Sensitometry with the Tobias Associates Wejex Model R

sanking · Dec 21, 2005

For many years I have done all of my film testing with a dedicated enlarger with the light source controlled by a Metrolux II timer. Precision is very great with this system since the Metrolux uses a light integrator, thus assuring almost absolute accuracy in the exposing process. This make it possible to accurately, and with considerable precision, compare films and developers for effective film speed, something that is virtually impossible without this type of exposure control.

However, since I am no longer use the enlarger for anything but film testing I thought it might be possible to save some space by converting to a sensitometer. So I bought a Tobias Associates Wejex sensitometer on ebay for a pittance. Two problems. First, the exposures are timed for 2.5 seconds (X-ray film is slow I guess), which gets you into serious reciprocity with some films, and second, the gray scale had only 11 steps, not the 21 which I consider necessary for accurate plottong of film curves. I figure that I can run the sensitometer through the metrolux to control timing, but the gray scale was a bigger problem as the existing scale was sandwiched underneath a thin piece of glass what was in turn heavily glued to the base. Removing it required breaking the glass, so I did that. Once the glass was broken I found that the Stouffer #T2115 sensitivty guide fits neatly over the opening.

The exposure of 2.5 seconds was for X-Ray film. Anyone know the effective ASA of X-Ray film? This would allow me to roughly calculate the eposure of regular film.

Sandy

Kirk Keyes · Dec 21, 2005

Why do it the round about way? Why not just take your light meter and make a direct measurement of the amount of light? I know you use an incident meter, if you have a flat diffuser for it, please read on-

I upgraded my light meters from a Minolta Flashmeter IV incident to a Flashmeter VI spot/incident, so now I keep the FM IV next to the enlarger baseboard to use as a lux meter. I have a flat diffuser and then I can calculate the lux by knowing the conversion factor from the owner's manual. By combining this with the exposire time, I can get lux-seconds and that allows me to graph my film tests. And lux-seconds are the same units that Kodak uses for their tests, last time I checked.

Kirk Keyes · Dec 21, 2005

Sandy - I know you have the sensitometer now, but in what ways do you think a sensitometer with a step wedge would be better for sensitometry than an enlarger with a step wedge, especially if you can dedicate an enlarger to this use?

sanking · Dec 21, 2005

Kirk,

I don't think the sensitometer is in any way better than my present set-up in which I have a dedicated enlarger with the exposures controlled by the integrator of the Metrolux. In fact, until I re-wire the Tobias to allow light integration control, which it presently does not have, there will be less accuracy with the Wejex sensitometer rather than more. It is simply a matter of space utilization. I don't use the enlarger at this time for projection printing and it takes up a lot of space that could use for other purposes, cabinets etc.

Sandy

Kirk Keyes said:
Sandy - I know you have the sensitometer now, but in what ways do you think a sensitometer with a step wedge would be better for sensitometry than an enlarger with a step wedge, especially if you can dedicate an enlarger to this use?

sanking · Dec 21, 2005

Jay,

I had forgotten that this is the sensitometer you use or would have contacted you directly about it.

About base exposure, I have the manual and see that it is possible to adjust the intensity of the light. However, I am not going to touch this as it would be easier, IMO, to make the adjustment with the timer of the light integrator, or with neutral density filters.

As for color balance, you can adjust the color of the tungsten lamp back to daylight with a cool blue filter, an 80a I believe.

Not sure I can agree with you about the sufficiency of the 11 steps for curve plotting. Some of the most important differences that show effective film speed are in the toe in the range from about log 0.06 to 0.15 and this discrimination is not available with the 11 stop wedge. But in any event I already substituted the 11 stop wedge with a 21 step guide .

Concerning the Winplotter software, unfortunately it will not accept direct input from a densitometer reading. Many densitometers will allow the direct input to Excel files, with software patches of course. Unfortunately, there is no program available at this time with all of the features of Winplotter. There may be some patch around this but I don't know enough about computer programs to make the connection. Perhaps Steve/Magic out in Washington, who is originally from Pocatello, might give some advice?

