Luna Pro exposure meter - please confirm my understanding

Bought used Gossen Luna Pro light meter. I believe it lost calibration on one (or both) scales, but I need to confirm that the same light intensity must give you the same reading [number] on top AND bottom scale. After all this is the number you should dial in to get a correct EV number e.g. combinations of apertures and shutter speeds, and this is a single value regardless of which scale was used. For instance, if I point light meter to a scene and the pointer shows "12" on top scale (low light range), once I switch to bottom scale (bright light range), the pointer still must read "12" value, albeit it now will be at the beginning of the scale on the left side. Since light scene (so light intensity into the meter) did not change, I still get the same combinations of aperture/shutter speeds on either scale, else one of them would have to be incorrect. Do I understand this right? Presently, this meter shows, say, 14 on bottom scale and only 11 on the upper scale. The pointer never reads above 12 on the upper scale even pointing on very bright objects which reads 20 or so on the bottom scale, as if it kind of saturates on the lower scale. One would think that you have to switch to upper scale only if the pointer shoots beyond "12" mark, but this never happens. I know it is mis-calibrated, just before fixing this I wanted to confirm that same light intensity should move pointer to the same number ("12" in this case) on either scale, and it should be possible to overrange Luna Pro (pointer moves beyond 12 on upper scale) if the subject is too bright for that scale. Yes, the power is stable 2.700 V (the meter was tested with external power supply). Thank you.

When the light is bright mine reads beyond 12 on the upper scale and when I push the lower button the dial will go to a higher number on the bottom scale. When moving the battery check it moves to the red spot. There is a calibration screw on the back. I forgot exactly the best way to calibrate but either someone will post but I’m sure you can find it online. When it is actually 12 it’s 12 on both upper and lower on mine.

I just recalled how to adjust. Remove the batteries adjust with the back screw to have the needle to green spot. Best to cover so no light enters.

Yes your meter is off calibration. Whether it can be calibrated or not I don't know as it seems to be off by a lot. On the top scale (with smaller numbers) the needle should peg to the right when you point at a bright scene and as you said it doesn't. Yes you're correct that if a scene reads 12 on the top scale it also should read 12 on the bottem scale.
I don't have the Luna Pro (I had it but I throw it away sometime ago) but I just check with my Minolta Spotmeter 9 which share number 9 on the 2 scales and there is some difference but very small.

jeffreyg said:

When the light is bright mine reads beyond 12 on the upper scale and when I push the lower button the dial will go to a higher number on the bottom scale. When moving the battery check it moves to the red spot. There is a calibration screw on the back. I forgot exactly the best way to calibrate but either someone will post but I’m sure you can find it online. When it is actually 12 it’s 12 on both upper and lower on mine.

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Thanks Jeffreyg, exactly what I needed to know. I have to recalibrate the unit so it reads 12 on both scales, and also capable of pegging beyond 12 on upper scale if the light is too bright for that scale. I just checked it out outside under direct sun, the diffuser cap off. It reads 22 on the lower scale, but still exactly 12 on the upper scale, the pointer never pegs beyond 12 on upper scale (in fact - incapable of pegging beyond 12 on either scale under direct sunlight). The power check lands pointer right in the middle of red swatch, so the power is OK. I've traces the schematic of the unit it is pretty basic circuit, so as long as the CdS cell itself is not degraded too far (possible but unlikely), I'll be able to recalibrate it. I'll post the results here if people are interested, could be useful for someone. Thanks again.

jeffreyg said:

I just recalled how to adjust. Remove the batteries adjust with the back screw to have the needle to green spot. Best to cover so no light enters.

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That screw is only zero position (null) adjustment, the pointer should lands on the black "zero" bar on the very left with battery removed. It does on mine, but this is very different issue - even without exactly nulling it should be able to peg beyond 12 mark (or 22 mark) if the light is too bright. It never does. BTW, there are no green spots on my Luna version.

Chan Tran said:

Yes your meter is off calibration. Whether it can be calibrated or not I don't know as it seems to be off by a lot. On the top scale (with smaller numbers) the needle should peg to the right when you point at a bright scene and as you said it doesn't. Yes you're correct that if a scene reads 12 on the top scale it also should read 12 on the bottem scale.
I don't have the Luna Pro (I had it but I throw it away sometime ago) but I just check with my Minolta Spotmeter 9 which share number 9 on the 2 scales and there is some difference but very small.

Click to expand...

Understood, thank you very much for checking on this.

The Luna Pro is what in Europe is called the Lunasix 3, and the folks at Learn Camera Repair have various documentation for it for sale:
There's also an interesting article here:

The pointer never reads above 12 on the upper scale even pointing on very bright objects which reads 20 or so on the bottom scale

Click to expand...

Can you get your money back?

