MARCH, 1940

Submarine Illumination in Photometric Units

C. L. UTTERBACK AND RAYMOND E. WILSON

Physics and Oceanographic Laboratories, Seattle, Washington(Received January 8, 1940)

The paper describes a method of obtaining underwater illumination in photometric units.The results are given as total horizontal illumination at different depths and as cell currentextinction coefficients of two bands in the visible spectrum. The effect of temperature on thesubmerged photometer is indicated, and a mean temperature coefficient between 0C and 17'Cis calculated.

INTRODUCTION

DURING the last few years the interest insubmarine light measurements to biologists

and physical oceanographers has resulted inobservations in the relative intensities of trans-mitted and scattered light beneath the surface ofnatural water bodies. The most satisfactorymethod at present for the measurement ofsubmarine daylight seems to be through the useof barrier layer type of light-sensitive cells. Thesecells are sensitive over a range of wave-lengthsextending beyond the short wave-length limitand beyond the long wave-length limit of thevisible spectrum. The current produced by thesecells is not an index of visual illumination sincethe maximum response of various cells liesbetween 520 and 580 millimicrons and since theirrelative sensitivities do not coincide with therelative sensitivity of the eye. Usually they havea relative response greater than that of the eyetoward each end of the spectrum. Also the highabsorption of the long and short wave-lengths of

FIG. 1. Region in which illumination measurementswere made.

visible light by water causes the spectral distri-bution of the light transmitted to lower layers tovary with depth. The spectral distribution alsovaries with the nature and amount of mattersuspended in the water. Current output, meas-ured with or without a color filter over the light-sensitive cell, permits the evaluation of "extinc-tion coefficients" which give some informationconcerning the content of suspended matter insubsurface layers. The extinction coefficient hasbeen defined by k in the equation

I = Ioe-kx,

in which lo and I are current outputs of the cellat two depths x meters apart. Also certainrelations between subsurface photosynthesis andthe vertical migration of plankton on the onehand, and relative transparency on the other, maybe inferred from the values of these extinctioncoefficients.

The purpose of this paper is to present theresults of a series of measurements made duringthe summers of 1938 and 1939 which givesubmarine illumination in footcandles, and alsothe current extinction coefficients of two spectralbands in the ocean and inshore waters of thePacific Northwest. No further reference to formerwork will be made as rather complete bibliog-raphies have been given by Atkins, Clarke,Pettersson, Poole, Utterback, and Angstrom ;Utterback and Jorgensen ;2 and Johnson andLiljequist.A

1 W. R. G. Atkins, G. L. Clarke, H. Pettersson, H. H.Poole, C. L. Utterback and A. Angstrom, J. Du ConseilInternational 13, 1, 37-57 (1938).

2 C. L. Utterback and Wilhelm Jorgensen, J. Opt. Soc.Am. 26, 257-259 (1936).

3 Nils G. Johnson and Gosta Liljequist, Svenska Hydro-grafisk-Biologiska, Kommissonens Skrifter. Ny. Ser.Hydrografi 14 (1938).

136

J. . S. A. VOLUME 30

IN PHOTOMETRIC UNITS 137

WAVE-LENGTH IN MILLIMICRONS

FIG. 2. Relative sensitivity curves and transmission offilters.

AREA INVESTIGATED

As it was desired to determine the visualillumination intensity beneath the surface in theopen ocean, in waters approaching bays andharbors and in the inshore waters, the regionshown in Fig. 1 was selected. It will be noticedfrom the position of the dots, which indicate thestations, that the measurements at variousdepths were made in the Pacific off Cape Flattery,in the Strait of Juan de Fuca, to the north inHaro Strait, in the San Juan Archipelago, inBellingham Bay, in the Strait of Georgia, and tothe south in Puget Sound.

At some of the stations, the surface intensitywas relatively high, while at other stations it waslow. In most cases the surface condition of thewater was suitable for such observations, the

FIG. 3. Current illumination curves for 0C and 17'C.

water being relatively smooth with little or norolling of the boat.

APPARATUS

The sea photometer and its method of opera-tion have been described by Utterback andHiggs.4 The method of making observations andthe necessary precautions in procedure have beendescribed by Utterback and Jorgensen.2

The relative spectral sensitivity of the light-sensitive cells employed in the photometer ex-tended from 300 millimicrons to about 800millimicrons, with a maximum response at 580millimicrons. In order to reduce the observationsto intensities in footcandles, or lux, it wasnecessary to provide the cell with a filter suchthat the effective spectral sensitivity of the celland filter correspond to the relative sensitivity ofthe light-adapted eye. Curve A in Fig. 2 shows

4 C. L. Utterback and P. M. Higgs, J. Opt. Soc. Am. 28,100-102 (1938).

PHOTOMETER CURRENTIN MICROAMPERES

600 800 1000

I

SUBMARINE ILLUMINATION

C. L. UTTERBACK AND R. E. WILSON

TABLE I. Temperature coefficients fromn 00 C to 17'C.

FT-C 1000 2000 3000 4000 5000 6000 7000 8000

lo 176 336 466 576 668 748 820 888

1 Al1 0.0108 0.0098 0.0092 0.0086 0.0078 0.0076 0.0069 0.0066

the relative sensitivity of cell and filter, whilecurve B is the average eye sensitivity. Thedeviations between A and B are less than thedeviations of individual eyes from the averageeye curve.

