International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 5

Number 1

March 2011

1 INTRODUCTION

1.1 Navigational visual condition at sea

The meaning of the navigational visual condition at

sea is the environmental condition by eye-sight at

sea. The visual perception of the target at sea in the

maritime traffic system is herein considered, based

on the following two measurement results:

1 horizontal illuminance inside the navigation

bridge

2 sky luminance of 2 degrees within the horizon

In addition, the visual perception of targets such

as aids to navigation and ships is directly related to

environmental physical characteristics.

1.2 A proper look-out

As you know, Rule 5 of the COLREG’s (Interna-

tional Regulations for Preventing Collisions at Sea,

1972) define “Look-out” as follows: Every vessel

shall at all times maintain a proper lookout by sight

and hearing as well as by all available means appro-

priate in the prevailing circumstances and conditions

so as to make a full appraisal of the situation and of

the risk of collision.

1.3 Marine accidents caused by the improper look-

out

An improper look-out is often pointed out as a cause

of marine accidents. Fig. 1 shows the annual change

of the total number of marine accidents and the ratio

of improper look-out, based on data collected by the

MAIA (Marine Accident Inquiry Agency) in Japan,

years 2000 ~ 2008.

Figure 1. Marine accident caused by improper look-out

Visual Condition at Sea for the Safety

Navigation

M. Furusho

Kobe University, Graduate School of Maritime Sciences, Kobe, Japan

K. Kawamoto

Kawasaki University of Medical Welfare, Okayama, Japan

Y. Yano & K. Sakamoto

Kobe University, Graduate School of Maritime Sciences, Kobe, Japan

ABSTRACT: To the navigation officers of the watch (OOW’s) on the navigation bridge the environmental

condition of visual acuity is the most important factor for keeping a proper look-out, as regulated by the IMO

COLREG’s (Rule 5). Navigation officers are required to keep a proper look-out for prevention of (ships) col-

lisions. There are many collision incidents at sea, and especially so under conditions of good visibility. This

paper has two topics related to the illuminance and luminance from sunlight, as follows:

The first topic is the introductory explanation of the illuminance inside the navigation bridge.

The second topic is the sky luminance condition as seen by the OOW from the navigation bridge.

These two topics are fundamental factors related to visual perception at sea. The OOW on the bridge has to

understand that every care must be taken, especially in fine weather conditions.

2 OBSERVATION

2.1 Horizontal illuminance in the navigation bridge

The Captain, navigation officers, look-outs and

helmsmen (hereafter termed “navigators”) on watch

on the navigation bridge are strongly affected by the

effect of natural lighting such as sunlight. The illu-

minance outdoors by day and by night has a wide

variance between 100,000 lx from direct sunlight in

fine weather and by 0.2 lx from the light of a full

moon.

The illuminance meter (model IM-3 made by

TOPCON Co. Ltd.) as a measurement device con-

nected with a recording printer was used as indicated

in Photo 1. There was no additional condition which

had a restriction, such as alteration of the course,

speed or others, during this measurement of illumi-

nance on the navigation bridge.

Photo 1, Measurement device

Table 1. Specification of the cooperative ships

___________________________________________________

Month Ship Purpose G.T. L.O.A. Speed H.E.

Tons m knots m

___________________________________________________

March C T 449 49.95 13.5 7

May A CF 3,611 114.50 19.5 14

June B CF 19,796 192.90 21.8 22

July C T 449 49.95 13.5 7

Sept D CF 3,597 196.00 25.0 23

___________________________________________________

Remarks

1) T: Training ship, CF:Car Ferry, H.E.:Height of Eye

2) Observing area was around Japan. Latitude: 35degrees N.

2.2 Sky luminance in 2 degrees from the horizon

One of the conditions to recognise a target, such as a

ship or aid to navigation, is the background lumi-

nance of the object. It is necessary for visible per-

ception that the difference of the luminance between

the background and the target should be more than

the value of the luminance difference threshold.

The luminance difference threshold means the

threshold limit value of the brightness, based on the

experimental studies by Blackwell, H.R., in 1946

and Narisade, K. et al, in 1977 and so on.

About a background when we look at a target, it

can be judged from the navigators’ characteristics of

eye movement at sea. This background is both the

sky luminance of 2 degrees above the horizon and

sea surface luminance of 2 degrees below the hori-

zon, but in this paper the sky luminance is taken into

consideration.

The measurement was carried out on board ship

C. The luminance meters (TOPCON’s meters BM-5,

BM-5A and BM-8: Photo 2) were set and directed

right ahead through the windscreen of the navigation

bridge according to the regular procedure by naviga-

tors. The specification of luminance meter BM-5A is

shown in Table 2. The weather conditions were fine,

or fine and cloudy, with direct sunlight.

