Just got an EG&G Sensitometer

Bill Burk · Mar 27, 2010

The E stands for Edgerton, who gave us stop-action studies of bullets piercing playing cards and balloons. His talent was controlling flash length. We are fortunate he could slow things down enough to give us 10-2 (1/100th-second) flash. But no, for long exposures (where I am likely to work shooting moving water in shady canyons), the EG&G won't take us there.
Shoot.
I wasn't worried about this until now.

ic-racer · Mar 27, 2010

Some more EG&G pictues. This is the ignition coil.

ic-racer · Mar 27, 2010

Here is the light box that sits on top of the bulb:

ic-racer · Mar 27, 2010

Here I am prepared to make the filter set. Since my clear filter that came with the unit was plexiglas, I used plexi for the other two that I made.

ic-racer · Mar 27, 2010

I used a saw to cut the plexi, then smoothed the edges with sandpaper.

ic-racer · Mar 27, 2010

ic-racer · Mar 27, 2010

This shows the two filters, painted black on one side, ready to have lines scraped into the paint.

Completed filters:

ic-racer · Mar 27, 2010

Stephen Benskin said:
No, I was thinking reciprocity failure on the other end of the scale - ie short. Either way, reciprocity failure doesn't fall within the parameters of the ISO standard. But it must be nice having the ability to test for long exposure reciprocity.

I just looked up Wejex. Sounds like a nice little device. They mention the choice of a blue or green filter. That sounds like it was designed for X-Ray film testing. Do you know the color temperature of the light source?

Steve

The current model blue/green Wejex is in a little different box than the older ones and the innards may be different. The older ones like mine use a white light source. Mine has a small incandescent bulb. All the electronics in side are run from 120 AC; there is no 'power supply' per say.

The size and configuration of the box is designed for evenness of illumination. The bulb is placed some distance from the diffuser to minimize cosine effects at the edges.

In mine there is a shutter on a stepper motor, keying off the 60hz mains and rotating about 30 rpm. It takes about 2 seconds to rotate, but the exposure through the shutter is about one second.

Light intensity is adjusted via a hefty AC rheostat. There are two test ports for an AC voltmeter to verify the voltage on the bulb. Realize this was sold as a 'control strip' maker, not an ANSI testing device.

Nonetheless, as long as the voltage on the bulb is checked before the run and is constant, one can make pretty good relative film speed determinations.

Lamp Voltage Test ports on bottom. The rheostat adjustment for light intensity is under the round cover:

Step wedge permanently mounted on the top panel under glass. I was able to take the top panel off and interrogate the wedge with my densitometer:

Filter drawer (not pulled out far enough to see the filter) I put an 80A in there for daylight correction:

ic-racer · Mar 27, 2010

Skipping a little back and forth between the Wejex and EG&G, here is a generic flash circuit of Edgerton's. The EG&G sensitometer follows it very closely.

sanking · Mar 27, 2010

Stephen Benskin said:
No, I was thinking reciprocity failure on the other end of the scale - ie short. Either way, reciprocity failure doesn't fall within the parameters of the ISO standard. But it must be nice having the ability to test for long exposure reciprocity.

I just looked up Wejex. Sounds like a nice little device. They mention the choice of a blue or green filter. That sounds like it was designed for X-Ray film testing. Do you know the color temperature of the light source?

Steve

Not the exact temperature, but it is a tungsten bulb so I think using an 80A filter would balance it pretty well to daylight.

Sandy

ic-racer · Mar 27, 2010

Here is a historic Wejex ad from the American Journal of Roentgenology:

Stephen Benskin · Mar 27, 2010

I find it interesting how the ad says that for X-ray film "exposure made just before processing eliminates latent image fade problem." That's because X-ray films are usually processed shortly after they are exposed so it's important to have conditions similar to those in actual use. Pictorial films are a different matter. They require a "hold time" which is easy to skip when using a sensitometer. You have the processing equipment all set-up. You then expose the wedge and immediately process the film. This can lead to inaccurate results because most people don't process their film immediately after exposing it. Latent image keeping is similar in nature to reciprocity failure - electrons are given off over time and silver atoms revert back to silver halide. Most of the effect is within seconds of exposure and it tends to plateau after a few hours, but it never stops. In other words, the effective film speed will continue to drop in relation to the length of time between exposure and processing. It is therefore important for the determination of accurate film speed to have a hold time that reflects usage. If you develop the film too recently after exposure, the speed will be unrealistically high.

