External mobile power may be used to recharge the
internal rechargeable batteries via a vehicle
’
s cigarette
lighter outlet, or the instrument may be powered directly
from the vehicle. On occasion, an instrument must be used
at a remote location where only a DC source of power is
available. Typical applications might be in space vessels or
in long term remote environmental stations that are solar
powered. Last but not least, we will mention the most
common power source: the locally available AC power from
the line outlet. Several choices of input line voltage make
the ILT1700 usable any where in the world.
3.1.1 AC Power
Section 2.2 described the location of the power socket
(connector) on the rear panel. The ILT1700 is supplied with a
two meter (7 feet) power cord that has the appropriate
connector to mate with the instrument on one end and a plug
that is approved in North America for compatibility with a
standard 115 VAC wall outlet . Before applying power, be
sure to check the rear
panel slide switch marked ‘AC
VOLTAGE’
. If the switch needs to be changed see 2.2.2 . The
instrument draws less than 11 watts, (about 10% of that
consumed by a standard light bulb), so it can be used on very
low power circuits
.
3.1.2 Internal Battery Power
For many applications, it is necessary to carry the instrument
around to survey different areas. The ILT1700 can be used
with 6 re-chargeable nickel cadmium batteries, available
from Internationa l Light as accessory number A417. (ILT
will install, charge and test the batteries prior to use when the
A417 is purchased.) These batteries will run the system for
about five (5) hours on one charge, which is approximatel y
50 measurement sessions. Recharging typically takes 1 4
hours and can be done overnigh t. Battery installation can be
accomplished b y following the directions outlined in section
2.2.1.
3.1.3 External Mobile or Battery Power
External DC (direct current) power may be applied
through pin
‘C’
of the accessory connector in the voltage
range of 8 to 15 volts, where pin
‘C’
is positive, and pin 1
(or pin A) is the ground pin. This input is protected for
reverse polarit y, and handles a very large range of direct
current. The ability to accommodate such a wide range
permits use of even a solar battery that is nominally rated at
12 volts, yet varies substantially. Mobile power may be used
to recharge the internal nickel cadmium batteries, making
extended field use possible where AC power lines are not
available. Anothe r reason for using the external power is to
avoid the automatic timed shut off. If field measurement s are
to be made throughout the day, an external battery (such as a
12 volt lantern battery or car battery) can be hooked up to the
ILT1700 to run the unit for about three weeks at 9 hours a
day.
10
3.2 C.W. Measurements
The
‘C.
W.
’
stands for
‘Continuou
s W
ave’
, which is also
referred to as D.C. Measurements (a carry over from the
electronics field, meaning Direct Current)or signal mode . In
other words, we are referring to those measurement s where
the magnitude of the light level remains reasonably stable
when averaged over any half second interval. The light
output from a fluorescent lamp actually consists of pulses
emitted 100-120 times per second (twice the line frequency),
but we consider that to be C.W. since our eyes cannot detect
the flicker. The instrument is similar to the eye, in that it
integrates for a half second, displaying the average over that
interval. If this average is repeatable, we will get the same
reading on the instrument over a long measurement interval
.
3.2.1 Zeroing
One of the most important controls on this instrument is
the
‘ZERO’
button. This button does not do an internal zero,
as some users might expect. It actually subtracts the present
reading from itself and from all future readings. It also
remembers this condition even if the unit is turned off. That
means if the previous user set the zero to subtract a large
amount of light, it will continue to subtract that same
magnitude until you correct it. This can be a very powerful
control if used correctly. For example, lets suppose you
want to make a measurement of a light source, but you do
not want to turn the room lights completely off. One method
would be to make a measurement with the sample light on,
and a second measuremen t with the sample off, and subtract
the two from each other to give you the difference, which
will be the value of the unknown source. Using the zero function
you can do this automatically in just one measurement, without the
external subtraction. Simpl y press the zero button when the
sample light is off. Turn the sample on and read its
contribution directly on the meter.
If your requirement is to read all the light present
and you want the best possible zero, you must cover the
detector with an opaque object (or place it face down on an
opaque surface), wait ten (10) seconds, and press the
‘ZERO’
button. It takes several seconds to produce a low
level zero due to the ranging over several decades plus the
automatic extension of the sampling time when the signal
gets small for increased sensitivity. This sampling can be as
long as two (2) seconds and requires several sample periods
to go down to its most sensitive range. If you are impatient,
you can watch the exponent drop down three (3) decades
below the level you want to measure. Then press
‘ZERO’
even though the number does not read zero, because it will
be less than 0.1% of the level you are about to read, which is
good enough for accuracy of one part in a thousand. For an
absolute zero, you must have the detector covered extremely
well to seal all light leaks, and then you must wait the full
10 seconds before the system settles down to the lowest
level, before pressing the
‘zero’
button. Another method to
restore the meter to absolute zero is to remove the detector and
press the zero button.
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