August 2010

Teachers' Guide: RSL Fiber Systems Challenge

SPOILER ALERT!! THIS RESOURCE IS FOR INSTRUCTORS ONLY! IT

DEFEATS THE PURPOSE OF PROBLEM BASED LEARNING IF THIS

INFORMATION IS SHARED WITH STUDENTS BEFORE THEY COMPLETE

THE PBL CHALLENGE!

The challenge: RSL Fiber Systems needs to provide a lighting system for the interior of

a submarine that is energy efficient and takes into account the impact of light on the

human circadian system.

What you need to know to solve the challenge (key concepts):

• When dealing with illumination, lumens (rather than watts) are the quantity of

interest. Illuminance is lumens per square meter (“lux”).

• LEDs can provide high luminous output with low energy consumption and are

available in a variety of formats that allow spectral control.

• Optical fiber can be used to bring light to enclosed spaces from light sources

located at a distance.

• The amount, spectral distribution, timing and duration of light important to

circadian rhythms are very different from light needed for vision.

• Color temperature is often used to refer to the hue of “white” light.

• (Optional calculations) Losses in fiber optic systems are usually given in decibels;

system losses can be used to determine total light throughput.

Submarine lighting

Submarines are totally enclosed environments that rely exclusively on artificial

lighting in work and living spaces. Submariners must remain alert and work effectively

despite a work/sleep schedule that is contrary to the natural twenty-four hour schedule

synchronized to the rhythm of day and night. Submarine watch-standers follow a 6-hour

on/12-hour off schedule for a total 18-hour “day”. During some of the 12-hour “off”

period, maintenance, cleaning, training and other duties occur; the remaining time is used

for sleep. This schedule is a compromise to accommodate 24/7 operations with the

limited crew on board.

The 6/12 submariner’s cycle has been described as instantaneously traveling east

across six time zones every 18 hours.1 In the short term, this can lead to exhaustion,

irritability, and decreased concentration. Reference [1] provides an excellent summary of

the difficulties of maintaining such a schedule. Current research is being undertaken to

determine the optimum operating schedule and how submarine lighting in all work and

rest areas might improve the efficiency of operations and the health of submariners.2

Reference papers [1] and [2] are available in pdf form in the Solution Additional

Resources; there is also a wealth of information on the U.S. Coast Guard web site

http://www.uscg.mil/ on Crew Endurance Management Systems.

August 2010

Circadian rhythms

Circadian rhythms, the 24-hour cycles most organisms are subject to, have been

known for many years. In humans, these include the sleep/wake cycle, hormone

production (e.g., melatonin), and core body temperature. Light is a critical input to the

system, but the exact mechanism was unknown until the early 1990s when scientists

discovered a new type of retinal photosensor, intrinsically photosensitive retinal ganglion

cells or ipRGC. Unlike rods and cones, these cells do not contribute to vision but instead

provide input to the brain to help regulate the body’s response to the 24 hour day/light

cycle. ipRGCs are also involved in other functions such as regulating pupil response to

bright light. While cones have peak sensitivity in the green part of the spectrum, ipRGCs

are most responsive to blue light. Some studies have indicated that light from above the

horizon is more effective at stimulating the circadian system than light from below,

perhaps not surprising since the blue sky is the primary source of daylight. Amazingly,

recent studies have shown that ipRGCs can work effectively even in subjects whose rods

and cones (and thus vision) have been destroyed.

The study of circadian systems is still in its infancy and the effect of circadian

disruption on human health is a topic of current research. The Rensselaer Polytechnic

Institute (RPI) Lighting Resource Center (www.lrc.rpi.edu/programs/lightHealth/) has

links to dozens of papers and research projects investigating “circadian light” and the

health effects of light on humans.

Color Temperature

Color temperature is based on the relationship between the temperature of a

blackbody radiator and the perceived color of the radiation at a given temperature. Color

temperature may be easily demonstrated by observing the change in an electric stove unit

as it heats in a darkened room. A discussion of the concept and an interesting color

temperature applet can be found on the FSU Molecular Expressions web site:

http://micro.magnet.fsu.edu/primer/java/colortemperature/index.html

Figure 1 shows the CIE 1931chromaticity diagram.3 Pure spectral colors are

mapped around the outside of the diagram (indicated by wavelength in nanometers from

400 nm to 700 nm). The various hues that result from combining colors make up the

gamut of human vision, for example, halfway between blue and red is purple The center

of the diagram is white, representing the combination of red, green and blue.

Figure 1. CIE chromaticity diagram showing how color temperature changes for a blackbody

radiator (the curved line with temperatures indicated). From Wikimedia Commons.