Photo 2. Luminance Meters

Table 2. Specification of the luminance meter(BM-5A)

___________________________________________________

Optical System Diameter 32 mm, F=2.5

Measurement Angle 0.1, 0.2, 1, 2 degrees

Photo acceptance Unit Electronic Light Amplifier

Wave Length 380

-780 n.m.

Range 0.0001～1200000cd/m2

Distance 520mm～∞

Sampling Time 2 sec

Size & Weight 355(L) X 130(W) X 169(H) mm ,

4 Kg

___________________________________________________

3 RESULTS

3.1 Horizontal illuminance on the navigation

bridge

The result of the measurement data of the horizontal

illuminance on the navigation bridge using the illu-

minance meter connected with a printer, as indicated

in Photo 1, is shown in Fig. 2. The vertical scale in

Fig. 2 shows illuminance in lux in logarithmic scale;

the horizontal scale shows Japan Standard Time

(JST) which has 9 hours difference from Greenwich

Mean Time (GMT). These data have dispersion

which depends on the different observing times in a

month.

Figure 2. Horizontal illuminance in the navigation bridge

3.2 Sky luminance of 2 degrees above the horizon

The sky luminance, dependent on the relative direc-

tion towards the solar direction in the open sea, was

measured. The result of these observations is shown

in Table 3. These experimental observations were

carried out in July during a research voyage of the

training ship.

Table 3 Sky luminance of the relative direction towards the so-

lar direction at open sea

___________________________________________________

Relative Angle Max.:A Min.:B Range:A-B

degrees cd／m

cd／m

___________________________________________________

0 27,420 7,070 20,350

45 20,900 6,610 14,290

90 6,627 4,723 1,904

135 5,500 3,960 1,540

180 6,165 4,166 1,999

-135 6,095 3,910 2,185

-90 6,170 4,249 1,921

-45 12,190 6,340 5,850

___________________________________________________

Remarks: Relative angle 0 degree means the solar direction.

“-(negative number)” means the left side of solar direction

The sky luminance, dependent on the relative di-

rection towards the solar direction at the time of

relative angle around 0 (zero) degrees, changes from

7,000 to 27,000 cd/m

. The range, which means the

difference of luminance between the maximum val-

ue at A and the minimum value at B, was found to

be approximately 20,000 cd/m

The opposite side, in the case where the relative

angle is -90 ~ +90 degrees of solar direction, pro-

duced a small change at 1/10th of the range.

Figure 3. Relationship between the sky luminance and the solar

altitude in degrees.

4 CONSIDERATION

4.1 Horizontal illuminance by the standardization

with using solar altitude

Fig. 2 shows the seasonal difference for times of

sunrise and sunset; also, the difference in hours of

morning or evening twilight at differing sea areas.

There is no affect by the different heights of eye

on the horizontal illuminance on the navigation

bridge. The illuminance change in daytime has a

wide value between 1,000 lx and 10,000 lx, but at

morning or evening twilight the illuminance changes

rapidly with time. This is a remarkable feature of

twilight – at this time there are functional changes of

both the cone and the rod of the visual cell.

The horizontal illuminance nearby the windshield

on the navigation bridge has various changes be-

tween 0.01 lx and 10,000 lx, according to the voyage

situation such as seagoing area, navigation time,

ship’s course and so on.

Because the illuminance change has seasonal

characteristics according to the times of sunrise,

sunset and the hours of twilight, standardisation of

illuminance based on the observation time might be

difficult. Therefore, the solar altitude is useful for

the standardisation – by using calculated solar alti-

tudes based on the observing time and geographical

position.

Fig.4 shows the results of the standardisation by

using solar altitude. This demonstrates that the solar

altitude is a suitable factor for explaining the change

of horizontal illuminance on the navigation bridge.

Figure 4 Horizontal illuminance on the navigation bridge by

the standardisation method using solar altitudes

4.2 Sky luminance on shore

For the purpose of comparing the luminance at sea

and ashore, the example of illuminance measure-

ment at Fukui prefecture in Japan is taken into ac-

count (Ref. Lighting Handbook published by the Il-

luminating Engineering Institute of Japan)

Fig.5 shows this example which had no observa-

tion data under 5 degrees on shore. In this figure, the

line on the celestial sphere via the sun and the (ob-

server’s) zenith is characterised as bilaterally sym-

metric, the maximum point being marked as “ X ”

near the sun and the minimum point, 90 degrees dis-

tant via the zenith, as“ ● ”.

4.3 Sky luminance at sea in fine weather

The sky luminance in the relative solar direction is

shown in Fig.6 according to the solar altitude. The

numbers indicated around this radar charted figure

show the relative angle in degrees and the numbers

indicated on the radial axes show the sky luminance

in cd/m

The distribution of sky luminance to each relative

solar direction in fine weather (with sunlight) gradu-

ally becomes concentric circles. When the solar alti-

tude is more than 60 degrees the variability of the

luminance is small, so the background condition of

the visual perception is uniform.

