Changing light output from enlarger bulb??

[Pasto]

While printing postcards for the recent APUG exchange, I noted that exposure across postcards was not even. At first I thought this was due to developer exhaustion but this was not the cause. When I make several prints back to back (as with postcards) it appears that the enlarger bulb produces a decreasing light output. After about 6 or 7 consecutive exposures of about 7 seconds each (with time enough to change paper between exposures), the last 1 or 2 prints in the sequence are very underexposed. I understand that with short exposures any difference in light output across prints is more likely to show. However, should the output vary this much? Should I let the bulb cool down before making the next exposure in a sequence? Mabe this is the first sign that the bulb is about to go??

Luigi

 

[pentaxuser]

If the bulb was about to go I wouldn't have thought that the decreasing dimness would have been anything like uniform which is what it seems to be. Are you saying that previously you could do 6/7 exposures and the prints would look identical but then on printing postcards whenever this was ,that you saw this progressive dimming?

If you have always experienced progressive dimness then I am totally puzzled


pentaxuser

 

[ic-racer]

What type of bulb? This is a common characteristic of cold cathode type bulbs. Unusual for a halogen lamp to have this behavior. I was easily able to document a decrease in intensity over time for my coldlight with a standard exposure meter. Maybe you could test your lamp output in a similar manner.

 

[Ian Grant]

You don't say what enlarger/bulb but with short exposures the warm up time of the bulb becomes more critical, this is worts with ordinary filament bulbs, and cold cathode heads. Stopping down further to get exposures over 10 seconds will usually help. bulbs do gae but not usually that quickly.

Ian

 

[Pasto]

The bulb is the standard 250watt/24volt ELC halogen found in Durst color heads. To clarify, yes the first 4 or 5 postcards are properly exposed then the next 2 or so in the sequence are underexposed. I've never had this problem before as I typically print in the 30 - 60 second range. With repeated small exposure the lamp warms up gradually. As it does the output seems to drop off quite rapidly after the lamp reached a certain temperature. I'll try running some tests with my hand held meter, and also increase my exposure times so that any warm up effect is a smaller portion of the total exposure. With many postcards to print, I thought I could save time with brief exposures. It ended up costing me more time in printing many postcards a second time....

Thanks for your input.
Luigi

 

[Ian Grant]

Those lamps don't really age significantly so it's almost certainly the short exposures causing the problems.

Ian

 

[pentaxuser]

Can you say what your exposure is? There may be little or no linkage with my Durst, a 605, with colour head in which I use a 75W bulb( 100W recommended) but my exposure for postcard is very short, say 5-6 secs without problems. Maybe with 250W your exposure for the same size paper is very short such as under 3 secs but if so then shouldn't this affect all postcards due to the portion of warm up time to overall time being quite large and not just postcard 6 and beyond?

If the lamp's output drops off after it reaches a certain temp then wouldn't this affect the long exposure prints i.e. the over 30 secs which are your usual exposures.

Do you have an analyser or EM10 probe? Leave it on any part of the neg projection in its probe mode with the lamp on and see what changes occur over a set time period such as 20 secs. If the theory of lamp output drop-off is correct then the recorded probe exposure will start to change.

Maybe my logic is faulty but I cannot reconcile the symptoms you get with the variation over a short period but only for prints 5/6 onwards.

Whatever is causing it but not affecting the first few prints and not affecting longer print exposures at all is maybe still eluding us. I am damned if I can work out why a 250W should do this but not my 75W or 100W when I used one.

If it is short exposures that dim the lamp after 6 prints then you'd imagine that this phenomenon would be known and if so what would Durst say about ensuring that small prints are consistent?

Surely it is not an inherent characteristic of Durst and 250W lamps or if it is then you'd think Durst would say: "Caution not designed for small prints. If you have to do postcards then add neutral density to ensure that exposures are above the X secs range"

pentaxuser

 

[Denis R]

shedding light on the subject

it may have to do with the halogen cycle

 

[Bob-D659]

It's most likely the relay contacts in the power supply are rather cooked. I just swapped the relay in mine, the contacts were cooked, and now all my standard filtering has to be adjusted, the light is noticeably brighter.

 

[Nicholas Lindan]

Ditto relay contacts. But also look at the bulb holder - the holders for halogen bulbs are service items and they do wear out and corrode, giving rise to all sorts of mysterious problems.

 

[ic-racer]

The bulb is the standard 250watt/24volt ELC halogen found in Durst color heads.

Ok, do you know which power supply. I know the TRA450 uses a triac, instead of a realay. So in that case the triac could be on its way out. Also, as Nicholas suggested, the bulb socket could be a problem. Many of the on-line bulb suppliers also carry replacement sockets. Its probably not a bad idea to replace the socket in an old head anyway. Also, the contacts on the connector that goes from the power supply to the head should be inspected for signs of arcing or carbon.

 

[Bob-D659]

Ic-racer, there are three versions of the TRA450, only one uses a triac, the older two only use a relay for switching the bulb supply current. Mine is the oldest style.

 

[ic-racer]

Ic-racer, there are three versions of the TRA450, only one uses a triac, the older two only use a relay for switching the bulb supply current. Mine is the oldest style.

Good point.
In fact, just last week I was cleaining up an Omega timer for a friend and was glad to see that the relay could be dissasembled to polish the contacts. The contacts were so bad, I was surprised it made any electrical connection at all.

 

[Pasto]

OK, I now have several suspect. I'll have the power supply checked by an electrician friend. I'll inspect the socket in the enlarger head, and maybe try replacing the bulb to see if this continues to occur. Regarding the power supply, I did note that a small light spark is visible from the top of the unit when I start an exposure. I have not noticed this before. This may be a sign that something is about to fail in the power supply....

Luigi