[sci.electronics] Homebuilt transmission densitometer?

ittfb@dcatla.UUCP (Thomas F. Blakely) (02/09/88)

A handy tool around the Black & White darkroom these days is a
transmission densitometer.  While relatively inexpensive ones are
available (ca. $200) it might be an interesting project to build
one.  Has anyone had any experience along these lines?  It should
be pretty straightforward, unless there's something I missed.
Since it's for monochrome, color sensitivity of the sensing element
(photodiode?) shouldn't matter.  How about connecting the output to
your PC (everybody has one, don't they)?  How about hearing from
those of you with experience, or even ideas?  (Or, for the sci.
electronics folks, maybe we could start talking about mercury-
filled speaker wires again :-).

Tom Blakely
(404)442-4866
gatech!dcatla!ittfb

larry@kitty.UUCP (Larry Lippman) (02/09/88)

In article <2659@dcatla.UUCP>, ittfb@dcatla.UUCP (Thomas F. Blakely) writes:
> A handy tool around the Black & White darkroom these days is a
> transmission densitometer.  While relatively inexpensive ones are
> available (ca. $200) it might be an interesting project to build
> one.  Has anyone had any experience along these lines?  It should
> be pretty straightforward, unless there's something I missed.
> Since it's for monochrome, color sensitivity of the sensing element
> (photodiode?) shouldn't matter.

	Here are a few suggestions:

1.	You MUST use an incandescent white light source; for general purpose
	use, a single monochromatic light source such as an LED is unsuitable.
	The transmission spectrum of B&W film stock is not flat, and varies
	from manufacturer to manufacturer; consequently, the use of a single
	monochromatic light source is an invitation to trouble.

2.	In order to achieve sufficient signal-to-noise ratio in measuring
	higher densities (< 10% T, > 1.0 absorbance), LED's will not provide
	sufficient luminous intensity with a photodiode or photovoltaic
	detector, anyhow.  So, you really need an incandescent lamp.

3.	The incandescent lamp MUST be powered from DC (use a 6 or 12 volt
	lamp), and MUST be regulated using a solid-state constant-current
	regulator.  I cannot overemphasize the importance of having good
	regulation for the lamp supply.

4.	You may wish to consider using fiber optics to direct the light
	source to the sensing head.

5.	The calibration control for setting 100% T (0 absorbance) can
	either vary photodetector sensitivity or vary lamp current; some
	commercial instruments use the latter approach (I personally
	don't care for it, though).  If you use a microprocessor, no
	potentiometer is necessary - just a switch to tell the uP to
	take a reference reading.

6.	Calibration of a meter for % transmission is simple, because it is
	a linear relationship.  If you want an absorbance readout, you will
	require a logarithmic scale; there are clever analog circuits for
	this, or you can do the conversion in a microprocessor.  In this
	day and age, I recommend the latter.

7.	I recommend a photovoltaic detector, but a photodiode will work -
	provided that it has decent visible light response.  Forget about
	using a CdS photoresistive detector.  A photomultiplier tube is
	used on most commercial densitometers, but this is probably too
	complex and frought with potential problems for a DIY project.

8.	Use a Kodak or equivalent density wedge to verify operation of your
	completed densitometer.

9.	Beware of ambient light interference resulting from poor optics
	design.
	
> How about connecting the output to
> your PC (everybody has one, don't they)?

	Not a bad idea - as long as you have some type of analog input
board.  You could then do the absorbance calculation (if you want to deal
in absorbance units) on the PC.

> How about hearing from
> those of you with experience, or even ideas?

