3.2.2 Range Selection
Most users will find the auto range mode the best choice
when using the Radiometer. This is the initializatio n default
mode which will display any magnitude automatically, as
long as a proper previous zero had been performed (see the
previous section) and the correct sensitivity factor is
programmed and selected (see the next section). There are
two other range choices, that may apply for special
circumstances. I f many repetitive measurements are to be
made, it is often desirable to turn the auto range mode off,
which essentially freeze s the range exponent, so the user is
alerted to a large change in signal by the fact that the
mantissa will either show several zero digits, for a low
reading, or will read
‘HI’, t
o indicate an upward range
change. In scientific notation, yo u could experience a
tenfold change in the light level, which might go unnoticed
because the exponent is the only obvious digit that changed.
By turning the automatic feature off, it becomes more
obvious that a significant change has occurred. Another
reason to turn the auto range off is to read a zero that occurs
quickly as described under
‘zeroing i
n section 3.2.1 above.
The third range choice is
‘percent mode’
, and is described in
section 3.2.4.
3.2.3 Programming
The Radiometer performs subtraction, as mentioned in
the zeroing section (3.2.1). It also calculates ratios as
described in the next section (3.2.4), but for most
measurement s it is required to scale the answers by a
sensitivity factor in order to read directly in your desired
optical units. This scaling is done internally by dividing the
electrical current coming from the input device by one of ten
(10) sensitivity factors, and then displaying the result in
floating point arithmetic, on the front panel. These answers
can range from 0.00x10
-19
to 9.99x10
+19
.
3.2.3.1 Current Measurements - If you want to read the
electrical current, you may select a factor of 1.00x10
+0
(which is 1). The answer will be displayed in amperes.
These are good units for any sensor that is not calibrated,
because a subsequent calibration would permit you to
reconstruct the absolute data. The best policy is to obtain a
calibration sensitivity factor for the detector assembly at the
time of purchase, so that direct readings can be obtained
immediately.
3.2.3.2 Entering Factory Calibrated Factors - The
factory calibration provides a sensitivity factor that appears
on the calibration certificate in units with amperes (A) in the
numerator and the desired readout units in the denominator.
When the instrument divides the current (amperes) by this
factor, the ampere term drops out leaving the final desired
optical units on the display.
You may store ten factors (in registers 0 to 9) for future
selection. Each factor is designated by a number in the factor
select window. (See 3.2.3.3 below) We suggest that you
store 1.000x10
+0
as one of the entries, for reading in units of
current(amperes)
To enter a factor, turn the instrument on, toggle
the ‘
DATA
DISPLAY
’
button to
‘
FACTOR SHIFT
’
, then increment the
‘
FACTO
R’
to read the register you wish to change (0 to 9).
Now use the three black buttons
marked ‘MSD’, ’LSD’
,
AND
‘EXP’
to enter the number in scientific
notation, just as it appears on the calibration certificate. By
holding a button in, the display will begin to roll faster, so
you can rapidly increment to a number some distance away.
As you approach the desired number, pulse the button one
step at a time
3.2.3.3 Sensitivity Factor Selection (0-9) - The last
window is the
‘
FACTOR SHIFT
’
readout. It shows the
user which of 10 stored factors has been selected. Each
detector/filter/optic combination has a unique calibration factor
therefore one detector can require several factors for
different combinations or readout units . Also at different
wavelengths there may be additional sensitivity factors, so
the wavelength would determine the
‘
FACTOR SHIFT
’
number used. This is changed by switching to the FACTOR
’
mode on the DISPLAY switch, and then incrementing the
‘
FACTOR SHIFT
’
button to arrive at the desired factor
number.
3.2.3.4 Self Calibration Technique - These factors may
also be used for customizing readouts in relative units that
pertain to your particular system. For example; an optical
throughput, in a hypothetical system, may be adjusted at the
factory to a particular magnitude, such as 150 (1.500e
+2
on
display). By selecting the right factor, you can make the
instrument also read 150 for that desired magnitude. The
ILT1700 can then be used for a subsequent quality control
check to adjust production units to have the same nominal
output. Another relative application could be to obtain a
precise correlation between departments. A standard source
would be used as a reference so all measurement systems
would be set to read the same from the same source. This
type of
‘i
n house
’
calibration can provide an accuracy better
than one percent, without having to refer back to the
reference source very often, especially if using a silicon
detectors which are generally more stable than standard
sources.
Self Calibrating Procedure: (ZERO METER FIRST)
A. Enter the ultimate display reading into one of the 10
factor registers, by pressing the DATA DISPLAY button
to
‘
FACTOR SHIFT
’
mode, and by using the MSD and
LSD buttons to enter the number (in scientific notation).
B. Switch back to the
‘D
ATA
’
mode and take a reading.
Record this on paper for the next step.
C. Enter the reading from step
‘B’
, into one of the 10
registers as you did in step
‘
A
’
. You can use the same
register, the last number is no longer needed.
D. Now switch back to the
‘D
ATA
’
mode again and read
the same optical signal. It should display the number
you were trying to calibrate to in the first place.
3.2.4 Percent Mode
There are two very common optical uses for the percent
mode, transmissio n and reflectance. Both measurements
require the use of a stable light source. By stable, I mean
that it will remain constant during the measurement
procedure. Modest accuracy can be achieved with a line
powered lamp, but for good long term repeatability, we
recommend an electronic regulator. (The visible output
from an incandescent lamp changes 3% for each 1% change
in voltage). The first step is to set up baffles so the light
travels directly from the lamp
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