Sandy

Kirk Keyes · Dec 21, 2005

OK - I see it's purely a space saving consideration to use the sensitometer vs. enlarger.

I'm curious about the light integrator use - I'm not directly familiar with your model of sensitometer, but I would hope that they would have some form of voltage control for the lamp. It's tungsten, right? If so, and it is a 120VAC bulb it may not be voltage controlled, but if it is 12V, then I bet it is. If they have a halfway decent electronic timer, you should be able to get +/- 1/60th of a second for the exposure. Most of the digital darkroom timers I've checked are listed as being that accurate. At around 1 second or more, that seems more than precise enough. That's +/- one thirtieth of a stop.

So if the lamp voltage is controlled and the timer is that accurate, do you need to worry about using an integrator?

I would also try to not use the lamp brightness variable resistor to change the lamp brightness. it will change the color temp of the bulb, and you probably want it to be at it's brightest setting to get the bulb as close to 3200K for the 80A to convert to daylight.

Jorge · Dec 21, 2005

Well yeah you should have posted your intention before you got it...

Mine is a Dupont sensitometer that had all kinds of scales but the best thing was that the light intensity and exposure time can be adjusted to give exposure of under a second. I have calibrated it for both 100 and 400 film exposures at 1/2 sec. All that for 20 bucks...sometimes E bay does work...

Kirk Keyes · Dec 21, 2005

Jay - "If the BTZS software cannot be integrated, it might be worthwhile to develop similar, more compatible software."

I don't have the BTZS software, but if you can open find out what the data base field order is, I bet you could scan with your scanning densitometer, output that info into Excel, crunch numbers/reorder data points, add additional film and development info, and then have Excel store the data into the BTZS database. And then you could view it all in the BTZS program. A macro could do all of that for you, once you figured out what needed to be done.

sanking · Dec 21, 2005

Kirk,

As best I can determine the Tobias Wejex operates at 60 Hz. A.C. at 115 Volts at .15 amps. No 12v DC here.

Do you think I get more stable results operating it through an AC/DC inverter?

Sandy

Kirk Keyes said:
OK - I see it's purely a space saving consideration to use the sensitometer vs. enlarger.

I'm curious about the light integrator use - I'm not directly familiar with your model of sensitometer, but I would hope that they would have some form of voltage control for the lamp. It's tungsten, right? If so, and it is a 120VAC bulb it may not be voltage controlled, but if it is 12V, then I bet it is. If they have a halfway decent electronic timer, you should be able to get +/- 1/60th of a second for the exposure. Most of the digital darkroom timers I've checked are listed as being that accurate. At around 1 second or more, that seems more than precise enough. That's +/- one thirtieth of a stop.

So if the lamp voltage is controlled and the timer is that accurate, do you need to worry about using an integrator?

I would also try to not use the lamp brightness variable resistor to change the lamp brightness. it will change the color temp of the bulb, and you probably want it to be at it's brightest setting to get the bulb as close to 3200K for the 80A to convert to daylight.

sanking · Dec 21, 2005

Well, you are right of course in that I should have made my intentions know. But hell, I bought the Wejex, in box as new, for less than the price of six-pack of Heineken, so not much lost if I can not make it do what I want.

Sandy

Jorge said:
Well yeah you should have posted your intention before you got it...

Mine is a Dupont sensitometer that had all kinds of scales but the best thing was that the light intensity and exposure time can be adjusted to give exposure of under a second. I have calibrated it for both 100 and 400 film exposures at 1/2 sec. All that for 20 bucks...sometimes E bay does work... [/QUOTE

Jorge · Dec 21, 2005

sanking said:
Well, you are right of course in that I should have made my intentions know. But hell, I bought the Wejex, in box as new, for less than the price of six-pack of Heineken, so not much lost if I can not make it do what I want.

