Coldlight Performance: Time, Intensity and Temperature

[ic-racer]

I amp posting some graphs that may be of interest to those thinking about using a coldlight.

These graphs were obtained from a Aristo 1212 with W45 Bulb (the old style bulb).

I'm no fan of coldlights, but this particular low-profile unit allows my 10x10 enlarger head to go all the way to the top, without hitting the ceiling of my darkroom.

 

[ic-racer]

Intensity vs Time

This graph shows the light intensity of the W45 lamp (as measured from a light meter on the baseboard) as a function of time. Per Aristo's recommendations, I limited the test to five minutes. The housing and lamp were pre-warmed with the lamphead's built-in heater.

This response (the decrease in intensity) is alluded to on Aristo's website, but its magnitude is not indicated.

As you can see (the blue line) the intensity drops one-half of a stop over five minutes, with most of the drop in the first few minutes.

Also, you can see that the lamp housing heats up during the exposure. I did not measure the temperature of the tube itself, I just used an infrared temp gun on the lamp housing just out of curiosity (ie, I am making no statements about lamp temp and intensity)

 

[ic-racer]

Significance of these results...

For me, since I am using a two-exposure printing technique, the results suggest some changes in technique that can improve the consistency.

My new heuristic is as follows:

1) Make the shortest exposure first (Green or Blue)
2) Use the same sequence (Green-first or Blue-first) for all test prints and the final print
3) Do the burning at the end of the initial Green-Blue exposure sequence.
4) Take a little break if I have a long "Framing and Focusing" period, so my exposure sequences can start with the lamphouse at the baseline temperature.

 

[ic-racer]

If the first exposure happens to be very long (over 1 minute) then the time between the Blue and Green exposures comes into play (as the lamp will be above the 39 degree baseline and progressively decreasing while preparing for the second exposure).

So, here is a graph of the lamphouse cooling over 10 minutes (following a 5 minute "on" period).

 

[George Collier]

And the V54?

It would be interesting to see this study with a V54 (newer) lamp. This is what I use, and don't experience inconsistency with split filter printing, but would still be interesting.
Does anyone in APUG have association with Aristo?

 

[jfish]

This is exactly why both the Zone VI stabilizer and subsequently the compensating timer were invented. Throw a photo cell in your head, hook up either one and while the actual time will change as the head first warms then gets to operating temp., the amount of beeps there are (1 beep for each "sec" you set on the timer) will never change. Redlight (?) timers used to use "LUX" as a reference for this instead of seconds so as not to confuse anyone. So...

just count the beeps!

 

[David A. Goldfarb]

Coldlight heads with a heater unit are more consistent than those without, and the ultimate solution is a compensating timer. I have both, and I really like my Metrolux. If I want to do something like a run of 40 postcards I can set it up for a very short exposure time, and I know they'll be absolutely consistent from the first to the last.

 

[ic-racer]

It would be interesting to see this study with a V54 (newer) lamp. This is what I use, and don't experience inconsistency with split filter printing, but would still be interesting.
Does anyone in APUG have association with Aristo?

I used conventional light meter to check mine. If you have a light meter, you can watch and see how much yours' changes with time. It would be interesting to know.

 

[ic-racer]

So, here is an example. The print with the edge burn had the burn done BEFORE the exposure to the center of the image. The lamp had decreased it's output by a fraction of a stop and the center image is therefore slightly lighter.

Certainly there is a perception component, but the enlargement shows it very clearly.

 

[ic-racer]

So, my conclusions are:

1) the effect is observable in real-life situations, though, it is subtle
2) you can almost always 'work around' the effect to eliminate it
3) I don't see a NEED for a compensating timer for my purposes (but there could be a cool electrical project in my future...)

Future experiments:

I wonder if setting the coldlight heater's thermostat to a higher temp will lead to less of a light output drop during the course of the exposure?

 

[Nicholas Lindan]

If this is the same enlarger you had posted to in the Large Format forum, do you still have the vent holes covered?

The reason I ask is that the covered vent holes will do two things: increase the temperature change when the lamp warms; slow down the time it takes for the lamp to come to a stable temperature.