ANSI / ISO used to stipulate a minimum two hour hold time but the revised ISO standard actually has two different times, one for amateur films and one for professional. As professionals tend to process their films soon after exposing, professional hold time is not less than four hours or more than 7 days. For general-amateur films, the hold times need to be not less than 5 days or more than 10 days.

Photo Engineer · Mar 27, 2010

Today, the LIK is not much of a problem. Most all Kodak films contain the chemistry which inhibits Latent Image Failure.

So, professional and amateur films are more alike.

In any event, all Kodak control strips for film and paper are held for a set period of time before freezing and shipping to customers.

PE

sanking · Mar 27, 2010

ic-racer said:
Filter drawer (not pulled out far enough to see the filter) I put an 80A in there for daylight correction:

Damn, my Wejex looks just like the one you show, but it does not have that neat little filter box? Is this original or did you add it?

Sandy

Kirk Keyes · Mar 28, 2010

sanking said:
ic-racer said:

Stephen, how did you make the mcs readings with your EG&G?

Click to expand...

This is where having an incident flashmeter like the Minolta Flashmeter IV or VI would be useful. Put the flat diffuser on, and lay the meter on the film exposure bed, and then take a reading. Minolta has a lookup table (and a formula) so that you can make a conversion from the meter reading in EV.

ic-racer · Mar 28, 2010

sanking said:
Damn, my Wejex looks just like the one you show, but it does not have that neat little filter box? Is this original or did you add it?

Sandy

Yes, mine came with that handy little filter slot.

You also mentioned a 2 second exposure with yours.

In mine, the lamp is indeed on for about 2 seconds, and it buzzes for about 2 seconds but the rotating shutter covers the lamp for half that time, resulting in a one second exposure. Are you sure your shutter is working OK?

You also mentioned you use ND filters. Does yours look like mine on the bottom? If it does, you can pop off that little cover plug an get access to the rheostat for adjusting lamp intensity.

ic-racer · Apr 1, 2010

Greg Davis said:
This thread has me interested in trying to build one of these machines. If I were to build a box with a flash unit on the bottom with a diffuser on the top, then a stepwedge and a lid, I would have a simple but working unit, correct? The flash duration is set by the flash unit, but how would I calculate the necessary "aperture"? If I used slits like ic-racer, would it just be trial and error until I got a minimum density on my film?

I made those 'slit' filters just to keep my EG&G historically acurate. In reality just use various neutral density filters for your homemade unit. I'd just use some Rosco ND sheets. If they fade in your lifetime of using the senstometer, I'd pat yourself on the back for doing a heck of a lot of testing and buy a new Rosco sheet for seven bucks.

Before I got my two units I was going to build one with a spare flash just like you mention. I searched for a good box and located non-working shell of a sensitometer but missed out on fleabay. Then a working Wejex came up and I won that, so I didn't have to make one.

Most important thing will be to get even illumination and good contact between the wedge and the film. If not pressed flat, light bleeds around the step wedge. It should be pretty easy.

Other somewhat easy things are to get a free, small enlarger and dedicate it as a sensitometer.
Also, those 4x5in and 5x7in contact printing boxes with a bulb inside (which frequently are thrown away as junk) might also make a good start for a home-made sensitometer.
Also, a cheap X-ray green-blue sensitometer should also work fine for most all 'home darkroom' testing purposes.