	This is the kind of stuff I have been designing for the past 18 years,
so you now have the benefit of some of what I've learned The Hard Way. :-)

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
<>  UUCP:  {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<>  VOICE: 716/688-1231        {hplabs|ihnp4|mtune|utzoo|uunet}!/
<>  FAX:   716/741-9635 {G1,G2,G3 modes}   "Have you hugged your cat today?" 

beyer@houxs.UUCP (J.BEYER) (02/09/88)

R
eferences: <2659@dcatla.UUCP>


There is an ANSI standard on transmission densitometry that you should
look up. While you could make a densitometer in ignorance of the standard,
and while you could calibrate it to serve your personal needs, your
numbers will not agree with anyone elses unless your machine conforms
fairly closely to the standard.
The light color is fairly important as is the sensitivity curve of the
detector. Of greater importance is the angle of acceptance of the
detector and the angle of illumination of the source. If you don't
get these right, you will not be measuring diffuse density, but something
else. THis will confuse you and everyone else.

korfhage@CS.UCLA.EDU (02/10/88)

Recently I was wondering about using my color meter as a transmission
densitomiter. I have the low end Beseler model, and it uses a photomultiplier,
seems quite sensitive and has a very small metering aperature.
The general idea, it seems to me, is to compare the unknown density with
some known references until you find a match, or until you get close enough
that you can make a good estimate. The question is how do you get your
reference? A calibrated Kodak step wedge runs about $81 at the local store,
and I didn't care to spend this much money on a whim. An uncalibrated wedge is
less (about $45, as I recall), but then you need to find a densitomiter you
can use to calibrate it. Even cheaper would be to make a series of negatives
of slightly varying exposure and calibrate those. You do have to check for
uneven density in the negatives, but you really need to calibrate only a
very small spot on the negative.

If anybody sees anything wrong with this scheme, I would like to know.
It would not be as accurate as a real densitomiter, but it would seem like
an inexpensive way to figure out 0.1 above film base + fog, or whatever,
with accuracy good enough for a hobbyist. And it would not require finding
a place to put another machine.

   Willard Korfhage

   ARPA : korfhage@cs.ucla.edu
   UUCP : {ucbvax,ihnp4,randvax,trwrb!trwspp,ism780}!ucla-cs!korfhage

donl@foist (02/10/88)

In article <2659@dcatla.UUCP>, ittfb@dcatla.UUCP (Thomas F. Blakely) writes:
> A handy tool around the Black & White darkroom these days is a
> transmission densitometer.  While relatively inexpensive ones are
> available (ca. $200) it might be an interesting project to build
> one.  Has anyone had any experience along these lines?  It should
> be pretty straightforward, unless there's something I missed.

I use an enlarging meter to measure density, under the enlarger light
source.  I either project the negative or lay it over the photocell.
The meter gives linear values (a function of time and paper speed), so
i take the log myself.

There is an outfit (which seems to be basically one man) called Science
& Mechanics Instruments or something like that, with plans for
densitometers.  The basic scheme is that they will sell you one of a
couple of different meters, and plans for a light source, which is
simply a wooden box with a bulb and brightness control, with a little
stand for the meter and an arm for the probe.

I agree that density measurements don't have to be very complicated,
and it should be easy to build a densitometer in any of several
configurations.  The meter i use, which is also handy for general
printing, is a Beseler i bought for about $75.  It has proven itself
plenty accurate and repeatable for my purposes.  It's a little clumsier
to use than a densitometer, because i have to multiply the ASA dial and
time and then take the log, but for the money, it serves me just fine!

- donl

{sun,pyramid,adobe,decwrl,ucbvax,allegra,research}!sgi!donl
donl@sgi.com

donl@foist (02/10/88)

In article <11237@shemp.UCLA.EDU>, korfhage@CS.UCLA.EDU writes:
> Recently I was wondering about using my color meter as a transmission
> densitomiter. I have the low end Beseler model, and it uses a photomultiplier,
> seems quite sensitive and has a very small metering aperature.

I recently bought a used Cosar Mornick color analyzer to reduce testing in
color printing.  I was quite excited that it had a density scale on it as
well as the time and filtration scales, though i am not yet sure how accurate
it is.  The density scale is basically just the log of the time scale, and
color filtration numbers are (supposed to be) density x 100.  If the density
scale were not present, the log of the exposure time scale would do fine.