Sandy

This is true....arent you worried about reciprocity with 2.5 sec exposures?

sanking · Dec 21, 2005

Jay,

You are absolutley right about the potential. And it is very frustrating that this potential is not available as a system, at least for use with Winplotter. Both of my densitometers, an X-Rite 810 and a Gretag D-200-II, have DB-25 interface for computer connection, and this works well with Excel, but not directly to Winplotter. If anyone can provide a fix around this I would be elated.

Sandy

jdef said:
The potential exists for a system that allows the user to quickly and efficiently calibrate film exposure and development to printing paper, and even determine printing exposure with great accuracy, it seems it's just a matter of puting the pieces together. The pieces, I think, include:

Jay

sanking · Dec 21, 2005

Yes, I am very concerned about reciprocity with exposures of 2.5 seconds. I thought I made that clear in my first posting.

Anything over about 0.5 second exposure is likely to result in reciprocity failure with most B&W films.

Sandy

Jorge said:
This is true....arent you worried about reciprocity with 2.5 sec exposures?

Jorge · Dec 21, 2005

sanking said:
Of course I am concerned about reciprocity with exposures of 2.5 seconds. I thought I made that clear in my first posting.

Anything over about 0.5 second exposure is likely to result in reciprocity failure with most B&W films.

Sandy

I guess you are stuck with tmy then....no reciprocity up to 3 secs....

So, how are you going to "calculate" the exposure of regular film if the gizmo is set to expose for 2.5 secs?

sanking · Dec 21, 2005

What I plan to do is re-connect the wires to bypass the built-in timer and operate the bulb directly through the Metrolux timer/integrator. Should be a fairly simple procedure. Course, you never know for sure until the "thing" is done.

Use of the Metrolux will allow exposures of down to 0.1 seconds, with accuracy of about 1/100 of a stop.

Sandy

Jorge said:
I guess you are stuck with tmy then....no reciprocity up to 3 secs....

So, how are you going to "calculate" the exposure of regular film if the gizmo is set to expose for 2.5 secs?

"

sanking · Dec 21, 2005

jdef said:
Keep up the good work, Sandy, and please keep us posted on your progress.

Jay

Thank you, Jay. And I will say the same to you. You are doing interesting work in developer formulaton and I commend your efforts to test and report the results with good sensitometry.

Best,

Sandy

Kirk Keyes · Dec 21, 2005

Jorge said:
I guess you are stuck with tmy then....no reciprocity up to 3 secs....

Or use Fuji Acros 100. But then 8x10 is the largest size for that film.

Kirk Keyes · Dec 21, 2005

sanking said:
As best I can determine the Tobias Wejex operates at 60 Hz. A.C. at 115 Volts at .15 amps. No 12v DC here.

Do you think I get more stable results operating it through an AC/DC inverter?

115 VAC - that's for the mains power I'm sure. It's possible the lamp is 120 VAC, but I bet it's not. Any way to see what kind of bulb it uses?

AC/DC Inverter? Not sure what kind you mean, but if you are talking about a regulated 120 VAC supply, then I bet that is just fine. But then the real question is how much does your household line voltage vary, and how often. I used to work in a photo lab and they had volt meters that plugged into the outlets permanently installed in the darkrooms. I used to glance at them and I don't think I ever saw them deviate from 120V. Either the voltage didn't change significantly, or they didn't really work... I'm guessing the former.

Not sure how "brown" a UPS would let your power go before it cuts in, but that may be something to try. It would certainly help filter any spikes that come down the line.

I would suggest that a cheap way to find out the quality of your home voltage is to buy a cheap digital volt meter for radio shack or lowes or whoever is close, and plug it into the outlet and find out. (Please be careful and not touch the bare electrodes or short them out. Also, make sure you are set to the proper volt AC scale, and not to Amps as I was one time. tripped the circuit and arc-ed across the tip of the electrode and melted it.) The level of meter needed to do this type of test is are really inexpensive these days.

