Last update: 2021/04/27 20:10
safety:pass
https://wiki.pangolin.com/doku.php?id=safety:pass
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Printed on 2022/07/28 09:15
voltage for the same amount of power from each separate wavelength).
The output of silicon photodiodes cannot be directly connected to PASS. Instead the photodiode output
must be amplified, and turned into a zero-to-5-volt analog signal, where zero volts represents no light
being detected and 5 volts represents the maximum voltage delivered to PASS. Note that PASS provides
a connector with 5 volts, so this is where the light sensor and amplifier can get its power.
Since PASS has adjustments for MINIMUM POWER and MAXIMUM SAFE POWER (see page 22), the output
of the sensor does not need to reach all the way to 5 volts when maximum light is coming out of the
projector. Likewise, the sensor may deliver 5 volts even before the maximum light power level is
reached. Any maximum-light-output scale factor can be used as long as around 4 volts or less represents
the maximum safe light level for audience scanning (which would generally be lower than 1 watt, but
could be more depending on the distance of the projector to the audience and other factors).
Under the most ideal circumstance, the photodiode should be amplified with a logarithmic amplifier (also
called “log-amp”), because when this is done, the voltage output from the amplifier will correspond to
light power (i.e. watts), not just light intensity. However, a simple op-amp amplifier may be used, as long
as it has enough dynamic range that a few milliwatts of power delivers a detectable voltage level (and
can satisfy the “MINIMUM POWER” adjustment) and also maximum safe light level is reached before
around 4 volts.
Presently, the only ready-made product that is readily available on the market, which has a silicon
detector with the “spectral flattening” filter attached and logarithmic amplifier board is the model SEL033
detector with model F “flat response” filer, and model A430 amplifier, all available from International
Light Technologies corporation. Note that these components are generally not stocked items, and
normally they have to be pre-ordered from the company. Also note that these are large and bulky, so
they may not be appropriate for compact projectors. And finally the combination of these parts does not
constitute a complete light sensor ready to be used in a laser projector. The sampling window and
mounting hardware are additional items that need to be fashioned within the projector to create a whole
light sensor package.
Using a light sensor with PASS can be tricky. When you enable the light sensor feature in PASS,
everything must be at least ready to operate correctly. Otherwise PASS will detect problems and “trip” –
forcibly shutting the projector down, requiring either a manual reset (i.e. by briefly shorting pin 3 to pin 4
on the Control Panel Connector) or the power being re-cycled.
PASS will forcibly shut the projector down under the following critical circumstances:
A voltage level less than the “MINIMUM POWER” adjustment indicates, being detected when the
software (via the color lines on the ILDA connector) commands light to be coming from the
projector.
A voltage level greater than the “MINIMUM POWER” adjustment indicates, being detected when the
software (through the color and intensity lines) indicates that the output is supposed to be blanked.
A voltage level greater than the “MAXIMUM SAFE POWER” being detected when the beam is
projected within an audience area. Of course the audience area may be the entire projected area,
or only the lower portion of the scan field, depending on the “PROTECTED AREA” switch setting
Using the light sensor with PASS affords the maximum level of protection, because if the laser power is
increased above the MAXIMUM SAFE POWER within the designated audience area (i.e. the entire scan