Also: What batteries are you using? What batteries does the meter say it requires? I used a voltage regulator circuit to give the meter 2.70 volts. Some have been converted by Gossen. Otherwise there are other solutions to get the battery voltage right.

Without the correct voltage you might not be able to calibrate

Just wanted to comment on the calibration article linked above to by tih - I saw this on a few websites and must say that
the schematic of the meter, aside being so blurry that components' references cannot be read, is not only incomplete
(no components values given), but is just wrong - it does not correspond to actual circuit on physical board, at least for
my Luna Pro on hand. I've traced actual onboard circuit and figured where the discrepancy is. May be Gossen modified
the the circuit and the previous or next version got published there?

BTW, I also came up with the battery adapter that provides 2.7V from a single CR1/3N 3.3V Lithium cell that fits into battery
compartment same way as two LR44 cells fit. I wonder if lack of good solution for former PX625 mercury cells replacement is still
an issue for Luna Pro / Lunasix users, or this was more-less conveniently resolved. I'm aware of the mods with a diode, Wien cells
and such, but there's really no drop-in replacement you can buy in any drug store, put in your meter and it works just like it used
to from the factory. Any comments on what people's solutions are?

Gossen has the very thing. Please see https://gossen-photo.de/en/produkt/battery-adapter-set/

336v said:

Just wanted to comment on the calibration article linked above to by tih - I saw this on a few websites and must say that
the schematic of the meter, aside being so blurry that components' references cannot be read, is not only incomplete
(no components values given), but is just wrong - it does not correspond to actual circuit on physical board, at least for
my Luna Pro on hand. I've traced actual onboard circuit and figured where the discrepancy is. May be Gossen modified
the the circuit and the previous or next version got published there?

BTW, I also came up with the battery adapter that provides 2.7V from a single CR1/3N 3.3V Lithium cell that fits into battery
compartment same way as two LR44 cells fit. I wonder if lack of good solution for former PX625 mercury cells replacement is still
an issue for Luna Pro / Lunasix users, or this was more-less conveniently resolved. I'm aware of the mods with a diode, Wien cells
and such, but there's really no drop-in replacement you can buy in any drug store, put in your meter and it works just like it used
to from the factory. Any comments on what people's solutions are?

Click to expand...

Did you buy the articles or is your comment based on the screenshots that you can see for free on Learn Camera Repair?

336v said:

Just wanted to comment on the calibration article linked above to by tih - I saw this on a few websites and must say that
the schematic of the meter, aside being so blurry that components' references cannot be read, is not only incomplete
(no components values given), but is just wrong - it does not correspond to actual circuit on physical board, at least for
my Luna Pro on hand. I've traced actual onboard circuit and figured where the discrepancy is. May be Gossen modified
the the circuit and the previous or next version got published there?

BTW, I also came up with the battery adapter that provides 2.7V from a single CR1/3N 3.3V Lithium cell that fits into battery
compartment same way as two LR44 cells fit. I wonder if lack of good solution for former PX625 mercury cells replacement is still
an issue for Luna Pro / Lunasix users, or this was more-less conveniently resolved. I'm aware of the mods with a diode, Wien cells
and such, but there's really no drop-in replacement you can buy in any drug store, put in your meter and it works just like it used
to from the factory. Any comments on what people's solutions are?

Click to expand...

The symptom of your meter doesn't seem to be one of wrong battery voltage. Another thing I recently open up a couple of meters (not a Gossen) and found that when you switch scale you do nothing electrically but rather switching in and out a mask to reduce light when reading the bright range. I wonder if the Luna Pro is the same?

Bill Burk said:

Did you buy the articles or is your comment based on the screenshots that you can see for free on Learn Camera Repair?

Click to expand...

No, I did not buy the articles for three reasons, and no, because I don't want to spare $3.23 for the download is
not one of them.
One is - I don't think anyone would purposely blur the snapshot of a single page from the manual to compel you
to buy the reproduction of the original they sell - it is very possible that what they cell is equally blurry, as it could
only be a photocopy or xerox copy of the true original factory manuals, which were internally published in late 60's.
Did anyone here see the downloaded "original" factory service manual for Luna Pro / Lunasix 3?
The second reason is - while the schematic I'd be interested in is blurry, it is very visible what info is missing even if
the reproduction would be very sharp, so it is of no use to me. Besides, as I mentioned, the snapshot if the factory
schematic shows that it is wrong, blurry or not.
And the third one - I took apart my Luna Pro and being an electrical engineer by trade, reverse engineered the thing,
reconstructed the actual schematic and after what I've learned how exactly this meter works and should be calibrated,
I don't think the factory manual will give me anything useful I don't already know about Luna Pro, at least the version
I have. My original inquiry was about the calibration procedure, I know now how this is done and calibrated mine.
What I wanted though is actual trim pot values Gossen used in its product, so they can be replaced with modern
equivalents - and that info is missing on the schematics where it normally belongs. I can always just measure the pot's
values in the physical unit, but if they have drifted over time, this would not be very useful - that is unless I can average
measurements of 3-4 Luna Pro samples which I don't have.
Bottom line - factory service manuals still may not have the info I was seeking, but it's no longer very relevant -
I've simulated the circuit in the software and know the limits of the pots which allow full deflection of the pointer
and complete calibration.

