What is
Light?
Sunlight arrives
at the top of the earth's atmosphere at a power level of
about one kilowatt per square meter. It is by this energy
that all life-processes on earth are ultimately driven.
Without the sun's constant energy input our planet would
quickly radiate away its own energy in short order, making
all life extinct. Light is referred to as electromagnetic
radiation because the true nature of light is based on tiny
electromagnetic fields, called photons. These photons of
light can have many different energy levels or wavelengths,
which are expressed in nanometres (nm). The most familiar
wavelengths are the visible ones. Every wavelength is represented
by a different colour. For example, the Sun appears as yellow
because its light is most powerful at the visible wavelength
of yellow. However, there are many other wavelengths beyond
visible light. All of them together are called the electromagnetic
spectrum. At the most powerful end of the spectrum are the
gamma rays, followed by X-rays, then ultraviolet light,
and then visible light which takes up only a tiny fraction
of the electromagnetic spectrum and is sandwiched between
ultraviolet and infrared light. Infrared light is familiar
to us as heat. The spectrum continues as microwaves and
ends in radio waves, the least powerful photons. Of the
entire electromagnetic spectrum, only ultraviolet light,
visible light and infrared light are important to us at
this time.
Visible
Light
Besides providing
us (and reptiles) with light to see properly, the indication
of daytime and night time (light and dark) is an important
function. The visible light spectrum ranges from 390 to
700 nm. The light registered by the eye, and the colour
of it, depends on the strength of each wavelength. The Colouring
Rendering Index (CRI) expresses the ability of a light source
to illuminate an object as compared to natural light, with
natural sunlight having a CRI of 100. Nowadays, every artificial
light source with a CRI above 95 is considered to be a full-spectrum
light, since it is able to light-up an object as it would
appear under natural light and thus receive a certain amount
of any wavelength within the visible spectrum. Closely related
is the colour temperature, expressed in Kelvin (K), to define
the colour of the light emitted. In describing colour temperatures,
a low colour temperature corresponds to a warm or a red-yellow
appearance like incandescent lamps, around 2500 Kelvin.
Fluorescent lamps, operating at 4500 Kelvin or higher, emit
a white-bluish light. In colour temperature, the higher
the Kelvin temperature, the whiter and bluer the light.
The standard average temperature for daylight is about 5600K,
although it can range from as low as 2000K at sunset, to
more than 18000K in overcast or humid conditions. To obtain
natural visible light conditions in the terrarium, it is
important to choose a light source with the highest possible
CRI and a colour temperature from around 6000K for optimal
colours in animals and plants. Terrarium plants benefit
from certain wavelengths within the visible light for photosynthesis.
This is a process by which plants use the energy from light
to produce sugar, the "fuel" used by all living things.
The conversion of light into usable energy is associated
with the green pigment Chlorophyll. A light source with
a high output in the 400-450 nm range promotes plant growth
and health.
Ultraviolet
Light
Ultraviolet
or UV light is a high energy portion of the electromagnetic
spectrum, just beyond visible light.
The UV-spectrum
is divided into three wavelength groups:
-
UVA -
Long wave ultraviolet A, ranges from 320-400 nm and
is of significant importance for reptiles.
-
UVB -
Medium wave ultraviolet B, ranges from 290-320 nm and
is the most important for reptile purposes.
-
UVC -
Short wave ultraviolet C, ranges from 180-290 nm and
is dangerous to all living organisms.
It has been
demonstrated that UVA can influence agonistic, reproductive,
and signalling behaviours in reptiles. As reptiles can see
into the UVA range (320-400 nm) it will affect the way they
see things. The colour of their food or their bodies will
appear different in a reptile’s eye then the way we see
it if exposed to UVA radiation. Signalling by exposing body
parts (e.g. Anolis sp.) or changing colours (e.g. Chameleon
sp.) is common in reptiles, these signals are perceived
and also interpreted differently by reptiles if UVA radiation
is absent. Failure to provide UVA to diurnal reptiles can
cause stress by altering the reptile's perception of its
surroundings and how it responds to it. This is crucial
for breeding or keeping them around for the length of their
natural life span. UVB is generally defined as the wavelength
band from 290-320 nm. In the wild, most reptiles synthesize
their vitamin D3 from the UVB component of sunlight. Vitamin
D3 is essential for the effective metabolism of dietary
calcium in reptiles. UVB reacts with the precursor of vitamin
D, 7-dehydrocholesterol, in the skin to produce provitamin
D3.
Depending on heat and the aid of a mechanism in the skin,
provitamin D3 is converted into vitamin D3 itself. The liver
and kidneys transform vitamin D3 into its active form, a
hormone (1,25, hydroxy-vitamin D) that regulates calcium
metabolism. Carnivorous and omnivorous reptiles get a high
proportion of their vitamin D3 requirement from their food.
