quantum of green light (500 nm) the energy can be calculated as:
E = hc/λ = [6.6×10-34J•s ×2.998 × 108 m/s] / 500 ×10-9 m
= 3.97×10-19 J
Light is reflected to some extent from all surfaces. There are two types
of reflection. Diffuse reflection occurs when rough surfaces scatter the light in
many directions. Specular reflection is more useful types of reflection; it is
obtained from very smooth shiny surfaces such as mirrors where the light is
reflected at an angle that is equal to the angle at which it strikes the surface.
Mirrors are used in many medical instruments.
Measurement of light and its units
The three general categories of light-UV, visible, and IR.
Wavelengths of light used to be measured in microns (1 μ = 10-6 m) or in
angstroms (1 Å = 10-10 m), but at present the recommended unit is the
nanometer (1 nm = 10-9 m). Ultraviolet light has wavelengths from about
100 to 400 nm; visible light extends from about 400 to 700 nm; and IR
light extents from about 700 to over 104 nm.
Visible light is measured in photometric units that relate to how light is
seen by the average human eye. In photometry the quantity of light
striking a surface is called illuminance and the intensity of a light source is
called its luminance. All light radiation, including UV and IR radiation,
can be measured in radiometric units. In radiometry the quantity of light
-3-
striking a surface is called irradiance and the intensity of a light source is its
radiance.
The wavelengths of light fit into the whole spectrum of electromagnetic
radiation. Note that light has wavelengths much shorter than TV and radio
waves but much longer than x-rays and gamma rays.
Applications of visible light in medicine
An obvious use of visible light in medicine is to permit the physician
to obtain visual information about the patient regarding, for example, the
colour of his skin and the presence of abnormal structures in or on his
body. It is quite easy for a physician to examine the skin under normal
lighting conditions, but when she wishes to look into a body opening she is faced
with the practical problem of getting light into the opening without obstructing
the view. Like a lot of tricks, this one is done with mirrors. The curved
surface focuses the light at the region of interest. More sophisticated
instruments, such as the ophthalmoscope for looking into the eyes and the
otoscope for looking into the ears, use basically the same principle.
A number of instruments, called endoscopes, are used for viewing
internal body cavities. Special purpose endoscopes are often given names
indicating their purpose. For example, cystoscopes are used to examine the
bladder, protoscopes are used for examining the rectum, bronchoscopes are
used for examining the air passages into the lungs, and Gastroscope used to
examine the esophageal & stomach. Some endoscopes are rigid tubes with a
light source to illuminate the area of interest. Many of them are equipped
with optical attachments to magnify the tissues being studied.
The development of fiberoptic techniques permitted the construction of
flexible endoscopes. Flexible endoscopes can be used to obtain information
-4-
from regions of the body that cannot be examined with rigid endoscopes,
such as the small intestine and much of the large intestine. Some flexible
endoscopes are over a meter in length. The image obtained with a flexible
endoscope is not as good as that obtained with a rigid endoscope, but often
the only alternative to a flexible endoscopic examination is exploratory
surgery. Flexible endoscopes usually have an opening or channel that
permits the physician to take samples of the tissues (biopsies) for later
microscopic examination.
Since light contains energy that largely appears as heat when it is
absorbed, there is a limit on the amount of light that can be used in
endoscopy. For endoscopy, the heating can be reduced by reducing the IR
light from the source with special IR absorbing glass filters. In this cold-light
endoscopy the light source contains very little IR radiation and the
heating of tissues is minimized.
Applications of microscopes in medicine
There have been few breakthroughs in science that have had as great
an impact as the invention of the microscope by Leeuwenhoek (1670). The use of
the microscope in the pathology laboratory is as common as the use of the
thermometer in the clinic.
The standard light microscope usually can be set at any of several
magnifications by changing the power of the eyepiece or of the objective lens. The
highest magnification that can be obtained is limited by the wavelength of visible
light. Since the wavelength of visible light range from 400 to 700 nm, the smallest
object that can be resolved is about 1 μm in diameter. Since most cells are 5 to 50
μm in diameter, this type of microscope is adequate for resolving all but subcellular
objects.
-5-
If you put a thin slice of tissue under a microscope you will not see much
because most cells are transparent to all wavelengths of visible light-red blood cells
are an exception. In order to distinguish different cells it is usually necessary to stain
them with a chemical that strongly absorbs certain visible wavelengths.
It is sometimes advantageous to use UV light or x-rays in microscopy. Since
our eyes cannot see wavelength shorter than those of visible light, it is necessary to
convert the image produced by UV light or x-ray beams into images that use visible
light.
Different types of light microscopy
The types of objects for microscopy can be very different. For
example, objects may have different degrees of transparency. In one
extreme case no light can penetrate an object at all, i.e. only its surface
can be imaged. In another extreme case the object is totally transparent,
i.e. under normal conditions it cannot be seen at all. For each case, and all
intermediate situations, a specific visualizing technique is available:
Bright field microscopy
is the standard type of microscopy in biology? Biological objects are
quite often more or less transparent but have some scattering or absorbing
subcellular structures. Such structures are called" amplitude objects"
since they modify the intensity of the light passing through it.