Chan Tran said:

The symptom of your meter doesn't seem to be one of wrong battery voltage. Another thing I recently open up a couple of meters (not a Gossen) and found that when you switch scale you do nothing electrically but rather switching in and out a mask to reduce light when reading the bright range. I wonder if the Luna Pro is the same?

Click to expand...

The voltage in my meter is correct, I fed it from the external power supply set to 2.700V to test, and later on used alkaline cells and Ge diode
to drop excessive voltage so that the battery check function lands the pointer smack on the middle of the red swatch - the voltage was stable
enough for the duration of the tests I wanted to do. And to answer your question - no, Luna Pro is different from the units you've described: when
you switch ranges, it does both - inserts neutral filter with 6 EV stops density in front of the CdS on high range, AND electrically switches to 2
different sets of trim pots, each set of two independently calibrated respective range. In Luna Pro there are 4 trim pots to do it (+ 5th one for
batt. check calibration). It is purely passive device, and the circuit is not a bridge based. There is so much misinformation about it on the web,
and unfortunately it's being replicated from site to site.

Would you please help us out; many of us probably still use Luna Pros and they are all old… what is the correct schematic and calibration process?

BrianShaw said:

Would you please help us out; many of us probably still use Luna Pros and they are all old… what is the correct schematic and calibration process?

Click to expand...

Let me make comments on the schematic to be presentable here, there are my few notes in CAD I have to delete to reduce clutter.
I'll post it here in about an hour. As of calibration, I also have notes on the schematics which trim pot does what, the procedure is
commented on the schematics, but it's too wordy to give full educational course here. Important thing is to understand what you're
seeing when you calibrate and why it behaves the way it does, so you know what to do. If this is reduced to a set of hard instructions,
it will generate a bunch of "how to" questions and I'm not in the position to educate here or present basics of linear circuits theory.
The process is basically iterative, the way the pots are arranged they affect each other - adjusting offset for bottom scale affects gain
(reading on top of the scale), and subsequent adjustment of gain to set correct gain will mess up offset again, but going back and forth
4-5 times will converge to the correct settings. The calibration itself is simple, most difficult part is to find a lighting reference to calibrate
against. What I did was to use my Nikon D200 DSLR to measure exposure of a gray house door I could open and close relative to sun
position, creating more or less shadow on it to "dial" particular EV settings corresponding to 3 and 12 on Low scale and 12 and 20 on
the high scale (for ISO=100), if I recall this was about 1s for F/5.6 to about 1/250 for F/5.6. I don't recall exact numbers but you can set
any combination of aperture and exposure to get EV-2 and EV7 as low (3) and high (12) on low scale respectively, and EV7 and EV15 for
low (12) and high (20) for high scale respectively. It is actually best to have a reference light source with known Lux intensity which
corresponds to an EV value as printed on the decal on the back of your Luna Pro. "Dialing" known EV value by opening/closing the door
is' course' poor man's substitute for the reference light source I did not want to build, but measured with the D200 it was accurate enough.
What I did was temporary unsoldered the CdS from the PCB and brought out extended wires, so just captured resistance of my CdS
for these particular light conditions, on low scale they were 460k for EV-2 (readout=3), 4.78k for EV7 (readout=12) and on high scale
(with filter in the light path) -112k (for EV7 (12 readout) and 883 Ohm for EV15 (20 readout). Then all I had to do is to keep connecting
fixed resistors of these values in the place of CdS to mimic those light conditions, and calibrate back and forth - far less tedious proposition
than run outside and swing the door open and close every time I adjust one of the trim pots. Your Cds' resistance under the same lighting
will be different, they were not made to the high tolerance and there's no need for that - the key is whatever characteristics of your CdS are,
they will be consistent, so calibration circuit is designed to accommodate "normal" variation of any Cd Gossen ever installed.
See - the post is getting way too wordy already, I better stop here...

Alright, for now I'm attaching the screen shots of the Luna Pro schematic. Gossen evidently has made a few
variations of the unit this may or may not be related to the US version vs. Lunasix 3, which is European version of
the gadget, both might have a few design differences. For instance, my unit s/n 517521 has filter plate that does
not have spring loaded metal ball indexing it after sliding to low and high range positions using side rocker switch.
It just have a wire spring with indent falling into small cavities in plastic in both extreme positions. I've also seen
photos of the board with a fixed resistor soldered in parallel to the CdS. Mine does not have it. The trim pots look
different too. I don't believe the circuit fundamentally got changed so what I have here does not apply to all Luna
Pros out there, but that is certainly a possibility, so don't assume that this schematic necessarily represents the unit
you may have in your possession.