However, plants do not contain D3 (cholecalciferol), instead
they contain D2 (ergocalciferol), which is far less efficient
in calcium metabolism than D3. Herbivorous reptiles are
therefore far more dependent upon the quantity and quality
of artificial lighting than carnivorous specimens. If inadequate
vitamin D3 is available, the animal will rapidly develop
the condition known as Metabolic Bone Disease. In this condition,
bone density suffers and various other serious metabolic
problems occur. Symptoms include swelling, lethargy, general
weakness, tremors and softening of the shell in turtles
and tortoises. Next to a UVB light source, adequate levels
of calcium must be present in the diet or must be provided
by means of dietary supplementation. Juvenile reptiles are
most at risk, although adults too can be affected if maintained
in a state of deficiency for a long enough period. Egg laying
females are also at great risk, due to the extra demands
in calcium necessary for egg production.
Infrared
Light
The exothermic
nature of reptiles (being cold-blooded) emphasizes the importance
of infrared radiation (heat) for thermoregulation. The infrared
segment of the electromagnetic spectrum occurs just below
or “infra” to red light and is not visible. It can, however,
be perceived as heat by the skin. The sun produces most
of its energy output in the infrared segment of the spectrum.
The best artificial
source of heat for diurnal reptiles is through an overhead
radiant source by means of incandescent light bulbs; all
emitting high amounts of infrared light (+700 nm).
The Intensity
The earth’s
climate is determined by the amount of solar radiation that
strikes the surface. Factors like the sun’s position, the
earth’s rotation, geographic location, the ozone layer,
clouds, air-humidity, elevation, environment, etc. influence
the intensity of light. Also within the microhabitat the
light intensity of both visual and non-visual light varies,
depending on the density of the vegetation or geological
conditions. The amount of light falling on a surface is
known as the illuminance and is measured in lumens per square
meter or lux. The illuminance of direct sunlight is approximately
100,000 lux, but normal daylight, which is filtered through
a cloudy sky, is between 5,000 and 10,000 lux, while moonlight
can be as little as 0.25 lux. Ultraviolet radiation is expressed
in microwatt per square centimetre (mW/cm2) and varies tremendously
from the poles (low) towards the equator (high). The amount
of UVB radiation received on the equator on a clear day
at noon lies around 270 mW/cm2. However, this high amount
of radiation decreases as the day passes, in the same way
that it had increased since sunrise, and taking into account
that not all days are clear. In the wild, basking activities
of most reptiles are limited to early morning and late afternoon.
The rest of the day is spent in the shade, either in burrows,
crevices or other shaded places or at various places in
leafy bushes, shrubs or trees. In tropical forests, home
to many types of reptiles and amphibians, only a little
direct sun penetrates the forest canopy and underlying layers
to reach the ground.
The UV radiation
and light levels to which the reptiles are exposed can vary,
depending on a variety of factors:
Habitat
Forest and
shrub areas provide more shade than plains and deserts.
Dense forests have many gradients of UV radiation, with
high
levels in the forest canopy to very low UV-levels on the
forest floor. Grasslands and savannahs provide the same
gradients for smaller species, whereas larger species are
more exposed. In deserts there is less protection from direct
sunlight, and UV levels can even be amplified by reflection.
Some mountainous regions have valleys,
meaning that sunlight may only penetrate the habitat several
hours after sunrise, considerably reducing the length of
exposure to UV rays.
Activity
Patterns
Diurnal (active
during daytime) animals receive higher levels of UV than
nocturnal species for obvious reasons. But even diurnal
reptiles do not spend all day in direct sunlight. Many species
seek cover during the hottest time of the day to avoid overheating.
Their basking periods are limited to morning hours and late
afternoon. These activity cycles may change in reptiles
from seasonal regions. Some nocturnal animals are exposed
to UV radiation as their resting location receives sunlight
and some even come out of their hiding spots to bask in
the sun for thermoregulation purposes.
Time of
Day
The sun is
at its highest in the sky around noon. At this time, the
sun’s rays have the least distance to travel through the
atmosphere and UVB levels are at their highest. In the early
morning and late afternoon, the sun’s rays pass through
the atmosphere at an angle and their intensity is greatly
reduced.
Time of
Year
The sun’s
angle varies with the seasons, causing the intensity of
UV rays to change. UV intensity tends to be highest during
the summer months. In the Northern Hemisphere, the sun shines
directly overhead at noon at the Tropic of Cancer on the
first day of summer, at the equator on the first day of
spring and autumn, and directly overhead at the Tropic of
Capricorn on the first day of the winter.
Latitude
The sun’s
rays are strongest at the equator, where the sun is most
directly overhead and UV rays must travel the least distance
through the atmosphere. Also the ozone is naturally thinner
in the tropics compared to mid- and high-latitudes, so there
is less ozone to absorb the UV radiation as it passes through
the atmosphere. At higher latitudes the sun is lower in
the sky, so UV rays must travel a greater distance through
ozone-rich portions of the atmosphere and, in turn, expose
those latitudes to less UV radiation.
Altitude
UV intensity
increases with altitude because there is less atmosphere
to absorb the sun rays.
Weather
Conditions
Clouds play
a big role in the amount of UV radiation reaching the ground.
On a cloudy day, depending on the shape and thickness of
the clouds, they can absorb and reflect 35-85% of the sun’s
radiant energy, and along with the other effects prevent
all but a negligible amount of radiation from reaching the
ground. Many reptiles seek the security of their burrows
or hiding places during rain, stormy and overcast conditions.