Dark field microscopy
is also suitable for amplitude objects. Here, for illumination the secondary
maxima of the Bessel function are used for illumination. The effect is that
the image appears as a type of negative of the corresponding bright field
image.
-6-
Phase contrast microscopy
Visualizes objects which are transparent but have an index of
refraction different from the environment. When a light wave enters an
object of higher refractive index, it is retarded by a quarter of a
wavelength. This wave, after having passed the phase object, can be
brought to interference with undisturbed light which is always present at
suitable illumination conditions. By negative interference the phase
difference is converted into an amplitude difference and the phase objects
appear darker than the environment.
Fluorescence microscopy
Fluorescence emission has a wavelength significantly different
from the wavelength of excitation. In addition, fluorescent light is emitted
in all directions of space, particularly also in the direction from where the
excitation light is coming. When fluorescence is observed from this side
one speaks of "epifluorescence microscopy". This allows one to separate
fluorescent light from the illuminating light using a filter set and on a
spatial basis. Under observation conditions close to ideal, fluorescence
may be observed before an almost dark background. This allows one to
visualize very faint structures and even individual molecules, provided
they contain a fluorophore which can be excited suitably and which reemits fluorescence.
Applications of ultraviolet light in medicine
Ultraviolet radiation (UV): Is an electromagnetic radiation with wave
length from 400 nm to below 290 nm, and freq. range from 7.5 x 1014 to
1015 Hz UV radiation lies between visible light and x-ray. UV radiation is
divided into three bands: UVA: 400-320 nm UVB: 320-290 nm UVC:
-7-
less than 290 nm. UVA: also known as long wave UV, is non-ionizing
and produce fluorescence in many substances. UVB: or middle wave UV
is also non ionizing, it produces the most skin erythema. UVC: short
wave UV, is ionizing and germicidal. Both UVA & UVB reach the earth
from the sun; however, UVC is filtered out by the ozone layer.
The intensity of UV radiation reaching the skin is greatest when a high
power lamp is used, when the loop is close to the pt. and when the radiation
beam is perpendicular to the surface of the skin. Penetration is deepest for
UV radiation with the highest intensity, longest wavelength and lowest freq.
thus UVA penetrate farthest and reaches through several millimeters of skin,
while UVB and UVC penetrate less deeply and are almost entirely absorbed
in the superficial epidermal layers. The penetration of UV radiation is also
less deep if the skin is thicker or darker.
Effect of UV radiation: 1. Erythema production: erythema or redness of the
skin due to dilatation of the superficial blood vessels caused by the release of
histamine, is one of the most common effects of exposure to UV radiation,
UVB is most potent than UVA, the precise mechanism is unknown,
however, it is known that this effect is mediated by prostaglandin release
from the epidermis, and it may be also related to DNA damaging effect of
UV radiation. 2. Tanning: a delayed pigmentation of the skin occurs in
response to UV radiation exposure. This effect is the result of increase
production and upward migration of melanin granules and oxidation of
premelanin in the skin. 3. Epidermal hyperplasia: a thickening of the
superficial layer of the skin occurs 3 days after exposure to UV radiation.
This effect is thought to be caused by the release of prostaglandin precursors
leading to increasing DNA synthesis by epidermal cells, resulting in
epithelial cell turnover and cellular hyperplasia. 4. Vitamin D synthesis: UV
irradiation of the skin is necessary for the conversion of provitamin D to
-8-
vitamin D, rickets may result of inadequate exposure to UV, inadequate
intake of vitamins or poor kidney function. For most individuals the exposure
to UV or sunlight is sufficient to maintain adequate level of vitamin D
production, UV exposure may be inadequate in some bedridden pt.
Ultraviolet irradiation induces keratinocyte proliferation and epidermal
hyperplasia through the activation of the epidermal growth factor receptor
Ultraviolet light with wavelengths below about 290 nm is germicidalthat is, it can kill germs-and it is sometimes used to sterilize medical
instruments. Ultraviolet light also produces more reactions in the skin than
visible light. Some of these reactions are beneficial, and some are harmful.
One of the major beneficial effects of UV light from the sun is the conversion
of molecular products in the skin into vitamin D.
Ultraviolet light from the sun affects the melanin in the skin to cause
tanning. However, UV can produce sunburn as well as tan the skin. The
wavelengths that produce sunburn are around 300 nm, just at the edge of the
solar spectrum. The amount of 300 nm light in the sun's spectrum depends
on the amount of atmosphere that the sunlight must pass through. Ordinary
window glass permits some near UV to be transmitted but absorbs the
sunburn component.