I can see the need for a light integrator for a light like a fluorescent that does not start up quickly or evenly, but for tungsten bulbs, I think that one would be overkill. But then I've never tried one... I just use the voltage regulator from the enlarger.

Since your are ripping the sucker apart - what about adapting a small photoflash like a Vivitar 283. Everyone has one of those laying around to do tests like this with, right? At it's highest power setting, it's probably around 1/1000 sec. It's at the other end of the spectrum as far as reciprocity curves go, but probably close enough and really conveinent to use.

Also, aren't most of your exposures in the 1 second range anyway? Mine often are by the time I start putting filters on the lens and stopping down.

Kirk Keyes · Dec 21, 2005

sanking said:
Use of the Metrolux will allow exposures of down to 0.1 seconds, with accuracy of about 1/100 of a stop.

Wow - that's pretty impressive. I'm not sure why you need 1/100 of a stop accuracy at 0.1 seconds though. Does it really do that at that short of a speed, or is it more like +/- 0.1 seconds and then with a 10 second exposure that's 1/100th of a stop?

Kirk - www.keyesphoto.com

sanking · Dec 21, 2005

Kirk,

Light integration is like water through a dam. Whether the duration is 1/100 of a second or one hour the amount of water that passes through the gate is easily measured.

However, the accuracy for my integrating system, as stated, is 1/100 of a stop at an exposures of 1.0 second, not 1/100 of a stop at 0.1 seconds. Sorry for the inaccurate data.

Sandy

Kirk Keyes said:
Wow - that's pretty impressive. I'm not sure why you need 1/100 of a stop accuracy at 0.1 seconds though. Does it really do that at that short of a speed, or is it more like +/- 0.1 seconds and then with a 10 second exposure that's 1/100th of a stop?

Kirk - www.keyesphoto.com

Kirk Keyes · Dec 22, 2005

jdef said:
To calibrate the Wejex one must expose a strip of the film to which one wishes to calibrate, process in a standard developer, like D-76 or Xtol, or whatever one chooses as the standard, measure the low densities with a densitometer, adjust and repeat as necessary until the target densities are acheived. Every time the light intensity is adjusted, one must take the cover off of the sensitometer, make the adjustment, and replace the cover for the exposure. Very inconvenient, to say the least

Jay - how does this calibrate the sensitometer? It looks like you've merely adjusted the amount of light for the exposure so that you get a good set of densities on the film. As far as calibration light levels to the real world, I don't see how you can do it like that...

You asked about my spreadsheet and interfacing with the densitometer earlier. I haven't done it with the Noritsu DM-201 I have, and like Sandy's Xrite and Gretag machines, it also has a serial port. There are "wedge" programs out there that can take RS-232 data and bring it into Excel and thus into a spreadsheet. I haven't bothered setting mine up yet, but I need to make a serial connection to the scanning spectrophotometer that I got last summer, and once I do that, then it should be easy to connect the densitometer.

If you know Visual Basic or whatever it's called now (.NET?), you could make a little program to access the RS-232 port and pull the data in. But I don't have those skills...

For another approach, you could use a flatbed scanner, make a scan of the film, and then use a program like Pixel Profile http://www.efg2.com/Lab/ImageProcessing/PixelProfile.htm to extract the RGB data from the scan. If you scan at the same resolution all the time for this and select a similar area to scan, then you can make a spreadsheet to pull data points at regular intervals out of the pixel profile data to coincide with your step wedge steps.

Calibrate your scanner by scanning in a calibrated step wedge, then you can make a graph of the scanner RGB values vs. the step wedge optical density in RGB, find the slope of the line, and then do the linear regression calcs, and you then should have a scanning densitometer!

(Photo Techniques this issue has an article that covers this but they use a NIH program to extract the data.)

