Lighting for High Speed TV and Film Shoots
When shooting high speed both the quantity and the quality of light are important considerations: as the frame rate increases, there is less time to expose the film or video frame, so more light is needed, and flickering not seen by the naked eye or cameras shooting at 25fps becomes all too apparent. To address the quantity issue, one cannot simply add more lights - not only can the heating effect cause practical problems, but also the flicker effect will remain. For those who have not seen it, flicker looks like the lights are throbbing or pulsating, which indeed they are as the filaments heat and cool in time with the 50Hz mains cycle.
Using Strobe Lights
When shooting with high speed film cameras, one often uses strobe lighting, such as those made by Unilux. They produce very sharp images because the very short exposure time (each flash lasts for about 1/100000th of a second) practically eliminates any motion blur. When using a strobe light, the shutter angle is irrelevant in terms of determining exposure, in fact the shutter acts really as a sort of capping shutter, and exposure is independent of frame rate, so 'ramping' special effects shots are easily achieved (in which frame rate changes during a take). However, it is important to arrange each flash to occur at some point during the open period of the shutter: this synchronisation is achieved by using the widest possible shutter angle (i.e. having the shutter open for as long as possible) and triggering the light source by the camera. The method of triggering varies from camera to camera, but is usually a pick-up attached to the camera motor shaft. Apart from dramatically reducing motion blur, strobe lights produce far less heat - which can be important if shooting living things (e.g. people!) and delicate models.
In fact, the images produced by strobe lighting can have so little motion blur that they look rather unpleasant - in the way that one can tell the difference between film and video because of a lack of motion blur, the footage can look like jarring 'bad' video. The effect is particularly noticeable if the objects are moving rapidly, because they have moved so far between frames. This can be alleviated by adding some motion blur back in - using a small shutter angle and introducing an additional steady light source (see below) with the right intensity.
Although at first daunting, metering strobe lights can be achieved quite simply. Most strobe lights have a 'metering' setting, giving a steady 60 flashes/sec, so by setting a light meter's gate to be 1/60s in the flash (strobe) mode, one can measure the light from a single strobe flash. Of course, one would take several readings to ensure that a single full flash was measured.
A minor detail to bear in mind is that is the 6000K colour temperature, which can be corrected using an 85 or 85-B ge, or of course one can colour correct at the lab or in post.
The main drawback of using Unilux lights is the cost: A typical shot, say a food commercial featuring falling coffee granules, would use two heads running off a single control unit. In the UK, hiring from Panavision this would cost £1380/day for the first head (an H3000) and control unit, plus £485/day for the second head and £385 for the operator. That's £2250/day. The other drawback is that it is not unknown for there to be a problem - when the film rushes return the next day an error in synchronisation can have dire consequences.
Steady Light Sources
There are three choices of steady lighting available to the TV & Film industry - hydragyrum medium-arc iodide (HMI), direct current (DC) lights such as the Longstrike or high wattage tungsten. The last might be surprising, but I will explain: at Pirate we have tested a wide range of tungsten lights using our digital high speed camera. Generally, the lower the wattage, the worse the effect. However, we have found that any tungsten light over 10kW gives a steady enough light that the effect cannot be seen. We believe this is due to the thermal inertia of such large filaments. In any event, most high speed shoots in our studio using our Phantom and Photron cameras have been lit using our 10kW lights, which give sufficient light to work with our 300ASA equivalent cameras - typically, we use two 10KW lights to illuminate a table top set-up. We have also used 5kW lights, but they do marginally flicker and should be avoided if possible.
Other lights which we have used that are flicker-free are the Biese 4kW and the Kinoflo's Vistabeam 600 lights.
Another approach is to use a triplet of lights with each light on a different phase, which produces a steadier overall light than an 18kW HMI for example. The approach is good for lighting large areas evenly. Whilst this is a very cost-effective solution to flicker-free lighting for high speed, the heat output can be a problem for people and models. Hence, at Pirate, we generally build rigs to perform pours, drips etc so that actions can be performed reliably and repeatably without stress.
And yet another approach is to use LED lights, which will not flicker as long as they are not dimmed.
And, finally, the Sun doesn't flicker ...