As the current produced by the light-sensitivecells is not proportional to the illumination,except at very low light intensities, it was neces-sary to calibrate the submarine photometer inorder that the illumination in footcandles could

be determined. Two color filters were employedin the photometer to determine the extinctioncoefficients for two spectral regions; their trans-mission curves, C and D, are also shown in Fig. 2.The filters employed were a Wratten No. 61 anda Schott BG 12. These curves show that the twocolor filters permit the determination of extinc-tion coefficients for these two bands withoutappreciable overlapping. Previous experiments inthese waters show that light in the region from600 millimicrons to the long wave-length limit ofthe visible spectrum is highly absorbed andcontributes but little to the intensity below thefirst few meters.

Since the effect of temperature on the currentoutput of the photometer was found to beappreciable at relatively high light intensities,

TABLE II. Illumination intensities and current extinction coefficients.

DEPTHSTATION INNUMBER METERS

3 5

10

15

20

25

30

35

40

45

CELL CURRENTEXTINCTION

COEFFICIENTS

GREEN BLUE

ILLUMINATION

IT{ I I STTTi

1 5 2518 233.90.085 0.142

10 1320 122.6.090 .087

15 821 76.3.113 .077

20 504 46.8.137 .098

25 263 24.4.106

30 138 12.8.114

35 83 7.7

40 50 4.6

45 31 2.9

.160

.167

.171

.195

.159

.156

.156

.161

.204

.207

.202

.183

3900

1345

490

254

108

43

18

8

3

362.3

124.9

45.5

23.6

10.0

4.0

1.7

0.7

0.3

DEPTHSTATION INNUMBER METERS

4

5

5

10

15

20

25

30

35

5

10

15

20

25

30

35

40

6 5

10

15

20

CELL CURRENTEXTINCTION

COEFFICIENTS ILLUMINATION

BLUE

0.318

.312

.366

.301

.296

i GREEN

0.225

.247

.242

.234

.222

.216

.244

.231

.170

.154

.149

.179

.656

.337

FT-C

2750

792

259

77

22

7

2

2530

703

240

89

30

14

6

3

4370

1400

71

15

LUX

255.5

73.6

24.1

7.2

2.0

0.7

0.2

235.0

65.3

22.3

8.3

2.8

1.3

0.6

0.3

406.0

130.0

6.6

1.4

I _I I-I-

138

. lo- I He .S

SUBMARINE ILLUMINATION IN PHOTOMETRIC UNITS

the magnitude of this effect was determined at0C, 110C, 17'C, and 22 0C for illuminationintensities up to 8000 footcandles. The currentillumination curves for 0C and 17'C are shownin Fig. 3. Each of these curves results from themean of many determinations made throughoutthe summer. Since the temperature range from00 to 17'C suffices for all conditions of use in thewaters of the Pacific Northwest, a mean tempera-ture coefficient was computed for this interval.

1 AlThis coefficient, defined by I A where I is the

photometer current in microamperes and t isthe temperature of the light-sensitive cells indegrees centigrade, is linear with illuminationintensity in the temperature interval from 00 to17'C. The values of the coefficient are given inTable I.

RESULTS

The results obtained are presented in Table II.The horizontal footcandle illumination and thecurrent extinction coefficients for the green andblue regions of the spectrum are given forstations No. 1, 3, 4, 5, and 6. These values,except for station No. 1, result from the mean ofat least two sets of observations, one taken withthe photometer descending and the other as itwas being raised to the surface. All data werecorrected for variations in the intensity of thelight incident on the surface. This correction wasusually a very small one.

By comparing the results given in Table II,one sees that the rate of extinction increases asone moves in from the ocean, up the Strait ofJuan de Fuca, and into the waters of the SanJuan Archipelago. A definite gradual increase inextinction indicates the transition from the ocean

water which contains a comparatively smallamount of suspended material in the upperlayers to the inshore water which contains arelatively large amount of such material. It is tobe noted that the numerical values are valid onlyfor the conditions under which the observationswere made. A comparison of the values of theextinction coefficients at the different stationsshows clearly the variations in the rate ofextinction of the blue and green spectral regionsas one approaches and enters inshore waterbodies.

The extinction coefficients as calculated fromthe cell currents are a function of the response ofthe particular cell-filter combination used. It isonly when the latter has a relative sensitivityequal to that of the eye that one obtains extinc-tion coefficients for visible light. As a result of thenonlinearity of the footcandle-current curve,Fig. 3, the extinction coefficients calculated fromcurrents are not equal to those calculated fromfootcandles, the difference being greatest forillumination intensities where the departure fromlinearity is greatest. Consequently, in order tocompute the intensity of illumination in foot-candles at a given depth from its value at someother depth, one must use the footcandle extinc-tion coefficients. Both sets of data from which theaverages in Table II were computed show dis-tinctly the "layering" of the water in EastSound.

We acknowledge the assistance of Alice Wilsonand E. Allan Williams in making observationsand of Dr. A. H. Taylor in the calibration of anauxiliary cell. One of us (C. L. U.) expresses hisappreciation of a Grant-in-Aid from the NationalResearch Council which made possible the con-struction of the photometer.

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