Here it is, whenever you are.
I am using a silicon phototransistor in current mode. The sensitive
area is no bigger than a pinhead. It has sensitivity from UV to IR, but
the response is not flat, being higher in the red.
The meter uses the exponential relationship between current through a
semiconductor diode and the voltage drop across it. I use two op amps,
each with a diode as feedback. In this mode, the current into the
summing junction of one op amp comes from the photosensor and that into
the other op amp comes from a variable resistor. The output of the
first op amp is proportional to the log of the photocurrent. The
resistor is used to set level. The outputs of the two op amps are
summed in a simple resistor network into a potentiometer, the arm of
which goes to a digital panel meter. This pot sets the constant of
proportionality between the log of the photocurrent and the meter
reading. The two pots act independently of one another in that I can
set the level without affecting the scale, or the scale without
affecting the level.
This arrangement allows me to set the scale to read zones for any grade
of paper and the level to make the zone readings come true, at least
for two points on the paper's scale. There is one scale setting that
will read density. So far it has been of great use for comparative
tests and in printing. The use of two log channels provides at least a
first order compensation of temperature effects. The residual is taken
care of by the fact that I don't want to be in my darkroom when it is
too hot or too cold.
The probe is a simple affair on about two feet of twin lead. In
measuring projected density there can be a problem with flare in the
projection lens. I put a mask in the negative carrier just large enough
to pass one step of the step wedge, and slide the wedge into the slit
to make each reading. I block off extraneous light from other sources.
When I use it for printing, The same flare that the photosensor may see
will also br seen by the printing paper. In this sense, it has an
advantage over densitometers that are outboard of the enlarger on
negatives that are to be enlarged. My meter could be adapted to work
over a light table to measure for negatives to be contact printed, and
it could be equipped with an attachment to read reflection density.
Excuse the length, but I thought some of you might be interested in the
thing.
I do not currently have any set of filters for special purposes. If I
had my druthers, I would have a fixed, flat, wide band response with a
set of filters to modify the response as needed. If anyone knows of a
phototransistor with very low dark current that fits the basic
requirement, I would like to know about it.
It is interesting to note that when I worked for NASA, much of our
simulation was done with a large analog computer.You could fit maybe 6
of the op amps into an average bathtub. Of course, they operated with
voltage swing of + or - 100 volts, but they required constant
attention. The rectifiers and amplifying elements in these things were
vacuum tubes,and if the air conditioner ever failed, we couldn't use
the thing. It took about 9000 watts to run it. There were 100
amplifiers, some of which were dedicated integrators. They were
programed by wiring plug boards to connect various amplifiers,
integrators, resistors, capacitors, pots, and gadgets dreamed up by
Gadget Gainer. The other guys said they could always tell which board
was mine because it had something hanging on it that didn't come with
the computer.
Enough for now.