LC6 User Manual – Working in 'Units of Light'

Printing exposures are traditionally based on time, measured from the moment the lights are switched on to whenever they are switched off. Time-based exposures are easy to measure, but inherently inaccurate because the intensity of all light sources varies over time. For example, some light technologies, e.g. fluorescent lights, take time to warm-up and reach full intensity. Other technologies, e.g. LEDs, are prone to overheating which reduces their intensity.

Light integrators allow photographers to make measurement-based exposures. These are based upon measurements of the actual intensity of light over the entire exposure. Measurement-based exposures are more accurate than time-based exposures, but require specialised equipment.

The Light Counter system uses a state of the art microprocessor, custom-designed digital sensor circuits and advanced software to bring measurement-based exposures within the reach of all photographers. The system was originally designed for platinum/palladium printing but will work for any ultraviolet-based or visible light-based printing process.

Light Counter systems are used by printers globally to maximise control when printing with platinum/palladium, cyanotype, carbon, silver gelatin and other processes.

 

Measuring Light with ‘Units of Light’

The light integrator measures light intensity in micro-Watts per centimeter squared (µW/cm2), but this is pretty meaningless to most photographers. Calibration allows the light integrator to convert this raw measurement into things called ‘units’. These are much easier to use when printing.

After calibration, one ‘unit’ is about one second of exposure when the lights are at full power. If the lights are a little bit dim, then the light integrator will record less than one unit. If they are a bit bright, then it will record more than one unit. In this way variability in the light source is accounted for, and ceases to be an issue when printing.

The light integrator does the necessary calculations and adjustments automatically, ensuring that your prints get the same exposure regardless of how the light intensity varies during exposure. You can even switch off your lights for a while during an exposure, and the light integrator will sort it out.

If the system has not yet been calibrated, then it will report the raw sensor measurement as units. For example, if the uncalibrated system measures 2,000 µW/cm2 then it will report this as 2,000 units.

‘Units’ are calibrated to a specific light source, so they are not comparable between exposure units. A 300 unit exposure under a low intensity light source is not the same as a 300 unit exposure under a high intensity light source. And because ‘units’ are an arbitrary measure, they are not comparable across systems made by different manufacturers.

 

Sensor Calibration

The calibration process adjusts the LC6 measurements so that 1 'unit' of light is about 1 second of time.

Calibration is beneficial for three reasons:

1. Calibration allows for variability between sensor chips. This helps if you ever need to replace a sensor
2. Calibration means we can deal with human-friendly numbers. For example, the raw measurements from my test LED light unit are about 4,400 per second or 264,000 per minute. It's much easier for us to deal with exposures of about 60 per minute
3. Calibration makes it easy to change from using time-based exposures to units-based exposures. If your old exposures were 5 minutes, then your exposures with a new LC6 will be about 300 units

Calibration data are stored on the sensor. These data are specific to the sensor, its physical position, and the light source it is measuring.

Sensor calibration is managed from the control panel.

 

But Can I Still Use Timed Exposures?

Of course! The LC6 includes a powerful darkroom timer, accurate to 0.1 seconds. This is especially useful for silver gelatin printing with an enlarger.

 

Please see the rest of the LC6 User Manual for information about how to use your LC6 system to maximise your control over your printing, or contact Ian Leake for advice.