Using LC2 System to Measure UV Intensity Across a Printing Area

One of my customers wanted to know whether their UV light source was even across their printing area. This article shows you how to check this using the Light Counter LC2 UV Light Kit.

Distribution of UV Light under Fluorescent Tubes
Distribution of UV Light under a Osram Vitalux Bulb

UV Intensity

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. This is a function of the amount of UV produced, the distance between the print and light source, and how much UV is blocked by the printing frame glass.

To properly understand what is happening during printing, we need to stop thinking about the electrical power consumed by the light source, and start thinking in terms of the UV energy that 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).

Using a Light Counter LC2 Light Integrator to Measure UV Intensity

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 LC2 light integrators for your measurements. This takes a bit more time, but eliminates errors due to light source fluctuations.

Here is how to do it:

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.

Run the light integrator calibration process. The calibration process allows for natural fluctuations in light intensity, for example during warm-up.

Once calibration is completed, the light integrator determines a calibration value which it shows on the bottom row of the display. The calibration value is proportional to the intensity of 365nm UV light that has landed on the sensor during calibration. It is used internally to compensate for fluctuations in the light source, but we can also use it to calculate UV intensity.

Simply multiply the calibration value by 0.63 to get the UV intensity in µW/cm² (if you are an early adopter of the Mark 2 sensor then you will need to multiply by 0.93 not 0.63). Note that this is the intensity measured by the sensor after the light has passed through a narrow band 365nm filter. This measurement is unlikely to match the intensity measured by a meter with a different sensor and filter.

For example, if I measure the light directly under the centre of my UV fluorescent tubes, I get a calibration value of 135.7, which when multiplied by 0.63 gives an intensity of 85.5 µW/cm²).

For the greatest accuracy you can run the calibration process several times, and average the results.

Using a Light Counter LC2 Light Integrator to Measure UV Intensity Across the Printing Area

To measure the distribution of UV intensity across your printing area:

  1. Lay down a sheet of paper across your printing area
  2. Mark out a grid of measurement points on the paper – in the example below I used a 10cm grid
  3. 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

Conclusion

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 use a UV light meter.

Using my Light Counter LC2 system eliminates measurement errors due to light source warm-up time and other factors, which makes the results more accurate and meaningful.

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