NOTES
AND
259
COMMENT
AN UNDERWATER LIGHT INTENSITY
METER
INTRODUCTION
We describe here an instrument
developed in connection with fish behavior
studies. For this purpose it is not necessary
to aim for high accuracy and precision,
since visual responses are known to vary
approximately with the logarithm of light
intensity rather than directly. On the other
hand, a very wide range of intensity has to
be measured to cover the varied conditions
at different depths by day and night. We
are indebted to Dr. B. H. Crawford, of the
National Physical Laboratory, for his help
and suggestions.
a-rubber
washer
’ rinqs
scale
Ins.
I
2
--brass case
3
PRINCIPLES
OF MEASUREMENT
4
The sensitive element used is a photomultiplier
tube (E.M.I. type 9524B). As
this is an 11 stage tube, its sensitivity varies
as a high power of the applied voltage. In
the equipment, means are provided for
varying, and measuring, this applied voltage, and also for measuring the output current due to the incident light.
To make a measurement, the cell is exposed to the unknown light intensity, then
the cell voltage is increased from its minimum until the output current reaches a
standard value-normally
100 pamp. The
applied voltage is then used as a measure
of the light intensity, the equipment being
previously calibrated.
UNDERWATER
5
6
junction
resistances
18
UNIT
The photomultiplier
cell and its associated chain of resistors is housed in a watertight brass case, as illustrated in Figure 1.
The tubular part is of drawn brass, and the
end plugs are turned from brass bar. The
plugs and windows are sealed by rubber
O-rings, and we have found it desirable to
make the grooves for these about 10 thousandths of an inch shallower than is normally specified by the manufacturers. The
photomultiplier
-- tufnol
“0’
mount
rinq
EYILqlass
--flat
FIG. 1. Watertight
case for photomultiplier.
Tubular section of drawn brass. End plugs turned
from solid brass bar. Length of tubular section
12 in., inside diameter 2Yz in.
box
t
rubber rinq
base
260
NOTES
AND
COMMENT
IO
K
DECK UNIT
FIG. 2.
Circuit
diagram
r
EMI
9524 B
U/W UNIT
of photometer.
length of the complete unit is 12 in. and
its outside diameter is 3 in. Provision is
made for fitting any color filters that may
be required over the window.
THE
DECK
UNIT
The circuit diagram of the complete unit
is shown in Figure 2. It consists of a transformer and a half-wave rectifier circuit to
give 2 ma at rather over 2,000 v. The variable resistance VR acts as a voltage control
allowing the output voltage to be swung
from about 300 v up to its maximum. The
Voltmeter V provides a measure of this voltage, which is applied to the photomultiplier. The microammeter A indicates the
photomultiplier
current, which is brought
to a standard value by adjustment of VR.
The deck unit is joined to the underwater
unit by 140 m of 3-core tough rubber
sheathed flexible electric cable (the conductor size chosen was 40/0.0076) which is
reasonably strong and easy to handle. This
size is convenient for sealing into the underwater unit by a standard compression gland
with a rubber filler.
I
400
I
I
800
1200
I
r600
I
2000
VOLTS
FIG.
3.
Typical
calibration
curve
of photom-
eter.
of this type of measurement is that it does
not call for an elaborate stabilized power
unit such as is normally required for phatomultiplier
work. The calibration is independent of the quality of the electrical supplies and of variation in the power unit
parameters. A typical calibration curve is
shown in Figure 3.
I-
CALIBRATION
Since the accuracy of the unit depends
on the characteristics of the photomultiplier
remaining unchanged, and would be affected by drift in the values of the resistors
of the photomultiplier
chain, its calibration
ought to be checked periodically.
It has
been found desirable to make weekly checks
during operations, using a stable light
source as a secondary standard.
It will be noted that the principal merit
52
*4-
5
*2-
k2
I
3000
4000
5000
A”
I
6000
7000
FIG. 4. Spectral response curve of photometer,
with Chance-Pilkington
filter
type OBA 2 in
position.
NOTES
I
00
I
I
I
2o”
4o”
60°
AND
\
ANGLE
FIG. 5. Directional response characteristic of
photometer.
SPECTRAL
SENSITIVITY
AND
DIRECTIONAL
PROPERTIES
Before interpreting
light measurements
in biological terms, some thought has to be
given to the quality of the light measured.
Since the present instrument is designed for
behavior studies on fish, it is desirable that
its spectral response should correspond as
261
COMMENT
closely as possible to that of the subject.
On the advice of Mr J. H. S. Blaxter, a bluegreen filter ( Chance-Pilkington
type OBA
2) is used. With this filter the overall response curve is shown in Figure 4, which is
presumed to approximate to that of many
marine species,
Since the calibration was carried out in
tungsten light, the calibration can be regarded as in “lux,” this unit being understood to refer to the selected response
curve. Thus a reading of 1 lux is recorded
at a distance of 1 m from a tungsten light
of 1 cp.
The directional properties of light are
also of importance in interpretation.
The
directional characteristics of this meter are
shown in Figure 5.
It was felt that for the purpose intended,
it would be adequate to measure the vertically downward component of light in the
sea, and to compare this from time to time,
place to place, and depth to depth. It may
be desirable later to fit an opal hemisphere,
as some other workers have done, in order
to give a less directional response. This
would somewhat reduce the sensitivity.
R. E.
CRAIG
AND
R. G.
LAWRIE
Marine Laboratory,
Aberdeen.
CONCERNING A COUNTING CHAMBER FOR NANNOPLANKTON
DESCRIBED
PREVIOUSLY
The simple counting chamber described
earlier (Lund 1959) has been in regular
use for the past 6 years. Experience has
shown that one method of determining its
depth is better than the others. Further,
the method of determining
the area by
weighing material of area equal to that of
the chamber (Lund 1959, p, 59) suffers
from the defect that it is difficult to obtain
transparent sheet of even enough quality.
The use of a dial gauge for determining the
depth is also unsatisfactory because it is
difficult to make sufficiently accurate meas-
urements with it. It will be seen, however,
that it is still of value. Lastly, a refinement
and a precaution need describing.
My
thanks are due to Mr H. C. Gilson and Mr
E. Ramsbottom for advice and help.
The depth is best determined as follows.
Weigh the slide empty and full of distilled
water as described previously.
Do this
several times to ensure that there is no significant error from evaporation or from an
excess of water outside the chamber. Determine the area in two ways. First, place
the chamber over a grid of millimeter