Solar UV light is also the major cause of skin cancer in humans. The
high incidence of skin cancer among people, who have been exposed to the
sun a great deal, such as fishermen and agricultural workers, may be
related to the fact that the UV wavelengths that produce sunburn are also
very well absorbed by the DNA in the cells. Skin cancer usually appears on
those portions of the body that have received the most sunlight, such as the
tip of the nose, the tops of the ears, and the back of the neck.
-9-
Applications of infrared light in medicine
Infrared radiations (IRR) are electromagnetic radiation that lies
within that part of electromagnetic spectrum between visible light and
microwave radiation. The radiation is characterized by wavelength
extended from 760 nm to 1 mm. IRR can be subdivided into 3 regions (A,
B & C) according to their absorption and their effect upon the tissue.
Classification of infrared radiation:
ABSORPTION Some radiations striking the surface of the skin will be
reflected and some will penetrate, scattered, refracted and ultimately
absorbed. Close to 95% of the radiation applied perpendicular to the skin is
absorbed. Water and proteins are strong absorbers of infrared. Any
radiation entering into the skin depends on: Structure, vascularity,
pigmentation of skin, wavelength of radiation.
About half of the energy from the sun is in the IR region. The warmth
we feel from the sun is mainly due to the IR component. The IR rays are
- 10 -
not usually hazardous even though they are focused by the cornea and lens
of the eye onto the retina. However, looking at the sun through a filter (e.g.,
plastic sunglasses) that removes most of the visible light and allows most of
the IR wavelengths through can cause a burn on the retina. Some people
have damaged their eyes in this way by looking at the sun during a solar
eclipse. Dark glasses absorb varying amounts of the IR and UV rays.
Two types of IR photography are used in medicine: reflective IR
photography and emissive IR photography. The latter, which uses the long IR
heat waves emitted by the body that give an indication of the body
temperature, is usually called thermograph. Reflective IR photography,
which uses wavelengths of 700 to 900 nm to show the patterns of veins just
below the skin. Some of these veins are visible to the eye, but many more
can be seen on a near-IR photograph of the skin. Since the temperature at
the skin depends on the local blood flow, a thermogram with good
resolution shows the venous pattern much like a near-IR photograph.
There is considerable variation in the venous patterns of normal
individuals. Even in the same individual the venous patterns in the two
breasts may be quite different. Cancer and other diseases can cause
changes in the venous pattern, but these changes can be masked by the
normal variations. Also, a layer of fat beneath the skin can reduce the
appearance of the venous pattern.
Infrared can also be used to photograph the pupil of the eye without
stimulating the reflex that changes its size. There are two types of
photography used in medicine
a. Reflective IR phototherapy:
It uses wavelengths of (700-900) nm to show the pattern of veins just
- 11 -
below the skin
b. Emission IR phototherapy (Thermography):
Infrared thermography is equipment or method, which detects
infrared energy emitted from object, converts it to temperature, and
displays image of temperature distribution. The image of temperature
distribution is called infrared thermograph and the method to be called as
infrared thermography. It is used to detect breast cancer, diabetic leg,
malignancy in arms and blood flow in brain.
Applications of Microwave radiation in medicine
The microwave technique is a currently very attractive technique
for medical applications. It uses non-ionizing electromagnetic waves and
has good penetration capability into human tissues (in the GHz range).
Absorption of M.W depends on water continent of the absorbing material
and the frequency of the incident M.W. The best frequency at which the
great absorption occur is 20 MHz. M.W is preferred for the muscle more
than for fatty tissue due to the highly water content of the muscles.
Hyperthermia: Microwaves as Cancer Treatment
Hyperthermia (or thermotherapy) is a cancer treatment that
involves heating tumor cells within the body. Elevating the temperature
of tumor cells results in cell membrane damage, which, in turn, leads to
the destruction of the cancer cells. Over a hundred years ago, doctors first
noticed that high heat, such as that resulting from a fever, killed cancer
cells and shrank tumors. Research into hyperthermia as a cancer treatment
began soon after. Today hyperthermia is used as an adjunct to radiation
therapy and chemotherapy.
- 12 -
Hyperthermia treatment of cancer requires directing a carefully
controlled dose of heat to the cancerous tumor and surrounding body
tissue. Cancerous tissues can be destroyed at exposure to a temperature of
about 108 °F for an hour. This high heat must be used wisely—too little
heat and the cancer will not be killed. However, if too much heat misses
the tumor target, the skin or other healthy tissues could be burned.
Microwave energy is very effective in heating cancerous tumors,
because tumors typically have high-water content. Such tissue heats very
rapidly when exposed to high-power microwaves. Furthermore,
microwaves can be delivered to tissue by special-purpose antennas that
are located adjacent to the patient’s body. Depending on the tumor size
and location in the body, one or more microwave antennas can be used to
treat the tumor. When a microwave thermotherapy antenna is turned
“on,” body tissues with high-water content that are irradiated with
significant amounts of microwave energy are heated. The temperature
rise in the tissue is due to the transfer of microwave energy into heat.
- 13 -