> The general idea, it seems to me, is to compare the unknown density with
> some known references until you find a match, or until you get close enough
> that you can make a good estimate.  The question is how do you get your
> reference? A calibrated Kodak step wedge runs about $81 at the local store,
> and I didn't care to spend this much money on a whim. An uncalibrated wedge is
> less (about $45, as I recall), but then you need to find a densitomiter you
> can use to calibrate it.  [...]

There are different sizes of Kodak step tablets; the #2 is about 3/4
inch wide, and about 5 inches long.  Mine is uncalibrated, and i think
it was about $23.  I've mounted one of these in a sheet of 4x5 film so
that i can project it when i want, which makes for quick paper-testing
exposures.

> If anybody sees anything wrong with this scheme, I would like to know.
> It would not be as accurate as a real densitomiter, but it would seem like
> an inexpensive way to figure out 0.1 above film base + fog, or whatever,
> with accuracy good enough for a hobbyist.

I think there is a tendency to be too worried about how accurate the
readings are in this sort of thing.  I suspect that shutter and
aperture variations, processing variations, metering problems,
differing amounts of flare, light falloff in the enlarger, etc. all add
up to a much larger variation than the inaccuracy of using an
inexpensive meter as a densitometer.  Reasonable tolerances suggested
by the master are .09 to .11 for film speed, which is 10%, which should
be pretty easy to meet.  If you can show that the meter you are using
doubles and halves pretty accuratly, the density readings you obtain
with it should be more than adequate.  I have found the uncalibrated
step tablet to be accurate enough for my needs; i have found steps that
are a little high or low using a rather inexpensive ($75) black-and-white
enlarging meter as densitometer, but the overall slope is plenty close.

- donl

{sun,pyramid,adobe,decwrl,ucbvax,allegra,research}!sgi!donl
donl@sgi.com

pwc@mitre-bedford.ARPA (Patrick W. Connors) (02/10/88)

In article <11237@shemp.UCLA.EDU> korfhage@CS.UCLA.EDU (Willard Korfhage) writes:
>Recently I was wondering about using my color meter as a transmission
>densitomiter. I have the low end Beseler model, and it uses a photomultiplier,
>seems quite sensitive and has a very small metering aperature.
>The general idea, it seems to me, is to compare the unknown density with
>some known references until you find a match, or until you get close enough
>that you can make a good estimate. The question is how do you get your
>reference? A calibrated Kodak step wedge runs about $81 at the local store,
>and I didn't care to spend this much money on a whim. An uncalibrated wedge is
>less (about $45, as I recall), but then you need to find a densitomiter you
>can use to calibrate it. Even cheaper would be to make a series of negatives
>of slightly varying exposure and calibrate those. You do have to check for
>uneven density in the negatives, but you really need to calibrate only a
>very small spot on the negative.
>
>If anybody sees anything wrong with this scheme, I would like to know.
>It would not be as accurate as a real densitomiter, but it would seem like
>an inexpensive way to figure out 0.1 above film base + fog, or whatever,
>with accuracy good enough for a hobbyist. And it would not require finding
>a place to put another machine.
>
>   Willard Korfhage

   In the section on film testing procedures in _The Negative_, Ansel
Adams states that a spot meter may be used as a densitometer.
According to Adams, every 1/3 stop mark on the meter scale is
equivalent to .1 density units.  To find .1 above fb+f, set your
negative on a light table and take a reading of an unexposed developed
and fixed negative, and that is your fb+f density.  All you need to
find now is the exposure which reads .1 more.  All readings are
relative to the fb+f frame so no other calibration is neccessary.
I have tried this with my Pentax Spotmeter V and it works fine.

