LIGHT THEORY:The Spectrum, Human Eye Response, Silicon Detector Response

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Chapter One

LIGHT THEORY

The Spectrum, Human Eye Response

Light is a form of energy. Virtually all the energy you use on a daily basis began as sunlight energy

striking the earth. Plants capture and store some the sun's energy and convert it into chemical

energy. Later, you use that energy as food or fuel. The rest of the sun's energy heats the earth's

surface, air and oceans.

White light disperses

color spectrum through a prism

Figure 1a

With the aid of a glass prism you can

demonstrate that the white light coming

from the sun is actually made up of many

different colors as shown in Figure 1a.

Some of the light falls into the visible

portion

the

spectrum

while

wavelengths, such as the infrared and

ultraviolet rays, remain invisible. The

human eye responds to light according to

the curve shown on Figure 1b. The

spectrum that lies just outside the human

eye red sensitivity limit is called "near

infrared" or simply IR. It is this portion of

the spectrum that is used by much of

today's

light-beam

communications

Figure 1b

systems.

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

As can be seen from Figure 1a, sunlight is a very powerful source for this band of light, so are

standard incandescent lamps and light from camera photoflash sources. However, many other man-

made light emitters, such as fluorescent lamps and the yellow or blue/white street lamps, emit very

little infrared light.

Silicon Detector Response

Just as our eyes are more sensitive to

certain wavelengths so are some electronic

light detectors. As shown in Figure 1c a

typical silicon light detector has a response

curve that ranges from the longer mid-

infrared wavelengths, through the visible

portion of the spectrum and into the

shorter and also invisible ultraviolet

wavelengths. The most notable feature of

the silicon detector's curve is its peak

sensitivity at about 900 nanometers. Also

note that at 600 nanometers, visible red,

the silicon detector response is about one

half that of its peak. It should therefore be

clear that any light source with a 900

Figure 1c

nanometer wavelength would have the

best chance of being detected by the silicon detector. Fortunately, as we shall see in the section on

light emitters, many of today's infrared light emitting diodes (LEDs) do indeed emit light at or near

this 900nm peak.

Units of Light

As shown in Figure 1d a standard

tungsten incandescent light bulb emits a

very broad spectrum of light. If you took

all

the

light

wavelengths

into

consideration, including all those that were

invisible to the human eye, the light bulb's

electrical power to light power conversion

efficiency

would

approach

100%.

However, much of the light emitted from

such a source takes the form of long

infrared heat wavelengths. Although still

considered light, heat wavelengths fall

well outside the response curve of both our

human eye and a silicon detector. If you

only considered the visible portion of the

Figure 1d

spectrum, the light bulb's efficiency would

only be about 10%. But, to a detector that was sensitive to heat wavelengths, the bulb's efficiency

would appear to be closer to 90%. This takes us to one of the most confusing areas of science. How

do you define the brightness or intensity of a light source?

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

It isn't enough to say that a standard 100 watt bulb emits more light than a tiny 1 watt bulb. Sure, if

you set a big 100 watt bulb next to a small 1 watt flashlight bulb, the 100 watt bulb would appear to

emit more light. But there are many factors to consider when defining the brightness of a light

source. Some factors refer to the nature of the emitted light and others to the nature of the detector

being used to measure the light.

For some light emitting devices, such as a standard tungsten incandescent light bulb, the light is

projected outward in all directions (omni-directional). When visually compared to a bare 1 watt

bulb, the light emitted from a bare 100 watt bulb would always appear brighter. However, if you

were to position the tiny 1 watt bulb in front of a mirror, like a flashlight reflector, the light

emerging from the 1 watt light assembly would appear much brighter than the bare 100 watt, if

viewed at a distance of perhaps 100 feet. So, the way the light is projected outward from the source

can influence the apparent brightness of the source. An extreme example of a highly directional

light source is a laser. Some lasers, including many common visible red laser pointers, are so

directional that the light beams launched spread out very little. The bright spot of light emitted

might remain small even after traveling several hundred feet.

The preferential treatment that a detector gives to some light wavelengths, over others, can also

make some sources appear to be brighter than others. As an example, suppose you used a silicon

light detector and compared the light from a 100 watt black-light lamp that emits invisible

ultraviolet light, with a 100 watt tungsten bulb. At a distance of a few feet, the silicon detector

would indicate a sizable amount of light being emitted from the light bulb but would detect very

little from the black-light source, even though the ultraviolet light could cause skin burns within

minutes. So which is brighter?

In order to define how much light a source emits you first need to specify what wavelengths you

wish to be considered. You must also assign a certain value to each of the considered wavelengths,

based on the detector being used. In addition, since many light sources launch light in all directions

you must also define the geometry of how the light is to be measured. Perhaps you only want to

consider the amount of light that can be detected at some distance away. The wavelengths you may

want to consider will depend on the instrument used to make the measurements. If the instrument is

the human eye then you need to consider the visible wavelengths and you will need to weigh each

of the wavelengths according to the human eye sensitivity curve. If the instrument were a silicon

detector, then you would use its response curve.

When doing research on light, you will come across many different units being used by various

light manufacturers. All the units are trying to describe how much light their devices emit. You will

see units such as candle power, foot candles, candelas, foot lamberts, lux, lumens and my favorite:

watts per steradian. Some units refer to the energy of the light source and others to the power. Many

units take only the human eye sensitivity into account. The light units can be even more confusing

when you consider that some light sources, such as a common light bulb, launch light in all

directions while others, such as a laser, concentrate the light into narrow beams. Rather than

confuse you even more by going into a long discussion of what the various units mean, I'm going to

try to simplify the problem. Let's just assume that each light source has a distinctive emission

spectrum and a certain emission geometry. You will have to treat each light source differently,

according to how it is used with a specific communications system.

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

In optical communications you only need to consider the light that is sent in the direction of the

detector. You also only need to consider the light that falls within the response curve of the detector

you use. You should regard all the rest of the light as lost and useless. Since all the light sources

discussed in this book rely on electricity to produce light, each source will have an approximate

electrical power (watts) to optical power (watts) conversion efficiency, as seen by a silicon detector.

You can use the approximate power efficiency and the known geometry of the emitted light to

calculate how much light will be emitted, sent in the direction of the light detector and actually

collected. Various sections of this book will give you some examples of such calculations.

Light Power and Intensity

The scientific unit for power is the "watt". Since the intensity of a light source can also be described

as light power, the watt is perhaps the best unit to use to define light intensity. However, power

should not be confused with energy. Energy, is defined as power multiplied by time. The longer a

light source remains turned on, the more energy it transmits. But, all of the light detectors discussed

in this book are energy independent. They convert light power into electrical power in much the

same way as a light source might convert electrical power into light power. The conversion is

independent of time. This is a very important concept and is paramount to some of the circuits used

for communications. To help illustrate how this effects light detection, imagine two light sources.

Let us say that one source emits one watt of light for one second while the other launches a million

watts for only one millionth of a second. In both cases the same amount of light energy is launched.

However, because light detectors are sensitive to light power, the shorter light pulse will appear to

be one million times brighter and will therefore be easier to detect. This peak power sensitivity

concept of light processing is a very important concept and is often neglected in many optical

communications systems published in various magazines.

Miscellaneous Stuff

Independent on how long the light remains on. The watt is more convenient to use since light

detectors, used to convert the light energy into electrical energy, produce an electrical current

proportional to the light power, not its energy. Detectors often have conversion factors listed in

amps per watt of light shining on the detector. Remember, energy is power multiplied by time.

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

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