For Large Format users (that want to test in the 1/2 to 2 second range), a Wejex style sensitometer can also be constructed with common parts. A 30 rpm stepper motor and a 120v 5W bulb are the major components.

ic-racer · Apr 1, 2010

Greg Davis said:
ND filters would certainly be easily replaced, but still, how would one go about calculating how much ND needs to be used for say 400 speed film? Is there a method? Or just trial and error?

Just trial and error. In the EG&G manual there is a chart because they know roughly how bright the thing was when new.

With respect to the Wejex, or any of the x-ray type sensitometers, or a home-made sensitometer you just have to adjust the intensity by trial and error so you can get the whole film curve.

sanking · Apr 1, 2010

Greg Davis said:
ND filters would certainly be easily replaced, but still, how would one go about calculating how much ND needs to be used for say 400 speed film? Is there a method? Or just trial and error?

It does not have to be merely trial and error. One you know the ND filtration that is needed for an ASA 100 film you should be able to adjust for an ASA 400 film by adding two stops more of ND filtration. One stop is the equivalent of log 0.30 of ND filtration.

Sandy King

sanking · Apr 1, 2010

ic-racer said:
Yes, mine came with that handy little filter slot.

You also mentioned a 2 second exposure with yours.

In mine, the lamp is indeed on for about 2 seconds, and it buzzes for about 2 seconds but the rotating shutter covers the lamp for half that time, resulting in a one second exposure. Are you sure your shutter is working OK?

You also mentioned you use ND filters. Does yours look like mine on the bottom? If it does, you can pop off that little cover plug an get access to the rheostat for adjusting lamp intensity.

I have not opened the Wejex in a long while but as best I remember there was no rotary shutter, only a bulb that turned on for 2.5 seconds. I know that it is possible to adjust the rheostat for intensity but I am ok with it as is since I dedicate the Wejex to testing film for long exposures. For most testing I use the EG&G.

Sandy King

ic-racer · Apr 1, 2010

sanking said:
I have not opened the Wejex in a long while but as best I remember there was no rotary shutter, only a bulb that turned on for 2.5 seconds.

Very interesting. So there may be quite some variability between Wejex units. I'll have to post some pictures of the inside of mine. The rotating shutter assembly is pretty cool.

ic-racer · Apr 4, 2010

http://cgi.ebay.com/VINTAGE-Kodak-P...emQQptZLH_DefaultDomain_0?hash=item53dc42bdb0

http://cgi.ebay.com/Vintage-ANSCO-M...emQQptZLH_DefaultDomain_0?hash=item53e08b0ee5

I think something like this would be a good starting point for a home-made sensitometer.

ic-racer · May 25, 2011

I'd seriously consider converting it over to a system like the EG&G posted in picture #55 above with just a plain piece of thicker glass. The EG&G, being a more advanced unit, allows freedom to use any type of step wedge. You are instructed to tape your favorite step wedge in place over the glass. The Wejex with the 'built-in' step wedge is more for an 'unskilled labor' setting where the intention was to be goof-proof.

ic-racer · May 25, 2011

Ok, after seeing the unit I have changed my thinking a little.
1) That thing is fantastic, I should have bought it ! The older Wejex is just a stepper motor driving a shutter, and a little light bulb.
2) I can't see the crack from the pictures.
3) I agree with trying to put it back to its original state. I was able to get spare parts for my older Tobias for my densitometers. I suspect they may have the replacement glass that you need. Service@tobiasinc.com

ic-racer · May 25, 2011

Fresh Kodak and/or Ilford film. Process it to ISO standard (D76 with appropriate gamma), per instructions and work backward from the equations in the manual to find the lux.

If you try to get a meter in there and calculate the lux that way, then when you test fresh Tri-X you will get some wonky result, so what do you do??

I think it is best to calibrate your sensitometer to some standard like "Fresh Tri-X processed to ISO standards = 400" and test all other films from there. I actually tested fresh TMY, Delta 100 and Delta 400 and averaged the results to calibrate mine.

Just got an EG&G Sensitometer

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