With that, one attached capture is complete schematic traced from the actual PCB also pictured here as CAD drawing,
and also two simplified schematics representing what the circuit becomes when switched to either low range or
high range; all unconnected components removed for clarity, including the battery check circuit.

I think I'll upload more material on the subject to my website and link to it from the pinhole camera
page. It's URL is mentioned on these captures for future reference. The page dedicated to Luna Pro is not created
yet, so I can't provide the link to it.

Have fun...

Thanks! That is a lot of information and very much appreciated.

Dan Fromm said:

Gossen has the very thing. Please see https://gossen-photo.de/en/produkt/battery-adapter-set/

Click to expand...

Yes, thanks, I've seen this one before, but looked closely now. The adapter looks like 3D-printed container,
one would think Gossen frankly could come up with injection-molded one.
On the electric side, It has no description of how it works, based on the images, the positive end of SR43
stack touches the metal cap of the meter just like original battery did.

This means if it reduces voltage, it does something on the negative side, most
likely integrates a series diode in the casing. The reason I'm saying this is in order to have active LDO
regulator you need a reference ground, e.g. solid negative connection to the meter, which this adapter
does not have - bottom (neg) side is insulated. That is, unless a -2.7V negative output regulator with ultra low
quiescent current is deployed here, but AFAIK, such a thing does not exist - the best device I saw has 3 uA
quiescent current, which will eat fresh SR43 in about 4.1 years. But it's possible Gossen still went with
such solution here - they don't say on this website.

Does anyone have this adapter to confirm if this is a passive or active device?

However it is designed, it works quite nicely.

I have the original version and it appears to be injection molded body. The new version has a nice additional feature… the indicator of how to load the batteries. It’s a passive device.

336v said:

Bought used Gossen Luna Pro light meter. I believe it lost calibration on one (or both) scales, but I need to confirm that the same light intensity must give you the same reading [number] on top AND bottom scale. After all this is the number you should dial in to get a correct EV number e.g. combinations of apertures and shutter speeds, and this is a single value regardless of which scale was used. For instance, if I point light meter to a scene and the pointer shows "12" on top scale (low light range), once I switch to bottom scale (bright light range), the pointer still must read "12" value, albeit it now will be at the beginning of the scale on the left side. Since light scene (so light intensity into the meter) did not change, I still get the same combinations of aperture/shutter speeds on either scale, else one of them would have to be incorrect. Do I understand this right? Presently, this meter shows, say, 14 on bottom scale and only 11 on the upper scale. The pointer never reads above 12 on the upper scale even pointing on very bright objects which reads 20 or so on the bottom scale, as if it kind of saturates on the lower scale. One would think that you have to switch to upper scale only if the pointer shoots beyond "12" mark, but this never happens. I know it is mis-calibrated, just before fixing this I wanted to confirm that same light intensity should move pointer to the same number ("12" in this case) on either scale, and it should be possible to overrange Luna Pro (pointer moves beyond 12 on upper scale) if the subject is too bright for that scale. Yes, the power is stable 2.700 V (the meter was tested with external power supply). Thank you.

Click to expand...

The extreme ends of the scales are somewhat nonlinear on these meters. The top of low range will not perfectly match the bottom of the high range even when correctly calibrated.

Yes, this device is electrically just a diode in series, connected to the negative side. I read this from
someone's review, who bought the adapter from B&H.

Yes, it drops the battery voltage by *approximately* fixed voltage amount. This drop on the forward biased
diode will slightly depend on the chosen measurement range (more for high range), and is also different
(bigger) for the BATT CHECK function. But the biggest drawback is the output voltage [decline] will
track cell voltage decline, and that curve, unlike for mercury cells, is not flat.

Finally, the adapter's (as any diode's) output is highly temperature dependent. If you can live with these
limitations, this is OK solution, though it is far cheaper to install the same diode inside the meter, which
then will take any two stacked CR44-sized cells, or better - a single CR1/3N cell without any adapters.

Thanks for the answer.

336v said:

... But the biggest drawback is the output voltage [decline] will track cell voltage decline, and that curve, unlike for mercury cells, is not flat. ...

Click to expand...

Instead of the SR44/357 battery I use the SR-43/386, where the discharge curve seems quite flat and the meter performance seems quite stable. The minor height difference between the two cells doesn't seem to matter. Even with the less-flat discharge curve of the 357, I'm not sure it matters much given the "allowable variance" of most photographic applications.

From the Renata spec: