I use UV fluorescent tubes for my printing. I have sixteen 18W tubes lined up above my vacuum frame, and I’ve used this set-up for years with only minor variations.
Fluorescent tubes are great because they are cheap, easy to install, last forever, and produce lovely, even light coverage.
But fluorescent tubes do have one drawback: when started from cold they can take quite some time to reach peak intensity. This can have a big impact on exposure times.
The chart shown in Figure 1 shows the intensity of UV A light as measured while calibrating my Light Counter LC2 system. The lights were cold when switched on. As you may expect, when switched on there was ‘instant’ light but it was far from full strength: it took over three minutes for them to reach full power.
By way of contrast, the chart in Figure 2 shows a second calibration exposure, made a few minutes afterwards. Again there was ‘instant’ light, but this time it reached full intensity after just a few seconds.
But rather than the light’s intensity at a moment in time, what really matters is the total light energy that has fallen on the print being exposed. To understand this we need to look at the cumulative light measurements. These are shown in Figure 3.
Figure 3 clearly shows that, for any exposure duration, the total exposure from the cold-start tubes is less than that from the hot-start tubes. The difference at one minute was about 2/3 of a stop; and at two minutes (when the hot-start calibration measurements finished) the difference was a little under 1/2 a stop. As exposure times increase, the effect of the cold-start gets smaller, but this is a disaster for short exposures.
Anyone who prints with fluorescent tubes soon learns to start a printing session by warming up their UV tubes. This reduces their variability a lot, and is good enough for most purposes. But it is only just ‘good enough’ because any pause in printing will allow the tubes to cool down again. This can be extremely frustrating when making fine prints or when you just want two prints from the same negative to look the same.
A better solution is to control your exposures with a light integrator. These devices continuously measure light intensity, and adjust the exposure time accordingly. A light integrator will easily cope with the situations shown in the charts, and will produce identical exposures. Although the exposure times would be different, the amount of UV energy landing on the print would be the same – and that is what matters.
So, once you have a light integrator, your exposures become much, much more precise.
… or do they?
Light integrators are great, but only if you are confident that they are consistent from exposure to exposure.
Every photographer understands that the exposure their camera measures doesn’t depend upon just on the sun’s brightness: it is also heavily influenced by where we point the camera. Pointing a camera at the sun will lead to a very short exposure, while pointing it only slightly away from the sun will lead to a longer exposure. It’s the same sun, and it’s producing the same amount of light, but the camera (or light meter) sensor is pointing in a different direction so it receives a different amount of light.
The same is true for light integrators. If the sensor moves slightly relative to the light source, then its readings will change. For example, I mount my sensors on the side of my light box pointing at the UV tubes. In that position, this sensor measures about 700 units of UV A every second when the fluorescent tubes are at full power. However, if I lay the sensor on the vacuum frame glass directly underneath the UV tubes, then it measures over 2000 units a second. The sensor is measuring the same UV tubes and they are producing roughly the same amount of light, but the readings are totally different.
Unless you can be certain that the sensor will never move relative to the light source, then it is essential to have a way to calibrate sensor measurements so that the 700 units measured in the former position is somehow made equivalent to the 2000 units in the latter position. This is a complex problem, and a significant amount of my development time was spent on solving it.
The calibration process I have built is highly accurate, and essentially eliminates the problem of moving sensors. It means that it doesn’t matter if you move your sensor, change your lights, or even move to a completely new light source: just recalibrate and you can be confident that your exposures will be consistent.
Best of all, the process is extremely simple – just press the calibrate button and wait for a few minutes. Unless you change your configuration there’s no need to calibrate the system every time you use it, because the calibration results are stored in permanent memory and are automatically reloaded when the system is switched on.
Enjoy your printing!