   There is a book called _Beyond the Zone System_ (I forget the
author), that gives a couple of other systems for controlling contrast
with an incident meter.  This book is very technical, but there is a
companion workbook that has plans for turning a spotmeter into a
densitometer.


  Pat Connors

kar@ritcv.UUCP (Kenneth A. Reek) (02/11/88)

In article <2659@dcatla.UUCP> ittfb@dcatla.UUCP (Thomas F. Blakely) writes:
>A handy tool around the Black & White darkroom these days is a
>transmission densitometer.  While relatively inexpensive ones are
>available (ca. $200) it might be an interesting project to build
>one.  Has anyone had any experience along these lines?  

	I have built one of these using a CDS cell from Radio Shack.  As was
pointed out, the spectral sensitivity of the cell (and the lamp, too, for
that matter) is of little interest to B&W workers.  The problem is to get
the answer in the right kind of numbers.

	I connect an ohmmeter to the cell and read its resistance in ohms;
this value is determined by the density of the portion of the negative I'm
reading.  Unfortunately, it is proportional to opacity rather than density,
so to use these values you need a step wedge of known densities with which
to "calibrate" your ohmmeter (e.g. knowing that 42,500 ohms corresponds to
 .65 density on the step wedge and 49,350 ohms is .8 density on the wedge
allows you to estimate pretty well the density that gives a reading of 46,000
ohms).

	I worked out my calibration in this way and got surprisingly good
results, verified by readings on a Macbeth densitomiter here at RIT.  Be
prepared to recalibrate the thing each time you use it, however, unless you
have a regulated power supply for the lamp, and have a lamp whose output
changes little during its lifetime.  The cell might be sensitive to
temperature, too, and God knows what else.

	To get a readout directly in density units, you need an amplifier
with a logarithmic characteristic to convert the resistance of the cell
(proportional to opacity) into density (log of opacity).  I've not yet
done that.
-- 
	Kenneth A. Reek
	Rochester Institute of Technology
	{allegra,seismo}!rochester!ritcv!kar

beyer@houxs.UUCP (J.BEYER) (02/11/88)

When I bought a T21 scale, it was about $6. The steps are supposed to be
about 0.15 density units apart with the 'clear' patch at about 0.06 density
units (should these be called Driffields? :-) ). I measured mine with a
MacBeth TD901 densitometer and I feel the densities listed are close enough
for most work.

I'd worry about exposing film and hoping the densities come out as expected.
The reason I bought my densitometer is to find out if they are as expected.
After over 10 years, I finally get what I expect sometimes ;-).

You might try using  Neutral Density filters, but, unless you get them
somewhere much cheaper than my local dealer, a useful set would cost more
than a calibrated wedge.

The T21 is about 1/2 inch wide and 2-1/2 inch long.

donl@foist (02/12/88)

In article <663@houxs.UUCP>, beyer@houxs.UUCP (J.BEYER) writes:
> R
> eferences: <2659@dcatla.UUCP>
> 
> 
> There is an ANSI standard on transmission densitometry that you should
> look up. While you could make a densitometer in ignorance of the standard,
> and while you could calibrate it to serve your personal needs, your
> numbers will not agree with anyone elses unless your machine conforms
> fairly closely to the standard.

I disagree.  I think that it's fairly easy to calibrate against full-stop
increases and decreases, and that if these are right and you can show that
the meter is pretty linear, you should be pretty gosh-darn close to any
body else's numbers.

> The light color is fairly important as is the sensitivity curve of the
> detector. Of greater importance is the angle of acceptance of the
> detector and the angle of illumination of the source. If you don't
> get these right, you will not be measuring diffuse density, but something
> else. THis will confuse you and everyone else.

Sounds like you're talking about reflection densitometers, which would
definitely be more difficult.  The angle of illumination for transmission
is pretty easy to arrive at -- straight through.  And i haven't even read
the ANSI standard!

