This article is an updated version of a previous blog post. It shows how you can measure and understand the distribution of UV light across your printing area.
It is quite easy for poorly adjusted lights to cause unwanted defects in your prints. The most obvious is ‘fall-off’ which is when the light intensity rapidly reduces at the edge of the printing area (this causes parts of your print to be under-exposed).
Therefore, when making fine platinum / palladium prints, it is important to understand how the UV light is distributed across your printing area.
What Does ‘UV Intensity’ Actually Mean?
We are all used to seeing light sources rated in watts of power, for example a 300W bulb or an 18W fluorescent tube, but this rating is only a guide to how much UV light it produces. And it tells us very little about how much of the UV light actually reaches the print, which is what determines the exposure time.
The amount of UV that lands on the print is heavily influenced by the light source’s power, of course, but it is also affected by the distance between the print and light source, how much UV is blocked by the printing frame glass, and other various factors.
A dim light source close to the print may produce the same exposure as a super-bright bulb a long distance away.
To properly understand what is happening during printing, we need to think in terms of the UV energy that actually lands on the print. This is called its intensity (or “irradiance”).
The intensity of light is defined as the amount of energy that falls on a specified area during a given time. Light intensity is usually measured watts per square meter, or more conveniently for iron-based printing: microwatts per square centimetre (µW/cm²).
The most practical way to determine the UV intensity is simply to measure it. You can use any UV light meter for this – just be careful to allow for natural fluctuations due to light source warm-up time and other factors. Alternatively you can use one of my Light Counter light integrators for your measurements. This takes a bit more time, but eliminates errors due to light source fluctuations.
Using an LC3+ Light Integrator to Measure UV Intensity
Here is how to measure UV intensity with an LC3+ light integrator (earlier models have a different process).
First place the sensor where you want to measure the UV intensity. It is important that the sensor is flat, pointing directly at the light source, and doesn’t move during the procedure.
Switch on your light source, and allow it warm-up so it is working at full power.
Touch the calibrate button, then the information button, and finally the Attached Devices button.
The attached devices screen shows the UV intensity being currently measured by each attached sensor.
Wait until the sensor reading is stable, then record the value.
Measuring UV Intensity Across the Printing Area
To measure the distribution of UV intensity across your printing area:
- Lay down a sheet of paper across your printing area
- Mark out a grid of measurement points on the paper – in the example below I used a 10cm grid
- Measure the UV intensity at each point on the grid
I use Microsoft Excel to produce a 3D chart that helps me to visualize the data. Because the absolute intensity measurement is not particularly useful for printing purposes, I also use the spreadsheet to convert the data to show the intensity relative to the maximum measurement (in stops). That makes it much easier to predict fall-off at the edges of the print area.
Interpreting the UV Distribution
When you look at the data, the first thing you will notice is that the light distribution is not even. It is highly likely that the area directly under the light source receives noticeably more UV than anywhere else. It is also highly likely that you will see rapid fall-off of light intensity at the edges of your printing area.
Neither of these are likely to cause problems, so long as there is a sufficiently large area of mostly-even light big enough to fit your print. (For my purposes, I define “mostly-even” as being within ⅙ of a stop of the peak intensity. You may prefer to use something different depending on your aesthetic and technical choices.)
The first chart above shows measurements I made across my vacuum frame under a bank of fluorescent tubes. The yellow zone shows where intensity is within ⅙ stop of the peak, the orange zone is between ⅙ and ⅓ stop, and the red zone is between ⅓ and ½ stop of the peak. The chart shows that there is a rectangular area, approximately 40x70cm, where the lighting is sufficiently even for printing. Outside that area the light rapidly falls off and will lead to under-exposure.
The second chart shows measurements made under a 300W Vitalux bulb suspended about 1m above my vacuum frame. It clearly shows that the area of even lighting is much smaller, and fall-off is more pronounced. In my opinion this is quite unsuitable for anything but the smallest prints. Point light sources need to be quite some distance away from the print before they give even lighting. (See Calvin Grier’s set-up for an idea of what is involved with high power point light sources.)
Other problematic things you may see include:
- Regular stripes across the print area – most likely caused by fluorescent tubes being too widely spaced
- One side of your printing area having significantly lower light intensity than the other – for example from a bank of fluorescent tubes that are not parallel to your printing area
- Uneven lighting from having multiple bulbs of different power, or poorly adjusted bulbs
Knowing how your light source distributes UV over your printing area helps you to better understand what is happening when you print, and this can help you solve some common printing problems.
You can do this by trial and error, but it is easier, faster and cheaper to measure it.
While you are here…
My Light Counter system of light integrators provides a state-of-the-art tool for controlling exposures for UV printing, in particular iron-based photographic process such as platinum/palladium printing.
You can find out more here: Light Counter System
And you can buy the LC3+ kit here: LC3+ Light Integrator Kit