How to dim a light without changing its color temperature

[Bill Burk]

Ok, thanks. I saw some wires and potentiometers and wondered if you were changing resistors or something.

Maybe I'm missing something, you are measuring the meters selenium cell's output directly in mA and V? Why not just observe the readings of the scale?

Gathering milliamps/volts that the needle requires separately from the milliamps/volts the cell outputs with the hope of “binning” (calibrating by matching components that match each other).

So far I suspect the coils in the meters have very consistent requirements, and it’s the cells that are falling weaker. The plain cells tend to be weak below usability but many of the Weston marked cells (you can see a W logo through the bubbles) have seemed correct but measured 1/3 stop low.

Now I think I found that the apparent third stop loss was simply a problem with my test geometry (distance from source to sensor) and color temperature (infrared influence).

Also a crazy vision. This is where I need engineer friends like you. Assuming the needle-coil assembly in the meter is an ammeter, I’m thinking a buck converter could reduce the voltage and increase the amperage.

Maybe we can come up with a switching power supply circuit (transistors, inductors and capacitors) that will raise the needle readings of cells that have weakened. Or maybe allow a 4700 K calibration for cells that are already up to spec. Maybe allow a green filter too.

 

[ic-racer]

Ok. Last I looked into replacement selenium cells (when I got my first Rolleiflex 2.8f with bad meter in 1985) I was under the impression I could cut them to size to adjust the output. Perhaps that is not the case.


In the more radical direction, maybe some type of "resto-mod" where the meter's outer case and galvometer are preserved and the insides are fitted with a modern photodiode connected to a micro arduino which would allow a perfect software correction for the meter's scale.

The arduino programming could be simplified with a table of 1/3 stop intervals; no complex equation. Then, using your setup, find, by trial and error, the numerical value to output voltage to the galvometer for each light value in the table. That wold be pretty cool because the needle would move in fixed 1/3 increments.

 

[ic-racer]

Just as I posted that above I realized I have boxes of spare parts from building the "Film Camera Tester"

I have enough stuff to make a prototype with a 'test' galvometer.

Another version would be for the arduino to 'intercept' the output from the existing selenium cell and then drive the existing Weston galvometer with a voltage to give the correct reading. Though, if going to all this trouble, I'd just replace the selenium cell all together with the modern sensor.

 

[Bill Burk]

Thoughts like those keep going through my mind.

The build quality of these Westons commands respect. I don’t want to put in a cheap solar cell.

Another thought I had was to use an op amp with variable resistors to calibrate and gets its power from a rechargable surface mount battery or supercapacitor that gets recharged whenever you hit the cell with more than 50 candles per square foot.

 

[ic-racer]

Looks like the Arduino, itself only has a PWM output, so some experimentation with a transistor and resistors would be needed to get the Arduino do power a galvanometer.

 

[Bill Burk]

I was thinking of a purpose-built circuit to fit the space below the magnet about .3” x .9” x .125” (the .125” dimension can be .2” between the neckstrap holes. So bigger surface mount device(s) like the super capacitor or a pair of variable resistors could fit the middle of the board).

 

[Chan Tran]

I don't know the characteristic of the selenium cell but the current that go into the galvanometer should be logarithmic unless they make the needle moves in logarithmic fashsion.

 

[koraks]

Looks like the Arduino, itself only has a PWM output

'The' Arduino is in fact a whole slew of uC experimenter boards, many of which also have DACs. For a project like this, you're furthermore not limited to an Arduino and can choose from any of the thousands of uC's on the market in a home-made circuit, and if that's insufficient, there are plenty of stand alone DAC chips to choose from.
But a PWM output with an RC filter will also work given the inertia of the needle.

 

[Chan Tran]

Bill Burk. How do you open up the Weston Master II. I like to check out the galvanometer as well as its selenium cell. For the galvanometer I would determine the resistance of the winding, full scale current and also if the galvanometer needle moves linearly with the current. For the selenium cell I would want to see the voltage generated with different level of light when measured with a very high impedance meter and when loaded with a low impedance like 100 Ohms or so. But first I need to know how to open it up. I can't see and screws.

 

[Bill Burk]

Find how close it is to working already. To creat a 100 foot lambert standard: illuminate a frosted panel from the back until a spotmeter tells you f/10 at 1/30 for ASA 100. (This is based on the “exposure formula” that Ansel Adams teaches).

The Weston should read 25 or 32 depending how close that you can bring the screen to the bubble glass and the color temperature and type of light source.

I find many Weston’s read only 13 and those are normally not candidates for cell resuscitation, though the “bones” may be good.

I have better luck with wholly non-responsive meters.

 

[Bill Burk]

There are two knurled screws in the back behind the serial number plate. I use them as indicators of tampering because you need a special tool to open without damaging. Even when I do open it with a tool the knurls are ever so slightly marred that you can tell it’s been opened.

A hardware store split friction pin is adaptable, I used a carbide scribe to etch ribs inside and ground the outside slightly for better clearance. But if you don’t care about looks it almost works as-is.

 

[ic-racer]

Gathering milliamps/volts that the needle requires separately from the milliamps/volts the cell outputs with the hope of “binning” (calibrating by matching components that match each other).

So far I suspect the coils in the meters have very consistent requirements, and it’s the cells that are falling weaker. The plain cells tend to be weak below usability but many of the Weston marked cells (you can see a W logo through the bubbles) have seemed correct but measured 1/3 stop low.

Now I think I found that the apparent third stop loss was simply a problem with my test geometry (distance from source to sensor) and color temperature (infrared influence).

Also a crazy vision. This is where I need engineer friends like you. Assuming the needle-coil assembly in the meter is an ammeter, I’m thinking a buck converter could reduce the voltage and increase the amperage.

Maybe we can come up with a switching power supply circuit (transistors, inductors and capacitors) that will raise the needle readings of cells that have weakened. Or maybe allow a 4700 K calibration for cells that are already up to spec. Maybe allow a green filter too.

Maybe we should have a thread on replacing photocells. Have you looked at these little ones DigiKey has (KXOB25-14X1F-TR).
It is about an inch by 1/4". This is the output. Do you think that would drive a galvanometer correctly?

 

[eli griggs]

I think a LED light-source can be dimmed without changing the color temperature.

I'm using a ND filter on my enlarger for small prints. Works fine.

Andi

The Lume Cube RGB LED Pro camera light, which is a small affair, and the Lume Cube RGB GO, both have colour temperature controls, however the GO has a Kelvin limit of 5600k 'daylight' while the Pro, which can be controlled by way of cell phone/PC bluetooth, hits a high of 7500k.

The top power level is calibrated to full power illumination as measured at .5 meter, (which you can see online) at whatever colour bias you make the light have and it keeps that colour temperature down to a 1% power level.

 

[Chan Tran]

Maybe we should have a thread on replacing photocells. Have you looked at these little ones DigiKey has (KXOB25-14X1F-TR).
It is about an inch by 1/4". This is the output. Do you think that would drive a galvanometer correctly?

View attachment 359408

The voltage is log of the irradiance which is good for exposure meter. You should try and see if it can generate sufficient current to drive the galvanometer.