- donl

{sun,pyramid,adobe,decwrl,ucbvax,allegra,research}!sgi!donl
donl@sgi.com

beyer@houxu.UUCP (J.BEYER) (02/12/88)

I checked last nite: it is a T14 scale and cost $6. a few years ago.

briand@tekig4.TEK.COM (Brian Diehm) (02/14/88)

>>It would not be as accurate as a real densitomiter, but it would seem like
>>an inexpensive way to figure out 0.1 above film base + fog, or whatever,
>>with accuracy good enough for a hobbyist. And it would not require finding
>>a place to put another machine.
>
>   In the section on film testing procedures in _The Negative_, Ansel
>Adams states that a spot meter may be used as a densitometer.
>According to Adams, every 1/3 stop mark on the meter scale is
>equivalent to .1 density units.  To find .1 above fb+f, set your
>negative on a light table and take a reading of an unexposed developed
>and fixed negative, and that is your fb+f density.  All you need to
>find now is the exposure which reads .1 more.  All readings are
>relative to the fb+f frame so no other calibration is neccessary.
>I have tried this with my Pentax Spotmeter V and it works fine.

This does indeed work, and work well.  However, I've found a few warnings from
my experience.

1)  Use a broad light source - the light table suggested above will work fine,
    as will a large globe light PROVIDED THE NEGATIVE AND METER ARE BOTH VERY
    CLOSE TO THE LIGHT.  If you don't fill the frame of the spotmeter with
    the light source, you will get specular density rather than diffuse
    density.  Because silver particles actually block photons in bunches with
    the grain, the specular and diffuse densities of B&W materials vary widely;
    with color materials the silver is washed out and the remaining dyes
    scatter the light rather than block it.  This is also why B&W negatives
    will shield your eyes while viewing a partial eclipse but color slides
    won't.  And why diffuse enlarging light (cold light) makes a difference in
    B&W printing but matters less when printing color.  Anyway, diffuse density
    is what most agrees with densitometers, in my experience.

2)  The fluorscent source of many light tables isn't corrected in any way for
    color balance; if yours isn't then you may have some spectral response
    mismatch between your meter and your source.  Theoretically this doesn't
    matter if you are comparing densities.  However, you might want to be
    cautious.  I have used tungsten with good matching to real densitometers.

The relationship between 1/3 stop and 0.1 density is very close to absolute;
you needn't take Adams' word for it.  Basically, it is because the base 10 log
of 1/3 is very nearly 0.1 - to about 4 places.  If you're getting into this,
it is a useful exercise to work out this relationship for yourself; you will
get a good understanding of the sensitometry and mathematical relationships.

Finally, one-hour labs all have process densitometers for color QC.  I
have gotten gray-scale density readings from these labs simply by taking in
my 4x5 negatives during low-demand hours; invariably they are willing to give
me a few readings for a smile.

-- 
-Brian Diehm     (SDA - Standard Disclaimers Apply)
Tektronix, Inc.
briand@tekig4.TEK.COM   or  {decvax,cae780,uw-beaver}!tektronix!tekig4!briand  

jtn@potomac.ads.com (John T. Nelson) (02/15/88)

The title of this discussion sounds like the title to a Tom Swift
book...

"Tom Swift and his Homebuilt Transmission Densitometer."






-- 


John T. Nelson			UUCP: sun!sundc!potomac!jtn
Advanced Decision Systems	Internet:  jtn@potomac.ads.com
1500 Wilson Blvd #512; Arlington, VA 22209-2401		(703) 243-1611

"Hi... My name is Hobbes.  I'm the product of a malicious 5-year old's
twisted and destructive imagination.  Would YOU like to be my friend?"

beyer@houxs.UUCP (J.BEYER) (02/15/88)

The trouble with makeing a transmissioni densitiometer at home involves
the Callier effect. If collimated light is passed through a diffusing
medium, such as exposed and developed film, it is scattered.
If a densitometer is made by passing collimated light through the
film and the sensor is 'far' from the film, only the unscattered
light is measured. This is measuring specular density.
If collimated light is passed through the film and all the transmitted
light is gathered by the detector (by using an integrating sphere or
suitable opal glass arrangement) the setup measures diffuse density.
[it is equivalent to pass diffuse light through the film and use a detector
'far' from the  film.]
If diffuse light is incident on the film and a detector measures all the
light transmitted through the film, doubly-diffuse density is being measured.