I've scanned my IT8 target and manually found the pixel values for each of the grey steps on the bottom edge of the IT8 target and the vertical grey column, and then looked up the L*a*b values from the IT8 calibration file and converted them to RGB data. The scanner when IT8 calibrated did a very good job of matching the calculated L*a*b > RGB values, and then I graphed the scanned data vs. optical density readings I get from my densitometer. There was a very linear relationship to them. (At least when I've done the IT8 calibration with the scanning software (Silverfast)).

Give it a try Jay and let me see what you think!

Kirk Keyes · Dec 22, 2005

OK - I just checked and the scanner was pretty linear until pixel values above 220 or so were reached. That's about 0.20 and lower in optical density. At the high end - RGB values less than 25 or so start to get noisy with my scanner (Umax Powerlook III). That's above 2.0 OD, so for some alt. people, that may not be sufficient.

sanking · Dec 22, 2005

I may need to qualify my comments about accuracy. The Metrolux II has a timer that measures light units in units that approximate 0.1 second. The system is calibrated first to the light unit, aftter which repeat exposures at 0.1 units give consistency of better than log 0.01 after development, and I suspect that any drift at all is due to lack of consistency in the developer, type of agitation and temperature rather than to any lack of accuracy with the integrator. In other words, an exposure of 0.1 units should always produce the same amount of light on the film, subject to the precision of the hardware and software.

Sandy

sanking said:
Kirk,

Light integration is like water through a dam. Whether the duration is 1/100 of a second or one hour the amount of water that passes through the gate is easily measured.

However, the accuracy for my integrating system, as stated, is 1/100 of a stop at an exposures of 1.0 second, not 1/100 of a stop at 0.1 seconds. Sorry for the inaccurate data.

Sandy

sanking · Dec 22, 2005

Kirk Keyes said:
Also, aren't most of your exposures in the 1 second range anyway? Mine often are by the time I start putting filters on the lens and stopping down.

Well, that is a good point, because most of my exposures are in the 1-10 second range, so it might actually make sense for me to calibrate to the 2.5 second exposure of the Wejex for my own personal work. But I don't believe it will be possible to adapt the unit to give it the versatility of the enlarger timed by the Metrolux since with it I can adjust for ASA merely by changng the apeture of the enlarging lens.

Sandy

Kirk Keyes · Dec 22, 2005

jdef said:
Do you mean something like measuring the light output of the sensitometer at the film plane, with a light meter, and finding the EV that corresponds to a 2.5 second exposure?

That's exactly what I mean. How do you determine your film speed unless you know how much light is hitting the film?

You don't need to measure through the lowest step on the wedge - you can just measure the light without the wedge there. But I guess you have your wedge mounted in the sensitometer, so I guess measuring through the lowest step would work, if your steps are large enough.

If you have an Minolta incident meter, most of them accept a flat diffuser which costs about $25 or so.

Or, head over to Boise and rent a meter for the afternoon and make the measurment. Here in Portland, you can rent a meter for about the same amount of money.

Once that's done, you can use the following relationship:
The ISO B&W film speed formula is:

.8/Hm = S

Where,
Hm = Exposure in meter candle seconds (i.e. lux-seconds) that gives a film density of 0.10 above film-base-plus-fog, and,
S = Film speed.

If you graphed your film tests so that you are using lux-seconds on the x-axis, you can find the value for Hm that gives 0.10 Base+Fog, and then use the equation above to get film speed. If your speed point was 0.008, then you get 0.8/0.008 = 100 ISO speed. Pretty easy.

To make the graph in the first place, you figure the exposure through each step. Make a light measurement through the step wedge, and then convert the value to lux by looking at your meter's calibration from the owners manual. Then simply multiply that base expsoure value (in lux) by the exposure time (in seconds). Since you know the density of each step, then next step is the base exposure you just figured out in the last sentence, and subtract the density of the wedge step, and then multiply the exposure time. Repeat each step. It's easy done than written.

Sensitometry with the Tobias Associates Wejex Model R

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