Reflection
Some surfaces,
such as sand (12%), grass (10%) or water (5%) can reflect
much of the UV radiation that reaches them. Because of this
reflection, UV intensity can be deceptively high even in
shaded areas.
Ozone
The ozone
layer absorbs some of the UV radiation that would otherwise
reach the earth’s surface. Ozone levels vary over the year
(even throughout the day) and from one geographical location
to another.
Incandescent
Light Bulbs
Incandescent
lamps are the most common sources of terrarium lighting.
Although incandescent bulbs are more suitable as a heat
source than as a visual light source, they are the perfect
form of complementary lighting as all reptiles need a form
of heat
radiation. In some cases, incandescent lamps are sufficient
as some terrarium animals do not need excessive visual light
based on their behaviour, for example night active reptiles,
arachnids or some amphibians. Some snakes will do well when
only these types of lamps are used, as they do not need
ultraviolet radiation. Incandescent bulbs fail to produce
UVB rays.
Fluorescent
Light Bulbs
The most important
feature of a fluorescent lamp is the ability to emit sufficient
ultraviolet B (UVB) light, a component of sunlight, whereas
an incandescent lamp only emits very little amounts of UVA-light.
It is impossible to accommodate a high visible light emission
with a high ultraviolet (UV) output. The more visible light
emitted, the less UV-radiation and vice versa. Other factors
to consider: not all reptiles or terrarium animals need
the same amount of UVB-radiation, nocturnal versus daylight
activity, geographical and climatological conditions (ex:
rain forests versus deserts).
There are
four important features that a fluorescent terrarium lamp
must possess:
-
UVB output
- critical to vitamin D3 synthesis and calcium metabolism
-
UVA output
- many reptiles are able to see into the UVA range (320-400
nm), and this is likely to have a profound effect upon
behaviour, and specifically, how they visualize food
items.
-
Correct
Colour temperature - nothing to do with heat, but rather
the colour from 'warm' red to 'cold' blue expressed
in degrees Kelvin. Daylight at noon is typically estimated
at 5,500 K. In the tropics the colour temperature can
reach 6,500 K.
-
High Colour
Rendering Index - Colour rendering is the degree to
which a light source shows the true colours of the objects
it illuminates. This is measured on a colour rendering
index, rated from 0-100. A normal fluorescent lamp,
for example, rates 54 on the CRI scale. High quality
fluorescent lamps designed for reptile use will rate
90-98 on the same scale. Colour rendering is very important
as many reptiles rely upon colour signals for reproductive
and feeding purposes.
The combination
of sufficient UVA content and a 'natural' >5,500°K colour
temperature is what improves activity patterns and feeding
when high quality full spectrum lighting is utilized in
terrariums. In addition to the quality of the lamp, its
proximity to the animal, its output intensity and duration
of use are also critical. The illumination intensity of
tubes is primarily dependent upon their size. A 24" (60
cm) tube produces less than half the light output of a 48"
(120 cm) tube. Do not expect to be able to provide adequate
levels of lighting in large terrariums using a single small
tube.
When installing
full spectrum or UVB producing tubes, it is absolutely critical
that nothing is placed between the envelope of the tube
and the recipient animal. UVB is greatly attenuated by glass,
plastic and ultra fine mesh. A normal mesh allows the highest
transmission, but the UVB rays are still reduced to about
90% of their normal power. The amount of UVB received also
diminishes with distance. It is generally recommended that
any Repti Glo UVB tubes be no further than 12" (30 cm) away
from the subject. At greater distances than this, the amount
of UVB actually received will be minimal. For reptiles with
very high UVB requirements, such as desert species, tubes
should be placed as close as 8-10" (20-25 cm) above the
basking site. Tubes also have a limited life and require
changing at least every 12 months in order to guarantee
continued UVB output. Although there may be no visible deterioration
in the performance of the tube, the invisible UV content
decays as the tube ages. It is a good idea to place a small
adhesive label near each fitting with the date the tube
was last changed clearly marked. Most fluorescent tubes
designed for reptile use are classified according to their
percentage UVB output. The most popular tubes offer 5% UVB
(Repti Glo 5.0). In the vast majority of cases the 5% tubes
are perfectly adequate, provided they are correctly sited,
changed regularly, and the number of hours of exposure is
sufficient. 10-12 Hours daily has proven a satisfactory
level of exposure for most species. Only animals living
in deserts (areas with high UVB levels) should be exposed
to 8% UVB bulbs (Repti Glo 8.0). Normal fluorescent or high
visible light emitting tubes (Repti Glo 2.0) emit very little
UVB light, in most cases not sufficient for vitamin D3 synthesis.
The higher the UV output (invisible light) the less light
(visual) is emitted. The light also gets a bluer appearance.
Therefore it is recommended to combine a tube with a high
UV output (Repti Glo 5.0 and 8.0) with a tube with a very
high visual light output (Repti Glo 2.0) for the best results.
Note: Fluorescent
tubes do not provide sufficient heat. A separate heat source
is required in addition (ex: incandescent basking lamp).
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