4. Outputs
4.1 LCD Displays
There are actually 12 positions that are computer
controlled on the LCD Displays, assuming you count the
signs and the decimal points. These are divided up into 8, 3,
& 1, for the Mantissa, the Exponent and the Factor selection,
respectively.
4.1.1 Mantissa (3 & 1/2 digits plus sign)
If we ignore the decimal points, the main LCD Display
presents numbers that range from (+/-) 0 to 1999, which is
commonly referred to as three and a half digits. The auto
ranging capability in the instrument is used to try to
maximize the readability of this display, by keeping the
number above 199. In this way, the readout is always going
to have a resolution better than 0.5%. If the auto ranging is
turned off, the number is allowed to drop down to zero. To
complete the scientific notation format, the mantissa display
presents the sign of the magnitude and always keeps one
significant digit to the left of the decimal point. When in
percent mode, the Mantissa display moves the decimal point
to any of three positions to display numbers that range from
00.00 up to 1999 percent.
4.1.2 Exponent (1 & 1/2 digits plus sign)
To be able to span a very great range of light
magnitudes, it is necessary to present the answer in scientific
notation. The mantissa above would be lost without the
exponential terms to determine the magnitude range in
powers of 10. The number 10 is raised to the exponential
value shown in the exponent window. It can range from -19
through 0 to +19, covering 39 decades of magnitude change.
This may sound like it is much more than needed, but we
have already had requirements that use 22 of the 39 decades,
due to the wide variety of light measurement units, and the
extremely wide dynamic range of user measurement needs.
4.1.3 Sensitivity Factor Designator (0-9)
The last window is the
‘
FACTOR SELECT
’
readout,
which shows the user which of 10 stored factors is selected.
These factors are held in 10 registers that remain stored even
when the power is turned off. The LCD factor designator
tells you which factor has been selected out of the 10
choices from 0 to 9. See section 3.2.3.2 for more on
changing these factors.
4.2 Recorder/Analog Output
The recorder output/analog output is on pin 8 with respect to
pin 1 or A on the accessory jack. Refer to 4.21 and 4.22 for
more information.
4.2.1 Voltage Range
The voltage is designed to be compatible with most strip
chart and X-Y recorders, and yet provide a large enough
signal to avoid excessive noise pickup. For these reasons,
we have selected a range from 0 to 1.0 volt (1000
millivolts). The output will be a voltage that is exactly 10%
of the reading of the mantissa. In other words, a mantissa
reading of 7.65 would produce .765 volts. If the unit auto
ranges (front panel readout of > 9.99 or < 1.000), the
recorder output increases or drops by a factor of ten, so the
chart scale will stay between 100 and 1000 millivolts, so as
to avoid chart recorder overrange. By keeping track of the
number of times the plot makes a 10 to 1 range change, it is
possible to know your absolute value on each range.
When in the 100% mode, the output reads 10 millivolts
per percent, if under 100% (99.0% = 990 mV). When over
100% the output drops a decade to read one millivolt per
percent (199% = 199 mV).
4.2.2 Auto Ranging Considerations
As briefly mentioned above in section 4.2.1, the chart
recorder will automatically stay between 100 to 1000
millivolts. If you are plotting a changing light level, or the
output from a spectral scan, the output will abruptly change
when either voltage level is reached. It is very obvious from
the plot what happened, and what magnitude to place on the
data. For example: if the plot is going down and hits the
100 mV lower boundary, the chart recorder will jump to the
full scale (1000 mV) position and the plot will continue to
come down from that new point of reference. Likewise if it
is rising when reaching the 1000 mV boundary, it will drop
down to 100 mV and continue on up from that point on.
This makes it very convenient for unattended plotting,
because you never have to worry about overranging the
recorder, or about losing valuable data. If you have an
application where you want the chart to go to the traditional
zero, you may turn off the auto ranging and the plot will
proceed down to zero without autoranging. (You should turn
off auto with the meter reading a signal that is close to max.
signal.)
4.2.3 Negative Readings
Since there is no such thing as negative light, we elected
to use a unipolar Digital to Analog (D/A) Converter. This is
fine if zero was properly set, and if your ambient conditions
do not change throughout the experiment. From our own
experiences, we have found that mistakes get made which
make the reading occasionally go negative. for example:
15
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