August 2010

The curved line shown passing through the white zone (the so-called Planckian

locus) indicates the progression of blackbody temperatures in Kelvins from low (<1500

K) to high (10000K and above) and the colors corresponding to these temperatures. The

colors along this curve represent daylight from morning (low temperature) to mid-day

(high temperatures) to evening (low temperatures). The submarine lighting solution

mimics this color temperature shift, providing simulated natural light in the work space.

RSL Fiber Systems Solution

RSL Fiber Systems has developed a number of optical fiber lighting systems for

the U.S. Navy. The advantage of fiber-delivered light is that the light source may be

located at a distance from where the light is needed, reducing the need for electrical

connections in the work area (Figure 2). Maintenance is also simplified since light from a

single source can be brought to several locations via fiber optic cable. The LED

illuminator light engine designed by RSL Fiber Systems can be used with one or two

fibers and one or two luminaires that direct the light onto the work space. The light

engine output is 110 lumens (lm), and the fiber is extremely low loss, only 0.012 dB per

meter. There is a 5% coupling loss at each end of the fiber where it attaches to the light

engine and luminaire.

Figure 2. RSLF Solution

The U.S. Navy has been requesting that contractors look for energy efficient

solutions wherever possible. LEDs reduce electrical energy usage and have very long

lifetimes. They are also resistant to vibrations and moisture, an important consideration

on submarines. The light output of LEDs has been increasing rapidly in recent years

making them an excellent choice for energy efficient lighting.

LEDs can can also be used to control the color temperature or light, mimicking

daylight from sunrise through midday to sunset. Two types of white LED are currently

available:

RGB multichip emitter – this is a single chip with separately controllable red, green

and blue high power LEDs (Figure 3).

White phosphor LED- this is a blue or ultraviolet LED in an enclosure coated with

phosphors that produce white light (the same method used with a fluorescent light).

The multichip emitter output can be continuously varied as needed by controlling the

RGB content. White phosphor LEDs have a blue tint and the spectrum is fixed (Figure 4).

August 2010

To vary the color temperature with the white phosphor LED, an amber LED is added.

This Challenge solution emphasizes the RGB multichip emitter, but the white and amber

LED solution is also possible.

Figure 3. Multichip emitter. Figure 4. Spectra of white LEDs. Left: Phosphor based.

Right: RGB multichip emitter

The illumination system was designed to mimic daylight by changing the color

temperature from “dawn” (red) to “noon” (blue) to “dusk” (red) over the work shift.

(Figure 5) That is, the color temperature rises from cool (red) to warm (blue) to simulate

sunrise, remains warm (around 6000 K) throughout the six-hour “day,” then returns more

slowly to a cooler red “sunset”.

Figure 5. Shifting color temperature with time of day over a six-hour work shift. Notice that the time

axis is not constant; the rate of change of color temperature is faster at the start of the shift than at

the end.

The lighting system also has the capability of shifting from day to day to help

submariners adapt to their work schedule by altering the start time and duration of the

sleep/wake cycles. Figure 6 illustrates human “energy production” over the course of a

24-hour day reflecting the typical need to expend energy during the day and sleep at

night. The graph reflects a mid-morning energy surge, a mid afternoon “slump” and

increase in energy in the early evening. The dip at night is termed the “red zone”, when

6000

5000

4000

3000

2000

00:00 00:20 00:40 01:00 02:00 03:00 04:00 05:00 05:20 05:40 06:00

Co

lor

tem

p,

K

Time

August 2010

energy production is lowest. Figure 7 illustrates a shift of red zone that can be

implemented by lighting coupled with social (meal time, bed time, etc) changes.

Figure 6. Energy production over a 24 hour day showing the red zone. From Crew Endurance

Management System (CEMS) Awareness Workbook, prepared by QSE Solutions for Pacific Marine

Towing Industry Partners.

Figure 7. Shifting the red zone to accommodate varying work-shift hours

Future lighting systems are being designed with red, green and blue lasers

supplying the illumination. Although more difficult to control electronically, lasers

produce much more light than LEDs so that fewer units are required for the same amount

of light (Figure 8).

Figure 8. Red, green and blue lasers (Left) combine to make white light (Right).

August 2010

MIL-SPEC Requirement (Loss Calculations)

In fiber optic systems, loss is usually expressed in decibels (dB). In this Challenge

we use both dB and percent loss. Using dB allows gain and loss in the system to be

calculated by addition and subtraction rather than multiplication as long as the optical

powers are expressed in dB units as well.

dB and dBm

If the input power (P1) and output power (P2) are known, the loss (or gain) in dB is

calculated from

dB = 10logP

2

P1

!

"#$

%&

For example, if a system suffers a 50% loss so that P2/P1 = 0.50, then

dB = 10log 0.50( ) = 10(!0.3) = !3

That is, a loss of 3 dB corresponds to a loss of one half the input power. The negative

sign is often not included but rather implied by the word “loss”.