These readings will all be different, especially in the higher density
areas. Now in most books and articles, diffuse density is what is referred
to. If your densitometer measures something else, you can get confused. If
you report something else, you may confuse others.
What density should you use? If making contact prints, you probably want
diffuse density. If enlarging with a diffusion enlarger, you probably want
diffuse density. If enlarging with a condenser enlarger, you could probably
use a specular density measurement, but that raises the issue of how specular.

The ANSI standard does define 2 kinds of specular density but these seem
to be related to movie and slide projection. It varies depending on the
aperture of the optical system.

beyer@houxs.UUCP (J.BEYER) (02/16/88)

While I don't normally measure density with an enlarger and enlarging
photometer, I think it is a pretty good way of controlling the process,
and even takes into consideration things like enlarger flare, Callier
effect, etc. However, the readings should not be equated to those measured
on a true densitometer. (They may even be *more* meaningful in some
applications, but they are not the same.) To get an idea of what is
going on, and to calibrate 3 new boxes of paper I just bought, I ran
some tests last weekend. Those results that I thought might interest others
are posted here. Relative speeds of the various papers are not, since
the box-to-box variations are the important ones that must be measured
individually.

I normally use a Macbeth TD901 transmission densitometer,
a Beseler 45MCRX enlarger with Aristo cold light head,
or Beseler condenser head, and sometimes use a Beseler PM2a color analyzer.
As a test, stimulated by recent postings here on netnews, I measured a Kodak
T-14 control scale four ways.
With the densitometer, over the color analyzer, and projected through a
Schneider Componon S 180mm f/5.6 lens.  When projecting, the scale
was 'pretty well' surrounded with an opaque (litho-film) mask.
When using the densitometer, I placed the emulsion side up (toward the
diffuser and detector). In all other cases, the emulsion side was down
(toward the detector -- where the paper would be). The "white" filter was used.

Step		D		D		D		D
Number		TD901		Scale on	Scale		Scale
		3mm aperture	Analyzer	Projected	Projected
				ColdLite	ColdLite	Condenser
-------------------------------------------------------------------------
 1		0.056		0.06		0.07		0.1
 2		0.184		0.2		0.2		0.27
 3		0.313		0.34		0.35		0.46
 4		0.463		0.5		0.52		0.69
 5		0.612		0.66		0.69		0.89
 6		0.764		0.82		0.85		1.08
 7		0.916		0.98		1.0		1.29
 8		1.08		*		1.16		1.48
 9		1.22		*		1.32		1.68
10		1.39		*		1.51		1.90
11		1.55		*		1.7		>2.0
12		1.71		*		1.88
13		1.86		*		>2.0
14		2.02		>2.0

* indicates too difficult to read (can't see where the steps are).
Assuming the TD901 to be the most accurate (I believe this assumption),
this seems to show that the PM-2A is pretty accurate, tracking the TD901
quite closely and linearly. Projecting with the cold-light head is also quite
close: I believe the slight upward curvature is due to light falloff at the
edge of the image where the higher densities on the control scale were. A
separate measurement seems to confirm this. The Callier effect is quite
pronounced with the Condenser head, but it seems to have more even
light distribution than the cold light head.

Makers of home-made densitometers can expect similar differences if they
are not careful about illumination and collection angles of their
instruments. Such makers are urged to plot the graphs, either from
my data, or (preferably) their own to see how much this difference is.