Comparing with the case on shore (Fig. 5), it is

understandable that items have similar conditions, as

follows:

1 There is a maximum point of the sky luminance

relative to the solar direction.

2 There is a minimum point in the opposite direc-

tion to the maximum point (when the relative so-

lar direction is near 180 degrees).

3 The line on the celestial sphere via the sun and

the zenith is characterised as being bilaterally

symmetric.

4 The distribution of the sky luminance has uni-

formity with no relationship to the relative solar

direction, nor to the solar altitude.

These items are the evidence to support the navi-

gators’ statements which explain that it is easy to

recognise targets visually when “the sun is behind

me, not in front of me”.

Figure 5 Example of the sky luminance on shore

Figure 6 Sky luminance in the relative solar direction

4.4 Sky luminance of the solar direction

The approximate curve as shown in Fig.3 is taken

into consideration. The fractional approximate curve

of the sky luminance of the solar direction related to

solar altitude can be explained by the formula as

shown hereunder (1).

Y = a/X + B (1)

where, Y=sky luminance in cd/m

; X=solar altitude

in degrees; a=105 (coefficient); b= -5,000 (constant).

According to Fig.4, this formula can be applied

when the solar altitude is more than 10 degrees.

4.5 Relationship between the solar altitude and

ship’s collision

Figure 7 Impact by the Solar altitude and the relative direction

on the ships’ collision

According to the result of analysing 1000 cas-

es((a)244, (b)455, (c)301) of ships’ collisions, Fig. 7

can be obtained as an impact by the solar altitude

and the relative direction toward the solar direction.

The solar direction in the case of less than 40 de-

grees of the solar altitude has a direct effect to the

ships’ collision.

The OOW should remind not only the solar direc-

tion but also fine weather. Generally speaking, we

believe that the fine weather is good weather, but

fine weather might have the blind spot so-called

“white hall” which means the restricted visual condi-

tion for the proper look-out by the OOW at sea.

5 CONCLUSIONS

The navigational visual, environmental background

condition of recognising targets (so-called ‘visual

perception’) by sight at sea has been taken into con-

sideration.

There are 4 conditions which should be consid-

ered, as shown hereunder, in order to discuss the

visual perception:

1 Luminance of a target

2 Luminance of the background

3 Adaptation condition of the (observer’s) retina

4 Equivalent Veiling Luminance from the near vis-

ual field

In this study the authors have focused on the

above item 2) – luminance of the background – be-

cause of the necessity for the field of study on board

ship.

5.1 Illuminance on the navigation bridge

The horizontal illuminance on the navigation bridge

can be standardised based on the solar altitude, as

shown below in table 4:

We can understand the illuminance condition of

foreside on the navigation bridge. The start and end

of navigational twilight is -9(minus nine) degrees.

Table 4 Horizontal illuminance on the navigation bridge

___________________________________________________

Solar altitude Horizontal illuminance

degrees lx

___________________________________________________

over 10 1000 ~10,000

10 1000

0 100

-3 10

-6 1

-9 0.01 ~ 0.1 *

-18 Start and end of astronomical twilight

___________________________________________________

Note * The horizon cannot be seen except in the solar direction

at the start and end of navigational twilight.

5.2 Luminance condition for the OOW

The sky luminance at 2 degrees above (and below)

the horizon is one of the most important composition

factors of visual perception at sea.

The authors obtained the remarkable features on

the sky luminance based mainly on the experimental

observations on board, and these are shown as fol-

lows:

1 The value of the sky luminance at 2 degrees

above the horizon is bigger than the sea surface

luminance, except in cases of sun-glitter on the

sea surface

2 The sky luminance of the solar direction has vari-

ous changes between approx. 7,000 and 27,000

cd/m

;

3 The sky luminance of the solar direction related

to the solar altitude can be explained when the so-

lar altitude is more than 10 degrees by this for-

mula:

Y = a/X + B

where, Y=sky luminance in cd/m2; X=solar alti-

tude in degrees; a=105 (coefficient); b= -5,000

(constant).

4 The distribution of sky luminance to each relative

solar direction gradually forms concentric circles.

When the solar altitude is more than 60 degrees

the variability of the luminance distribution is

small and the background condition of the visual

perception can be said to be uniform

5 The distribution of the sky luminance on the op-

posite side of the solar direction has uniformity,

with no relationship to the relative solar direction

nor to the solar altitude. This is the evidence to

support navigators’ statements which say that it is

“easy to recognise targets when the sun is behind

the observer”.

6 The OOW should think of the “white hall” which

means gimmick of the proper look-out especially

in fine weather.

ACKNOWLEDGMENT

This work was supported by the Grant-in-Aid for

Scientific Research (B) (KAKENHI) No21300211.

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