It would be interesting to monitor the lamp's temperature. A thermocouple held on with a dab of RTV to the center of the tube is the usual method. To measure the time for the lamp to come to full operating temperature it is only needed to measure the current to the lamp: when the current stops dropping the lamp is warm. The lamp voltage can also be used for the meausrement - when it stops rising the lamp is warm. Lamp voltage gives a rough idea of the pressure in the lamp and hence the temperature - the relationship isn't as linear as one would like because the mercury in the lamp is vaporizing as the lamp warms.

The mercury vaporization is of interest because it makes for a dramatic shift in the lamp's output spectra when the mercury goes into the plasma. This causes problems for compensating timers unless the timer's filter is very well matched to the materials. With MG paper the contrast grade you get with a cold lamp and a hot lamp may differ.

For precise work with a fluorescent/cold light head it is best to leave the light on all the time and use a shutter to control the exposure. To save electricity some systems keep the lamp idling at a temperature where the mercury is just vaporized, bring it to full brightness, wait a few seconds for the lamp to stabilize and then open the shutter to make the exposure.

 

[ic-racer]

Yes, good pickup, the vent holes are still covered (one of the reasons I checked that "return to baseline temp." data, as I am planning on a comparison with the holes uncovered.)

I appreciate you comments on the way tube works.

Do you know if there is some reason the built in heater does not bring the tube to 'full operating temp?' Is there something else, besides temp, that comes in to play in the first 30 sec. of operation?

I ask this because it looks like just upping the thermostat to 45C would have the same effect as keeping the lamp on all the time. (But I suspect this is not the case, though I don't know why)

 

[Nicholas Lindan]

Do you know if there is some reason the built in heater does not bring the tube to 'full operating temp?' I ask this because it looks like just upping the thermostat to 45C would have the same effect as keeping the lamp on all the time.

The heater should turn off when the lamp turns on. The heater thermostat needs to be set at the steady-state lamp-on-forever temperature. Aristo has (hopefully) calibrated the thermostat so that it turns off as the lamp turns on. There may be a relay that turns down the heater when the lamp is turned on to assist in this action.

Covering the vent holes has probably upset this relationship. I would suggest uncovering the holes. Changing the thermostat setpoint will most likely not help matters as Aristo has (again, hopefully) made the system work acceptably as originally designed.

A thermocouple attached to the lamp will give you an indication if the system now holds the lamp temperature somewhat constant. Radio Shack sells cheap digital volt meters that come with thermocouple probes. Obviously, if the light output is steady from turn-on then the heater is doing its job and you don't really need to measure the temperature.

If uncovering the holes fixes the warm-up problem then an external light baffle is the solution to lessening the stray light.

I am not familiar with this enlarger or this Aristo light source, but Aristo grid lamps are sometimes designed to mount inside of or as a part of the existing original enlarger head. Aristo may have been using some part of the original lamp housing to baffle the light from the vents.

An email to Aristo may help clarify matters.

 

[ic-racer]

Nicolas,
Thanks for your reply. This Aristo unit head is just a box with the tube in it (the transformer is in a remote box). The heater is just 4 power resistors shorted to the mains. Only control is via a switch and the thermostat, (which would function as you suggested, ie, turns it off when the light is on, based on elevated temp).

Opening up the vents on top would mean creating a light trap, but I am prepared to do that to improve the performance. Its easy enough to test it without the tape on the holes. One thing that may cause it to not make any difference is that there are no 'inlet' holes for proper convection. So, it may be that all the cooling will be shown to be from conduction to the enlarger frame and radiation from the coldlight box. Only way to know is to test it.

 

[ic-racer]

I found this 'hidden' in the instructions for the VCL4500 on the Aristo web site:

The lamps do generate moderate amounts of heat on their own. Excessive on time can cause light loss. This is a temporary condition. When turned off and allowed to cool to the thermostat setting (95°F on the top of the lamp head) the light level will increase and return to normal.

Again, confirming this light loss is an expected behavior. And again highlighting that opening my cooling vents could speed up the return to temp.

95 degree Fahrenheit = 35 degree Celsius, so it seems my thermostat is set OK.