For speedy printing, I don't bother with the color analyzer (in B+W, that is),
I just set the enlarger for an 8x10" print, put the 4x5" negative in,
and print 10 seconds at f/11. Sometimes this comes out right. Other times
it is close enough so the next print is correct. This is only true for
"average" scenes. I do use the analyzer when changing sizes, but one does
have to calibrate each box of paper.

donl@foist (02/17/88)

In article <191@ritcv.UUCP>, kar@ritcv.UUCP (Kenneth A. Reek) writes:
> In article <2659@dcatla.UUCP> ittfb@dcatla.UUCP (Thomas F. Blakely) writes:
> >A handy tool around the Black & White darkroom these days is a
> >transmission densitometer.  While relatively inexpensive ones are
> >available (ca. $200) it might be an interesting project to build
> >one.  Has anyone had any experience along these lines?  
> 
> 	I have built one of these using a CDS cell from Radio Shack.  As was
> pointed out, the spectral sensitivity of the cell (and the lamp, too, for
> that matter) is of little interest to B&W workers.  The problem is to get
> the answer in the right kind of numbers.

I like this because it's cheap and it works!  A handy programmable calculator
or equivalent might be an easier way to get from the wrong numbers to the
right numbers.  If the response of the cell is linear, you should be able to
come up with an equation to turn it into proper density values.  I set up
mine using the first value as zero; it gets subtracted from all the rest as
film base + fog.

- donl

{sun,pyramid,adobe,decwrl,ucbvax,allegra,research}!sgi!donl
donl@sgi.com

mikkel@cg-atla.UUCP (Carl Mikkelsen) (02/17/88)

In article <2659@dcatla.UUCP> ittfb@dcatla.UUCP (Thomas F. Blakely) writes:
>A handy tool around the Black & White darkroom these days is a
>transmission densitometer.  While relatively inexpensive ones are
>available (ca. $200) it might be an interesting project to build
>one.  Has anyone had any experience along these lines?  

	I have built an analogue enlarging meter/densitometer that I
use routinely for both B&W and colour enlarging, although
theoretically it should not work as well for colour.  I have a CDS
cell on a probe that measures the amount of light transmitted through
the film and delivered to the paper.  There is a "pot" to null out the
intensity of any particular portion of the negative (such as base,
point of full blackness, etc), so that by subsequently probing the
highlight areas, can read "stops of negative contrast" directly.

	Based on negative contrast, I pick a polycontrast filter (yes,
I am not a purist).  With the proper filter, I set the pot to a
calibrated position, and adjust the f-stop to null the meter reading.
At this point, I have a pretty good (often "near perfect") selection
of both contrast grade and exposure.  From this point on, I adjust for
special effects desired, dodge and burn, etc.

	Internally, the meter contains two (2) crude log-amps, a
differential amplifier, and a metering circuit.  The whole thing is
powered by two 9v alkaline cells (that have so far lasted for over two
years), that also drive two small lights to illuminate the meter.

	BTW, although probably obvious to you all, I did not
anticipate a need to turn off the safelights when making meter
readings.  I modified my enlarger timer to kill the safelights by
adding a DPST NO relay energized by the enlarger circuit switching the
safelight circuit.

	All parts came from Radio %$#@ or my junk box.  I have also a
Beseler Analite 400, and find it to be total trash compared to this
meter.

	May your good shooting come out fine in the soup!


      +--------------------------+--------------------------------+
      | Carl Mikkelsen           |   ..!ism780c\                  |
      |                          | ..!cbosgd!ima>!cg-atla!mikkel  |
      | Compugraphic Corporation |   ..!ulowell/                  |
      | 200 Ballardvale St.	 |   ..!decvax/                   |
      | Wilmington, Ma. 01887    | (617) 658-5600 x 5220 (voice)  |
      |                          | (617) 658-0200 x 5220 (TT-auto)|
      +--------------------------+--------------------------------+