Re: Meaning of Brightness for TFT monitors


"Gernot Hoffmann" <hoffmann@xxxxxxxxxxxx> wrote in message
news:1152287326.253061.208430@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Results so far:

<snip>


The instrument was always calibrated with the white
reference and for darkness.
Dark noise is typically less than 0.1% of monitor white.


This describes a reflectance factor calibration but not a spectral radiance
calibration

Further specs:
Optical resolution 5nm (used in graphic and data file)
Bandwidth 20nm
Aperture 1.5mm x 2mm
ADC 12 bit


Ok - here is one area of concern. Is the ADC unipolar or bipolar? If the
latter then the signal is mapped as -2048 to +2047 (4096 total steps) In
that case if you have a high signal level then when the ADC reaches +2048 it
wraps around and returns a negative count for the large, over-range signal.
Try adding the absolute value of the negatives to 2047 and see if the
signals then make sense.

I don't expect accurate measurements for the TFT
by Spectrocam, but I'm still wondering where the
negative values are coming from.
The ADC should deliver unsigned data (not negative
numbers), using a small analog offset which allows
to detect small negative errors in the data acqui-
sition system.


The ADC will return only positive values if the device is unipolar. Many
ADCs are in fact bipolar. I have a StellarNet (competitor to Ocean Optics)
CCD spectrometer that has a 16 bit ADC and it too will return negative
values when it is over-range.

<snip>

Best regards --Gernot Hoffmann

Danny


.

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