Calculations are simplified by expressing the output of light sources in dBm where

the subscript “m” indicates “referenced to one milliwatt”. That is,

dBm= 10log

P (in mW)

1 mW

!"#

$%&

A light source that emits 20 mW can also be described as a 13 dBm source:

dBm= 10log

20 mW

1 mW

!"#

$%&= 10log 20( ) = 13 dB

m

With source strength expressed in dBm and loss (or gain) in dB, calculating system

output power becomes a matter of addition and subtraction, since

log(AB) = log A + logB

For example, suppose a source emits 20 mW and undergoes the losses and gains shown

in Figure 9. Corresponding power and dB measurements are shown in each block. Using

power units and percent loss and gain:

P

2= 20 mW 0.50( ) 1.50( ) 0.50( ) = 7.5 mW

In dB and dBm:

P

2= 13 dB

m! 3 +1.76 ! 3 = 8.76 dB

m

The two answers are equivalent, but one is obtained by addition and subtraction, which is

usually easier to perform especially without a calculator. This is a good application to

answer the question “What are logs good for?”

Figure 9. dB calculation example

20 mW

P1

13 dBm

50%

loss

-3 dB

150%

gain

+1.76 dB

50%

loss

-3 dB

P2 = ?

August 2010

Calculations for the RSL Fiber Systems Challenge

According to the Problem Statement for this Challenge, The MIL-SPEC for lighting

in this work area specifies 14-15 foot-candles. (Note that the SI unit for illuminance is

lumens/m2 but foot-candles are still occasionally used.) To determine the light level

delivered to the work-station, it is necessary to determine the loss in the cable and at the

couplings to the light engine and luminaire. The fiber loss is given as 0.012 dB/m. The

distance from source to luminaire is unknown, but assuming it is no more than ¼ the

length of the submarine this gives a loss of

Loss =1

4i115 m

!"#

$%&

0.012dB

m

!"#

$%&= 0.345 dB

That is, the initial source luminous power suffers a 0.345 dB loss due to the fiber. The

total submarine length (115 m) is taken from typical values found on the Internet.

Since other losses are given in percent and we want the final result in lumens, we

can convert this dB loss to a percent:

dB = 10 log (P

2

P1

),

P2

P1

= 10dB

10

P2

P1

= 10!0.345

10 = 0.92

That is, 92% of the entering light is transmitted by the fiber. We are also given that 5% of

the light is lost at each end of the fiber where it couples to the light engine and the

luminaire. The light available from a single light engine coupled to a single fiber is then:

Luminous Power = 110 lm 0.95( ) 0.92( ) 0.95( ) = 91 lm

We can now find the area that can be effectively illuminated by 91 lm and compare it to

the size of the workstation. Since 1 foot-candle = 10.764 lux (where 1 lux = 1 lm/m2), to

satisfy the MIL-SPEC we need approximately

14 foot-candles x (10.764 lux) ~ 151 lux = 151 lm/m2.

We can determine the area that can be illuminated to the MIL-SPEC requirement by:

151 lux =91 lm

A

A = 0.6 m2

Using one fiber and luminaire per light engine provides sufficient illumination over

0.6 m2, which is about the area of one of the work-stations.

Explorations/Applications

Ergonomic lighting has applications beyond submarines. It can be applied to shift

workers, people who live at extreme latitudes where there is little sunlight in the winter,

and even students who may travel early in the morning from home to their artificially

lighted classrooms without ever seeing the blue sky. There is much current research on

lighting, circadian rhythms and human health and disease and the RPI Lighting Center is

a good place to start.

Separate red, green and blue LEDs are reasonably priced and students might be

interested in trying to create light of varying color temperature by combining them so that

August 2010

individual brightness may be controlled. Light from red, blue, and green LEDs can be

easily coupled into inexpensive plastic optical fiber and the fibers combined to produce

white light. Measuring LED spectral output is usually done with fairly expensive and

sophisticated instruments such as integrating spheres, but an inexpensive light-integrating

device may be constructed from an inexpensive Styrofoam cooler. The light sources or

fiber outputs, are placed in the bottom at the center of the cooler, and a small observation

hole cut in one side near the top so that the observer (or instrument) does not have a

direct view of the source.

References

[1] Loring J. Crepeau, John D. Bullough, Mariana G. Figueiro, Steven Porter, and Mark

S. Rea, “Lighting as a circadian rhythm-entraining and alertness-enhancing stimulus in the

submarine environment”, Presented at the 2006 Undersea Human Systems Integration

Symposium, Mystic, CT.

[2] C. A. Duplessis, J.C. Miller, L. J. Crepeau, C. M. Osborn, J. Dyche, “Submarine

watch schedules: Underway evaluation of rotating (contemporary) and compressed

(alternative) schedules”, Undersea & Hyperbaric Medicine 2007 Jan-Feb;34(1):21-33.

[3] http://en.wikipedia.org/wiki/CIE_1931_color_space